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Eukaryotic Cell, August 2006, p. 1215-1228, Vol. 5, No. 8
1535-9778/06/$08.00+0     doi:10.1128/EC.00037-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Analysis of Mitogen-Activated Protein Kinase Signaling Specificity in Response to Hyperosmotic Stress: Use of an Analog-Sensitive HOG1 Allele

Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, California 94720

Received 6 February 2006/ Accepted 23 May 2006

	ABSTRACT

Top Abstract Introduction Materials and Methods Results Discussion References

 
When confronted with a marked increase in external osmolarity, budding yeast (Saccharomyces cerevisiae) cells utilize a conserved mitogen-activated protein kinase (MAPK) signaling cascade (the high-osmolarity glycerol or HOG pathway) to elicit cellular responses necessary to permit continued growth. One input that stimulates the HOG pathway requires the integral membrane protein and putative osmosensor Sho1, which recruits and enables activation of the MAPK kinase kinase Ste11. In mutants that lack the downstream MAPK kinase (pbs2) or the MAPK (hog1) of the HOG pathway, Ste11 activated by hyperosmotic stress is able to inappropriately stimulate the pheromone response pathway. This loss of signaling specificity is known as cross talk. To determine whether it is the Hog1 polypeptide per se or its kinase activity that is necessary to prevent cross talk, we constructed a fully functional analog-sensitive allele of HOG1 to permit acute inhibition of this enzyme without other detectable perturbations of the cell. We found that the catalytic activity of Hog1 is required continuously to prevent cross talk between the HOG pathway and both the pheromone response and invasive growth pathways. Moreover, contrary to previous reports, we found that the kinase activity of Hog1 is necessary for its stress-induced nuclear import. Finally, our results demonstrate a role for active Hog1 in maintaining signaling specificity under conditions of persistently high external osmolarity.

	INTRODUCTION

Top Abstract Introduction Materials and Methods Results Discussion References

 
Many external cues initiate appropriate cellular responses via mitogen-activated protein kinase (MAPK) cascades (45). Although a eukaryotic cell contains multiple MAPK modules, the stimulus for each signaling pathway elicits the proper output and does not inadvertently evoke incorrect responses. The molecular basis of this specificity and the mechanisms that insulate one MAPK pathway from another are under intense study in many organisms.

Budding yeast (Saccharomyces cerevisiae) has multiple MAPK pathways. Increases in the external solute concentration stimulate the high-osmolarity glycerol (HOG) pathway, resulting in production of glycerol to prevent dehydration (61). Secreted peptides initiate the pheromone response pathway, which prepares cells for mating (60). Invasive growth by haploids and pseudohyphal growth by diploids are triggered when cells are limited for carbon (haploids) or nitrogen (diploids) (59). These three signal transduction pathways have some components in common, in particular, the MAPK kinase kinase (MAPKKK) Ste11.

Two distinct inputs lead to activation of the MAPK, Hog1, of the HOG pathway. Under isosmotic conditions, a transmembrane protein-histidine kinase, Sln1, acts through a relay of phosphoproteins to hold two highly related MAPKKK isoforms, Ssk2 and Ssk22, in the inhibited state (39). Moderate osmolyte concentrations (e.g., 0.1 to 0.4 M NaCl) somehow block Sln1 function, thereby relieving inhibition of Ssk2 and Ssk22 (29). These enzymes, in turn, phosphorylate and activate the MAPK kinase (MAPKK) Pbs2, which then phosphorylates both Thr174 and Tyr176 in Hog1 and thereby activates this MAPK (10, 50). When cells are exposed to higher dissolved-solute concentrations (e.g., 0.5 to 1.0 M NaCl), another integral membrane protein, Sho1, also participates in the activation of Hog1 (28). Sho1 directly binds the alternative MAPKKK Ste11 (66) and, via its C-terminal Src homology 3 domain, also binds Pbs2 (40). Pbs2, in turn, has a high-affinity docking site for Ste11 (40, 50). In this arm of the HOG pathway, association of Sho1 with a transmembrane mucin-like protein, Msb2 (33), and association of Ste11 with the small adaptor protein Ste50 (41) are also required for optimal signaling. Some osmolyte-induced change in the modification state, conformation, and/or composition of this membrane-tethered complex permits Ste11 to be phosphorylated and activated by a protein kinase of the PAK family, Ste20, which is itself activated and membrane localized via its interaction with GTP-bound Cdc42 (anchored in the plasma membrane via its C-terminal geranylgeranylation) (44, 47). Activated Ste11 phosphorylates and activates Pbs2, which then activates Hog1 (18). Once phosphorylated by either the Sln1- or the Sho1-initiated route, Hog1 is transported into the nucleus (via the importin ß homolog Nmd5) (20, 46), where it affects the transcription of 600 genes, including those needed for glycerol production and other events that restore osmotic balance (35, 38, 42).

Ste11 is also essential for response to mating pheromones (60). In this case, Ste11 is brought to the plasma membrane, where it encounters Ste20, by its binding to a scaffold protein, Ste5 (19). Ste5 is recruited to the plasma membrane, in part via its interaction with the membrane-anchored Gß complex (Ste4-Ste18) released from the G subunit (Gpa1) of the heterotrimeric G protein that is coupled to pheromone receptors (Ste2 and Ste3). G-protein dissociation is triggered when the appropriate agonist (-factor or a-factor, respectively) is bound to its cognate receptor. Ste5 also binds the MAPKK Ste7 and the MAPK Fus3, allowing efficient downstream signal propagation. Activated Fus3 translocates into the nucleus, relieves repression of the transcriptional activator Ste12, and thereby induces expression of mating-specific genes (48). Likewise, Ste20, Ste11, Ste7, and a different MAPK, Kss1, are needed for the change in budding pattern and elongated morphology necessary for the filamentous growth observed when diploid cells are deprived of nitrogen and for the invasive growth displayed by haploid cells when starved for carbon (59). Activated Kss1 promotes expression of genes under the joint control of Ste12 and another DNA-binding transcription factor, Tec1 (2, 12). As in the HOG pathway, Sho1, Msb2 (15), and Ste50 (58) are components of the upstream sensing mechanism necessary to stimulate Ste11 for filamentous growth.

How Ste11-dependent signal propagation normally is confined to the HOG, pheromone, and filamentous-growth pathways to elicit only the proper response is not completely understood. One important clue about how such specificity is maintained between the HOG and pheromone response pathways is the observation that pbs2 and hog1 mutant cells, when subjected to hyperosmotic stress, form mating projections and express a specific reporter of pheromone response, FUS1^prom-lacZ (34). This so-called cross talk is dependent on Sho1 (and not on Sln1). Thus, Hog1 (or one of its downstream effectors) somehow prevents inappropriate stimulation of the pheromone response pathway by the pool of Ste11 activated when Sho1 is subjected to an elevated solute concentration (36).

Essentially all of the studies demonstrating cross talk have been performed with hog1 null mutants, which chronically lack Hog1. The capacity for cross talk displayed by hog1 cells could arise from some rewiring of the cell resulting from the adaptations necessary for the cell to cope over the long term with the complete absence of Hog1. Hence, the behavior of hog1 mutant cells might be misleading with regard to the physiology of normal cells. Moreover, only a single kinase-dead hog1 point mutant has been described (20, 46). Although such cells should also chronically lack Hog1 catalytic activity, when examined for cross talk, they displayed an intermediate, and hence ambiguous, response (34). Thus, the first objective of the studies presented here was to devise a means to acutely inhibit Hog1 activity at will to determine if its catalytic function or just the presence of the polypeptide itself is required to prevent cross talk. Such a tool also provided the means to reexamine the role of the catalytic activity of Hog1 in its primary physiological function (resistance to hyperosmotic stress) and in its subcellular localization (translocation to the nucleus in response to hyperosmotic stress). Finally, we could examine whether the kinase activity of Hog1 is important both for the initial response to a hyperosmotic challenge and for sustaining signaling capacity and maintaining pathway specificity during long-term adaptation to elevated osmolyte concentrations.

	MATERIALS AND METHODS

Top Abstract Introduction Materials and Methods Results Discussion References

 
Strains, media, and genetic techniques. Yeast strains (Table 1) were cultivated in standard rich medium (YPD) or in defined synthetic complete medium (49) lacking appropriate nutrients and, where necessary, containing 5-fluoroorotic acid (9) to select for plasmids or integrative transformants. When used, D-sorbitol (Sigma Chemical Co., St. Louis, MO) was present at a 1 M final concentration. DNA-mediated transformation was conducted by the Li acetate procedure (51). All gene replacements and disruptions were confirmed both by phenotypic analysis and via PCR with appropriate gene-specific primers.

The FUS1::lacZ::LEU2 and FUS1::lacZ::leu2 reporter genes have been described previously (34). To generate the integrating TEC1::lacZ::URA3 plasmid, the 5' untranslated region of TEC1 was amplified by PCR from genomic DNA of strain 1278b DNA with primers with XhoI sites. The resulting 1-kb product was inserted into the polylinker in pJM133 (a plasmid carrying CYC1::lacZ::URA3 from the Herskowitz lab plasmid collection), yielding pPW45. The appropriate fragment was excised from pPW45 with HindIII and used for DNA-mediated transformation of appropriate yeast strains.

Integrated versions of HOG1, hog1-as, hog1(K52R), and hog1(D144A) tagged with Aequorea victoria green fluorescent protein (GFP) were created with the GFP-marked modules described by Longtine et al. (26) along with HOG1-specific primers. To install a C-terminal three-hemagglutinin [(HA)₃] tag on Rck2, the HA-marked modules described by Longtine et al. (26) were used along with appropriate RCK2-specific primers. To tag a nucleoporin, Nup1, and the karyopherin Nmd5 with mCherry, a derivative of monomeric Discosoma sp. red fluorescent protein (52), the coding sequence in pRSET-mCherry (gift of R. Y. Tsien, University of California, San Diego) was amplified with primers with PacI and AscI sites, respectively, and the resulting product was inserted into PacI- and AscI-cleaved pKT209 (gift of K. Thorn, Harvard University), thereby replacing the GFP module in this vector (53), yielding pPW58. Nup1-mCherry and Nmd5-mCherry fusions were then generated with the mCherry-containing module in pPW58 and integrated into the desired yeast strains as previously described (53).

To generate pPW86 expressing Ssk2N (28), the sequence of SSK2 encoding Met1173 to the COOH terminus was amplified by PCR with primers containing XbaI and HindIII sites, and the resulting product was inserted into the polylinker in YEp352-GAL (5).

Measurement of reporter gene expression. Expression of ß-galactosidase from lacZ-marked reporter genes was assayed with a colorimetric substrate as described previously (56). Values presented (A₄₂₀) represent the average of at least three independent trials, each conducted in triplicate. The enzyme sources were samples of cultures of exponentially growing cells that were collected by centrifugation and resuspended in fresh YPD medium in the absence or presence of 4-amino-1-tert-butyl-3-(1-naphthylmethyl)phenylpyrazolo[3,4-d]pyrimidine (1-NM-PP1) (8) lacking or containing 1 M sorbitol, as indicated, for various times (up to 5 h). To select the inhibitor concentration used, the growth of hog1-as cells and that of HOG1⁺ cells were compared on YPD without and with 1 M sorbitol over a range (5 to 20 µM) of 1-NM-PP1 concentrations. The 1-NM-PP1 was added from a stock dissolved in dimethyl sulfoxide (DMSO), and controls always received the appropriate volume of DMSO alone. We found that 8 µM 1-NM-PP1 was the minimum concentration that completely blocked the growth of hog1-as cells on 1 M sorbitol but not on YPD alone, yet even 20 µM 1-NM-PP1 had no discernible effect on wild-type cells on either medium (data not shown). On this basis, and to ensure complete inhibition, we routinely used a final concentration of 12 µM 1-NM-PP1 in both liquid and solid media.

To determine how fast 1-NM-PP1 acts to block the kinase activity of Hog1-as, we conducted the following comparison on the basis of our observation (see Results) that 1-NM-PP1 blocks the hyperosmotic stress-induced nuclear import of Hog1-as. One sample of an exponentially growing culture of cells expressing Hog1-as-GFP was pretreated for 10 min with 12 µM 1-NM-PP1, collected rapidly by centrifugation, and then exposed to 1 M sorbitol in the continuing presence of 12 µM 1-NM-PP1, and another sample of the same cells was treated simultaneously with 1 M sorbitol and 12 µM 1-NM-PP1. The two samples were then examined immediately and over the course of the next 10 min, by which time Hog1-as-GFP shows pronounced nuclear accumulation in >90% of the cells challenged with 1 M sorbitol in the absence of inhibitor. Whether inhibitor was added 10 min before 1 M sorbitol or added simultaneously, the fraction of the cells in which nuclear import was blocked was the same within sampling error (95% versus 93%). Hence, by this criterion, 1-NM-PP1 blocks kinase activity in <1 min.

Microscopy. Exponentially growing cells were resuspended in fresh YPD medium in the absence or presence of 12 µM (final concentration) I-NM-PP1 for 10 min and then exposed or not exposed to 1 M sorbitol for various times (up to 20 min) without or with 1-NM-PP1, as indicated, before viewing. For routine observation, yeast cells were visualized under the 100x objective of an epifluorescence microscope (BH2; Olympus Microscopes, GmbH, Hamburg, Germany) and images were captured digitally with a charge-coupled device camera and MagnaFire SP software (Olympus). For certain experiments, optical sections of cells were viewed by confocal fluorescence microscopy on a DeltaVision Spectris system (Applied Precision LLC, Issaquah, WA) with a 100x objective lens, and Applied Precision SoftWoRx software was used for image capture, deconvolution, and adjustments to brightness and contrast. To visualize DNA, cells were treated prior to examination with 2 µg/ml (final concentration) 4',6'-diamidino-2-phenylindole (DAPI). For image clarity in data presentation, cropping and equivalent adjustments to brightness and contrast were made with Photoshop CS (Adobe Systems, Inc., San Jose, CA).

Cell extracts and immunoblotting. To estimate the cellular content of both total Hog1-GFP protein and the fraction of activated (dually phosphorylated) Hog1-GFP (or of the corresponding Hog1-as-GFP, Hog1(K52R)-GFP, and Hog1(D144A)-GFP variants), a sample (A₆₀₀ of 0.5) of exponentially growing cells in 5 ml of YPD (without or with 1 M sorbitol and without or with 12 µM 1-NM-PP1, as indicated for the microscopy experiments) was concentrated in a microcentrifuge. After removal of culture medium by aspiration, cell pellets were resuspended in 1x loading buffer (5% sodium dodecyl sulfate [SDS], 0.1 M Tris-HCl [pH 7.5], 5% glycerol, 0.07 M 2-mercaptoethanol, 0.02 mM bromophenol blue) and boiled for 5 min. After clarification in a Microfuge, a sample of the supernatant solution (10 µl) was resolved by SDS-polyacrylamide gel electrophoresis (PAGE) on an 8% gel and transferred to a sheet of nitrocellulose filter paper. The filter was incubated in Odyssey blocking buffer (Li-Cor Biosciences, Inc., Lincoln, NE) for 1 h at room temperature and then overnight with a mouse monoclonal anti-GFP antibody (Roche Diagnostics, Inc., Basel, Switzerland) and rabbit monoclonal anti-phospho-p38 antibody (Cell Signaling Technology, Inc., Danvers, MA) in the same buffer. Filter-bound antibodies were detected with Alexa Fluor-conjugated secondary anti-mouse and anti-rabbit anti-immunoglobulin G antibodies (Molecular Probes, Inc., Eugene, OR) and visualized with an Odyssey infrared imaging system (Li-Cor). To detect HA epitope-tagged Rck2, mouse monoclonal anti-HA epitope antibody 6E2 (Cell Signaling Technology) was used.

	RESULTS

Top Abstract Introduction Materials and Methods Results Discussion References

 
Construction and characterization of a functional hog1-as allele. We generated a conditional allele (dubbed hog1-as) susceptible to inhibition by the cell-permeable adenine analog 1-NM-PP1 on the basis of principles developed by Shokat and coworkers (4, 7, 8, 55). By using sequence alignments and modeling of the back of the ATP-binding pocket of Hog1 on the crystal structure of its closest mammalian ortholog, p38 (21), we determined that a T100A substitution in Hog1 should yield the desired allele (alteration of the equivalent residue to Ala in other protein kinases has sometimes been referred to as an "-as2" allele). The corresponding mutant DNA was inserted into the chromosome in place of the normal locus under the control of its native promoter. Indeed, the hog1-as allele behaved exactly as anticipated. Like HOG1⁺ cells, but unlike hog1 mutant cells, hog1-as mutant cells grew under conditions of hyperosmotic stress (1 M sorbitol) (Fig. 1A). Unlike wild-type cells, hog1-as mutant cells failed to grow on 1 M sorbitol and 12 µM 1-NM-PP1, indistinguishable from the inability of hog1 mutant cells to grow on 1 M sorbitol (Fig. 1A).

View larger version (36K): [in this window] [in a new window]   FIG. 1. Phenotypic characterization of the hog1-as allele. (A) Tenfold serial dilutions of the indicated strains [HOG1, hog1, hog1-as, hog1(K52R), and hog1(D144A)] were spotted onto plates containing YPD (left), YPD plus 1 M sorbitol (middle), or YPD plus 1 M sorbitol and 12 µM 1-NM-PP1 (right) and incubated at 30°C for 2 days. (B) The same HOG1, hog1, or hog1-as mutant cells as in panel A were transformed with empty URA3-marked vector YEp352-GAL (upper half) or the same vector expressing Ssk2N (lower half). Representative transformants were streaked on selective medium (synthetic complete medium without uracil) containing glucose (upper right), galactose (lower left), or galactose plus 12 µM 1-NM-PP1 (lower right) and incubated at 30°C for 2 days.

To determine if inhibition of Hog1-as by 12 µM 1-NM-PP1 was as severe as the loss of catalytic activity caused by point mutations in conserved residues known to be critical for protein kinase activity (23), we used site-directed mutagenesis to generate first a previously described kinase-dead mutant, Hog1(K52R) (20, 46), which alters a Lys required for proper tertiary structure and binding of the -phosphate of ATP, and a novel kinase-dead mutant, Hog1(D144A), which eliminates the putative general base catalyst for the phosphotransferase reaction. Indeed, the Hog1(D144A) mutant was unable to grow on 1 M sorbitol, just like hog1 mutant cells and hog1-as-carrying cells in the presence of 1-NM-PP1 (Fig. 1A). By contrast, Hog1(K52R) retained residual activity that permitted slow (but readily detectable) growth on 1 M sorbitol (Fig. 1A). This residual growth on 1 M sorbitol was eliminated in the presence of 1-NM-PP1. We presume that the K52R mutation, because of its effects on kinase conformation, makes this already crippled enzyme somewhat susceptible to the inhibitor.

As yet another independent measurement of the catalytic competence of Hog1-as and its inhibition by 1-NM-PP1 in vivo, we examined the Hog1-dependent phosphorylation of a known downstream target, the MAPK-activated protein kinase (MAPKAP kinase) Rck2 (6, 57). In normal cells under isosmotic conditions, the bulk of Rck2-(HA)₃ migrated as a hypophosphorylated form (band 1, Fig. 2A), with a trace of a slightly slower-mobility species (band 2, Fig. 2A). Upon exposure to hyperosmotic stress, no detectable band 1 remained, most of the Rck2-(HA)₃ migrated as band 2, and a third, even slower-mobility form appeared (band 3, Fig. 2A). In hog1 mutant cells, the bulk of the Rck2-(HA)₃ remained hypomodified (band 1), even in the presence of 1 M sorbitol. By contrast, in hog1-as cells in the absence of 1-NM-PP1, Rck2-(HA)₃ underwent conversion to bands 2 and 3 under hyperosmotic challenge, just as it did in HOG1⁺ cells, whereas in the presence of inhibitor, Rck2-(HA)₃ modification was completely abrogated (Fig. 2A). It should be noted that, even when bound to 1-NM-PP1, Hog1-as was still an efficient substrate for the upstream kinase Pbs2 (Fig. 2A, bottom).

View larger version (17K): [in this window] [in a new window]   FIG. 2. Biochemical characterization of the hog1-as allele. (A) HOG1, hog1, and hog1-as mutant cells carrying an integrated copy of (HA)3 epitope-tagged Rck2 were grown to mid-exponential phase in YPD, pretreated in the absence (–) or presence (+) of 12 µM 1-NM-PP1 for 10 min, and then not exposed (–) or exposed (+) to 1 M sorbitol, as indicated. After 10 min, the cells were lysed as described in Materials and Methods and samples of the resulting extracts were analyzed by SDS-PAGE and immunoblotting with an anti-HA epitope antibody. 1, unmodified Rck2; 2 and 3, hyperphosphorylated Rck2. (B) The same strains as in Fig. 1A, each carrying an integrated copy of the FUS1prom-lacZ reporter gene, were grown to mid-exponential phase and then resuspended in YPD (lanes 1), YPD plus 1 M sorbitol (lanes 2), or YPD plus 1 M sorbitol and 12 µM 1-NM-PP1 (lanes 3), as indicated. After 5 h to allow for gene expression and enzyme synthesis, samples were assayed for ß-galactosidase content as described in Materials and Methods.

Catalytic activity of Hog1 is needed to prevent cross talk. Inappropriate activation of the mating pheromone response pathway does not occur when Hog1 is present but does occur when Hog1 is absent (34). Loss of signaling specificity upon hyperosmotic stress can be measured semiquantitatively by monitoring the induction of a diagnostic transcriptional reporter of pheromone response, FUS1^prom-lacZ. In HOG1⁺ cells, little or no expression of ß-galactosidase was observed when the cells were grown on rich medium (Fig. 2B, lane 1), on the same medium containing 1 M sorbitol (Fig. 2B, lane 2), or on l M sorbitol and 1-NM-PP1 (Fig. 2B, lane 3), whereas a hog1 mutant displayed robust reporter expression when subjected to a hyperosmotic challenge (Fig. 2B, lanes 2 and 3). The hog1-as-carrying cells showed no induction of the reporter under normal growth conditions or under conditions of hyperosmotic stress, like wild-type cells, but did so to the same extent as hog1 mutant cells when challenged with 1 M sorbitol plus 1-NM-PP1 (Fig. 2B, lane 3). This latter result provides yet another independent demonstration that the inhibitor concentration used completely abrogated Hog1-as kinase activity whereas wild-type Hog1 was unaffected. The cells expressing one of the catalytically inactive point mutants, Hog1(D144A), behaved just like hog1 mutant cells. By contrast, the other putative kinase-dead allele, Hog1(K52R), seemed to retain partial activity (and thus the ability to prevent the full appearance of cross talk) because the degree of reporter expression upon hyperosmotic challenge was reproducibly and significantly less than that displayed by hog1 mutant cells or cells expressing either Hog1(D144A) or the hog1-as allele in the presence of 1-NM-PP1 (Fig. 2B, lane 2). Moreover, the Hog1(K52R) mutant displayed further elevation in reporter expression in the presence of 1-NM-PP1, equivalent to that displayed by hog1 mutant cells (Fig. 2B, lane 3). These observations provide independent evidence that Hog1(K52R) retains significant residual activity (as shown in Fig. 1A) and that conformational flexibility caused by the K52R mutation makes the enzyme somewhat susceptible to the inhibitor because this mutant always displayed a greater degree of cross talk in the presence of 1-NM-PP1 than in its absence.

Efficient dephosphorylation of activated Hog1 requires its catalytic function. Upon hyperosmotic stress, Hog1 is activated by phosphorylation on T174 and Y176 in its activation loop by the MAPKK Pbs2. Dual phosphorylation of Hog1 was readily detected by immunoblotting with a commercially available rabbit monoclonal antibody raised against activated (dually phosphorylated) mammalian p38 (22, 54). To permit convenient quantitation of total Hog1 (or its variants) and its visualization in live cells in real time, it was fused to GFP (65) and substituted for the normal chromosomal HOG1 locus by a PCR-based method (26).

Upon exposure to 1 M sorbitol, Hog1-GFP was phosphorylated rapidly, with modification peaking at 10 to 20 min and then declining as cells adapted, as shown previously for epitope-tagged Hog1 (10) (Fig. 3A, left). By 90 min, the amount of activated Hog1 declined to 10% of the maximum, as quantified from the relative band intensities measured with an infrared detection system (Odyssey; Li-Cor, Inc.). Likewise, the hyperosmotic stress-induced phosphorylation pattern for Hog1-as-GFP was virtually indistinguishable (Fig. 3B, left). However, under the same conditions but in the presence of 1-NM-PP1, phosphorylation of Hog1-as-GFP was sustained throughout the time course of the experiment (Fig. 3B, right) whereas the kinetic behavior of wild-type Hog1-GFP phosphorylation was unaffected (Fig. 3A, right). In agreement with the conclusion that the kinase activity of Hog1 is required to promote its own dephosphorylation after hyperosmotic-stress-induced activation, both kinase-dead point mutants Hog1(K52R)-GFP and Hog1(D144A)-GFP displayed a pattern of persistent phosphorylation just like Hog1-as-GFP in the presence of inhibitor (Fig. 3C and D).

View larger version (14K): [in this window] [in a new window]   FIG. 3. Kinetics of Hog1 phosphorylation in response to hyperosmotic stress. Cells expressing C-terminally GFP-tagged derivatives of wild-type Hog1 (A), Hog1-as (B), Hog1(K52R) (C), or Hog1(D144A) (D) were grown to mid-exponential phase in YPD and then exposed to 1 M sorbitol or 1 M sorbitol and 12 µM 1-NM-PP1, as indicated. At the times shown, samples of each culture were withdrawn and frozen immediately in liquid N2. For subsequent analysis, the samples were thawed on ice, lysed, resolved by SDS-PAGE, and subjected to immunoblotting with an anti-phospho-p38 antibody to detect active (dually phosphorylated) Hog1 (upper part of each panel) and an anti-GFP antibody to detect total Hog1 (bottom part of each panel).

During growth under isosmotic conditions, Hog1-GFP, the Hog1-as-GFP mutant, and both kinase-dead point mutants Hog1(K52R)-GFP and Hog1(D144A)-GFP were all found primarily in the cytosol (Fig. 4, left). Within 10 min after exposure to 1 M sorbitol, Hog1-GFP and Hog1-as-GFP were found almost exclusively in the nucleus (Fig. 4, right), confirming the rapid hyperosmotic stress-induced translocation observed before (20, 46). However, in contrast to previous claims that nuclear import of Hog1(K52R)-GFP occurs to the same extent and at the same rate as that of wild-type Hog1-GFP in response to exposure to 1 M sorbitol (20, 46), we found that this catalytically compromised variant expressed at its endogenous level did not enter the nucleus efficiently (Fig. 4, right). In agreement with this conclusion, our novel, completely nonfunctional point mutant Hog1(D144A)-GFP displayed even weaker nuclear accumulation. Close inspection indicated that Hog1(D144A)-GFP did not actually enter the nucleus but remained at the nuclear periphery (Fig. 4, right). This impression was confirmed by examination of the cells by confocal microscopy (data not shown; see also below and Fig. 6). Thus, contrary to previous studies, we found that when the catalytic activity of Hog1 is compromised, nuclear import is blocked.

View larger version (19K): [in this window] [in a new window]   FIG. 4. Hog1 catalytic activity is required for its hyperosmotic stress-induced nuclear import. Cells expressing the same GFP-tagged versions of wild-type and mutant Hog1 as in Fig. 3 were grown to mid-exponential phase, prestained for 30 min at 30°C with 2 µM (final concentration) DAPI to visualize DNA, and then resuspended in YPD (left) or YPD plus 1 M sorbitol (right). After 10 min at 30°C, the cells were examined under an epifluorescence microscope. Arrows indicate the positions of representative nuclei.

View larger version (16K): [in this window] [in a new window]   FIG. 6. Nmd5 is required for nuclear import of Hog1 but does not itself enter the nucleus. (A) NMD5+ (top) and nmd5 mutant (bottom) cells expressing C-terminally GFP-tagged wild-type Hog1 were grown to mid-exponential phase in YPD and resuspended in YPD (left) or YPD plus 1 M sorbitol (right). After 10 min, the cells were viewed under a confocal fluorescence microscope. Representative 0.2-µm optical z sections are shown. (B) Cells coexpressing C-terminally GFP-tagged Hog1-as and fully functional C-terminally mCherry-tagged Nmd5 were grown to mid-exponential phase in YPD and resuspended in YPD (left), in YPD plus 1 M sorbitol (middle), or in YPD plus 1 M sorbitol and 12 µM 1-NM-PP1 (right). After 10 min, the cells were examined with the appropriate cutoff filters to view GFP (top) or mCherry (middle). The merged GFP and mCherry images are also shown (bottom).

View larger version (13K): [in this window] [in a new window]   FIG. 5. Analog inhibition blocks nuclear entry and does not stimulate nuclear exit. (A) Cells expressing C-terminally GFP-tagged wild-type Hog1 or Hog1-as were grown to mid-exponential phase in YPD, prestained with DAPI as described in the legend to Fig. 4, resuspended in YPD plus 12 µM 1-NM-PP1 for 10 min, and then exposed to 1 M (final concentration) sorbitol. After 10 min, the cells were examined under an epifluorescence microscope. (B) The same cells as in panel A were resuspended in YPD (left) or YPD plus 1 M sorbitol. After 10 min, samples of each were viewed with an epifluorescence microscope to confirm that hyperosmotic-stress-induced nuclear translocation had occurred in the sample exposed to 1 M sorbitol (middle). A portion of the sample that received 1 M sorbitol was then adjusted to 12 µM (final concentration) 1-NM-PP1. After 2 min and various times thereafter (up to 30 min), the cells that were exposed to 1 M sorbitol and 12 µM 1-NM-PP1 were examined again (right).

View larger version (8K): [in this window] [in a new window]   FIG. 7. Nmd5 and kinase activity are required for passage of Hog1 through the NPC. (A) An nmd5 nup133 double mutant coexpressing C-terminally GFP-tagged wild-type Hog1 and C-terminally mCherry-tagged Nup1 was grown to mid-exponential phase in YPD and resuspended in YPD plus 1 M sorbitol. After 10 min, the cells were examined with the appropriate cutoff filters to view GFP (left) or mCherry (middle). The merged GFP and mCherry images are also shown (right). (B) Cells expressing C-terminally GFP-tagged Hog1-as and C-terminally mCherry-tagged Nup1 were grown to mid-exponential phase in YPD and resuspended in YPD plus 1 M sorbitol and 12 µM 1-NM-PP1. After 10 min, the cells were examined with the appropriate cutoff filters to view GFP (left) or mCherry (middle). The merged GFP and mCherry images are also shown (right). In both the top and bottom parts, representative 0.2-µm optical z sections are shown.

Hog1 kinase activity is needed continuously for maintenance of the adaptive response to hyperosmotic stress. Our findings with the Hog1-as and Hog1(D144A) alleles demonstrated, as had not been shown before, that Hog1 catalytic activity, and not the mere presence of the Hog1 polypeptide, is required both for mounting an initial response to a hyperosmotic challenge (10) and for preventing inadvertent activation of the pheromone response pathway (34). The hog1-as allele also provided a unique tool with which to explore another feature of HOG pathway regulation, namely, whether Hog1 kinase activity is required for maintenance of pathway signaling specificity, even after relatively long-term adaptive changes have occurred.

Exposure to a high external dissolved-solute concentration causes an 1-h delay in cell cycle progression during which the osmotic balance is restored and reassembly of the actin cytoskeleton occurs, permitting resumption of active growth (11, 13, 64). To determine if Hog1 catalytic activity is still needed to prevent cross talk even after growth resumes, cells expressing Hog1-as-GFP and containing the FUS1^prom-lacZ reporter were exposed to 1 M sorbitol in the absence and presence of 1-NM-PP1 and both cell density and reporter gene expression were monitored. As expected, the onset of cross talk was readily detected within the first hour after hyperosmotic stress in the culture that received 1-NM-PP1 initially but not in the control (DMSO-only) culture (Fig. 8A). By 3 h after exposure to 1 M sorbitol, the culture lacking inhibitor had resumed growth, as judged by a significant increase in its A₆₀₀ (data not shown), whereas the culture that received inhibitor had not, also as expected. At that point (Fig. 8A, arrow), 1-NM-PP1 was added to the control culture that had not previously received the inhibitor. Expression of the cross talk reporter commenced almost immediately, and the onset of its expression was only slightly slower than when the Hog1-as mutant was treated with 1 M sorbitol and the analog at the same time. Thus, the catalytic activity of Hog1 is required persistently to prevent inappropriate cross talk between the HOG and pheromone response pathways.

View larger version (13K): [in this window] [in a new window]   FIG. 8. Persistent Hog1 function is required to prevent cross talk and maintain the adapted state. (A) Cells expressing Hog1-as were grown in YPD and, at time zero, resuspended in YPD plus 1 M sorbitol in the absence (filled circles) or presence (filled squares) of 12 µM 1-NM-PP1. After 3 h (arrow), 12 µM (final concentration) 1-NM-PP1 was added to a portion of the culture that was exposed to YPD plus 1 M sorbitol only (filled triangles). At the indicated times, samples were withdrawn and the level of expression of the FUS1prom-lacZ reporter was measured as described in the legend to Fig. 2B. (B) Cells expressing a C-terminally GFP-tagged version of wild-type Hog1 (gray bars) or Hog1-as (black bars) were grown in YPD and, at time zero, resuspended in YPD plus 1 M sorbitol. After 90 min (arrow), 12 µM (final concentration) 1-NM-PP1 was added to each culture. Samples were withdrawn at the indicated times, and the ratio of phospho-Hog1 to total Hog1 was determined as described in the legend to Fig. 3.

Hog1 catalytic activity is necessary to prevent cross talk between the HOG pathway and filamentous-growth-signaling pathways. Connections between the HOG pathway and the filamentous-growth-signaling pathway have been noted. For example, in diploid strains of the 1278b lineage (which display robust filamentation in response to nitrogen deprivation), cells lacking the Sho1 osmosensor (34) or the mucin-like protein (Msb2) with which it interacts (15) display markedly reduced pseudohyphal growth. Conversely, it has been reported that, when subjected to hyperosmotic stress, hog1 (or pbs2) mutant cells express genes normally expressed only under conditions that promote filamentous growth and require components of the filamentous-growth response pathway (e.g., Ste7 and Kss1) to do so (16). Indeed, in our experiments, haploid hog1 and pbs2 mutants of non-1278b lineage displayed robust invasive growth on 1 M sorbitol but HOG1⁺ cells did not (P. J. Westfall, unpublished results). Thus, as observed for the pheromone response, Hog1 seems to be required to prevent inadvertent activation of the filamentous-growth pathway by hyperosmotic stress.

The hog1-as allele allowed us to assess whether this cross talk behavior was manifested because of alterations of cell physiology that arise in hog1 mutants because of the chronic absence of Hog1 or is also observed in otherwise wild-type cells upon acute inhibition of Hog1 catalytic activity. To assess the degree of this kind of cross talk, we attempted to use a well-accepted reporter of the pseudohyphal growth pathway, FRE(Ty1)-lacZ (27). However, as we have observed before (14), this reporter displays a rather high basal level of expression in the 1278b background. Therefore, we sought a different reporter that would give very low basal expression when introduced into the 1278b background. The TEC1 gene is specifically induced under filamentous-growth conditions and has been shown to be a reliable and specific indicator of this signaling pathway (15, 16, 25). Hence, we constructed a TEC1^prom-lacZ::URA3 fusion which was targeted for integration in single copy just upstream of the resident TEC1 gene at its normal chromosomal locus. In this way, 1278b cells containing this reporter remained TEC1⁺ and thus capable of mounting a normal filamentous-growth response. We then prepared an isogenic set of haploid and MATa/MAT diploid cells of the 1278b lineage containing this reporter and either wild-type HOG1 or the four hog1 mutants we used throughout this study. For all intents and purposes, the haploids and diploids behaved essentially identically (Fig. 9A and B). Wild-type (HOG1⁺) cells did not express TEC1^prom-lacZ when grown in YPD before or after exposure to 1 M sorbitol. In contrast, in hog1 mutant cells, basal expression of the reporter was somewhat elevated on YPD alone and its expression was robustly induced (8- to 10-fold) in medium containing 1 M sorbitol (Fig. 8). Likewise, and in agreement with the other evidence that the Hog1(D144A) allele is a complete loss-of-function mutation, it behaved like hog1 mutant cells. Correspondingly, and in keeping with the other evidence that Hog1(K52R) retains partial function, the degree of cross talk activation of the TEC1^prom-lacZ reporter was reduced reproducibly. Finally, Hog1-as allele-carrying cells behaved like wild-type (HOG1⁺) cells in the absence of 1-NM-PP1 but displayed readily detectable expression of the cross talk reporter when exposed to 1 M sorbitol in the presence of 1-NM-PP1. We noted, however, that, at least by the criterion of the results obtained in this assay format, 1-NM-PP1 was not causing complete inhibition of Hog1-as. We presume that 1-NM-PP1 was less efficacious because cells of the 1278b lineage are less permeable to the analog, have larger competing intracellular adenine pools, or have a different spectrum of ABC transporters that extrude the compound more effectively (or some combination of the three), compared to the other strains we used (all derivatives of EG123).

View larger version (12K): [in this window] [in a new window]   FIG. 9. Hog1 catalytic activity is required to prevent cross talk with the filamentous-growth-signaling pathway. Isogenic TEC1+ haploid (A) and MATa/MAT diploid (B) derivatives of strain 1278b carrying an integrated TEC1prom-lacZ reporter and either wild-type HOG1 or the four mutant alleles described in the legend to Fig. 1A, as indicated, were grown to mid-exponential phase in YPD and then resuspended in YPD (gray bars) or YPD plus 1 M sorbitol (black bars). A portion of the hog1-as mutant cells was treated in the same way but also in the presence of 12 µM 1-NM-PP1, as shown. After 5 h, samples were assayed for ß-galactosidase content as described in the legend to Fig. 2B.

	DISCUSSION

Top Abstract Introduction Materials and Methods Results Discussion References

To determine whether analog-inhibited Hog1-as is as nonfunctional as point mutants that alter catalytically critical residues, we constructed Hog1(K52R) and Hog1(D144A), a previously described and a novel kinase-dead allele, respectively. Despite prior claims to the contrary, we found that Hog1(K52R) retained detectable (albeit low) activity, on the basis of phenotypic criteria, whereas Hog1(D144A) behaved indistinguishably from hog1 or hog1-as mutant cells in the presence of 12 µM 1-NM-PP1. Thus, all of the latter three represent the true null condition.

A primary motivation for generating the hog1-as allele was to address the question of whether the failure of hog1 (or pbs2) mutant cells to maintain signaling specificity and avoid inadvertent cross-activation of the mating pheromone signaling pathway resulted from adaptive changes to cellular physiology that arose during long-term propagation of such strains because they chronically lack Hog1 activity (or the Hog1 polypeptide itself). Using the chemical genetics approach made possible by the hog1-as allele, we clearly demonstrated that it is the catalytic activity of Hog1 per se that is required to prevent improper cross-activation of both the pheromone response and filamentous-growth pathways, the other two MAPK cascades initiated by the MAPKKK Ste11. Thus, the target for blocking cross talk is presumably a substrate of Hog1 itself or of a downstream kinase (Rck2) that Hog1 activates. However, we have ruled out the latter possibility. When challenged with 1 M sorbitol, a rck2 single mutant or even a rck1 rck2 double mutant does not display cross talk (as monitored in the standard way via expression of a FUS1^prom-lacZ reporter), suggesting that these MAPK-activated protein kinases (MAPKAP kinases) are not necessary to prevent cross talk (Westfall, unpublished). However, to make certain that these MAPKAP kinases are not necessary to evoke cross talk in the first place, we constructed a hog1 rck1 rck2 triple mutant and found that it displays robust cross talk when challenged with 1 M sorbitol (Westfall, unpublished). Thus, the target for preventing cross talk is likely to be a direct substrate of Hog1 and not of Rck2.

In this regard, in addition to Rck1 and/or Rck2, we have evidence that rules out the following other gene products as the Hog1 target necessary to prevent cross talk between the HOG and mating pathways: Sho1; Msb2; Opy2, a single-pass integral membrane protein implicated in initiation of HOG signaling (62); and Gpd1 (NAD⁺-dependent glycerol-3-phosphate dehydrogenase), the key enzyme of glycerol synthesis (1). For example, following the same logic as mentioned above for the MAPKAP kinases, when challenged with 1 M sorbitol, an msb2 mutant does not display cross talk whereas a hog1 msb2 double mutant does (Westfall, unpublished). As another example, Gpd1 cannot be the target because its sequence lacks any consensus MAPK phosphorylation site (-SP- or -TP-). Sho1 and Opy2 are signaling components essential to elicit the HOG response. Therefore, to test whether Hog1-dependent phosphorylation of these proteins is necessary for preventing cross talk, point mutations that convert the Ser or Thr in each MAPK phosphorylation site to Ala were constructed (Opy2 has one site, and Sho1 has four). Cells expressing Opy2 lacking its MAPK phosphorylation site and cells expressing Sho1 lacking any or all four of its MAPK phosphorylation sites did not display constitutive cross talk in wild-type cells and, moreover, did not display cross talk when challenged with 1 M sorbitol, whereas the hog1 derivatives of the same strains showed robust cross talk (Westfall, unpublished).

We show here that Hog1 kinase activity is required to insulate the filamentous-growth pathway from the HOG pathway by employing a novel reporter gene (TEC1^prom-lacZ) for filamentous-growth signaling. The TEC1 promoter reportedly contains pheromone response elements (which bind Ste12 alone) (32), as well as bona fide filamentous response elements that bind Tec1-Ste12 heterooligomers (67). However, expression of the reporter in MATa/MAT diploid cells, which lack components of the circuitry essential for the pheromone signaling pathway (including the MAPK scaffold Ste5 and even the MAPK Fus3), was indistinguishable from that observed in haploids, confirming that TEC1^prom-lacZ was measuring exclusively the output of authentic, Tec1-dependent filamentous-growth signaling.

One factor common to both pheromone and filamentous-growth signaling but not required for the HOG response is the MAPKK Ste7, which contains 10 consensus MAPK phosphorylation sites. Hence, Ste7 is another potential target by which Hog1 might prevent cross talk. However, when a fus3 kss1 mutant strain expressing both Hog1-as and a functional, C-terminally c-Myc epitope-tagged derivative of Ste7 (3) was exposed to 1 M sorbitol, no detectable shift in Ste7 mobility was observed (Westfall, unpublished), unlike the nearly complete conversion to multiple hyperphosphorylated Ste7 species produced upon pheromone treatment of FUS3⁺ KSS1⁺ cells (30). Thus, Ste7 seems not to be a direct substrate of Hog1. Furthermore, when wild-type MATa cells were exposed to 1 M sorbitol for 30 min and then exposed to 5 µM -factor, pheromone was still able to elicit strong induction of the FUS1^prom-lacZ reporter (Westfall, unpublished), indicating that signaling through the mating pathway, which requires both Ste11 and Ste7, has not been compromised by Hog1 action. Hence, the Hog1 target that blocks improper activation of the other two MAPK cascades remains elusive.

Prior studies examining the kinetics of Hog1 phosphorylation during response and adaptation to hyperosmotic stress indicated that no phospho-Hog1 remained by 30 to 60 min postshock (20, 46). However, we consistently observed a readily detectable pool of phospho-Hog1 (i.e., active Hog1) that persisted at a level clearly above that seen in cells maintained under isosmotic conditions for at least 180 min after cells were exposed to 1 M sorbitol. Perhaps the sensitivity of the infrared imaging we used to examine immunoblots allowed us to acquire this new insight. What physiological reason might make it advantageous for the cell to maintain a residual pool of phospho-Hog1? Our data suggest that this threshold is a necessary part of the long-term adaptive response. First, even after cells adapted, addition of 1-NM-PP1 to hog1-as mutant cells resulted in improper expression of the FUS1^prom-lacZ reporter nearly immediately. Thus, the residual phospho-Hog1 is required to maintain the block to cross talk. Second, when 1-NM-PP1 was added to hog1-as mutant cells after they had adapted, growth ceased. Hence, persistence of some active Hog1 is necessary for maintenance of those processes (e.g., continued glycerol production) that sustain osmotic balance and permit growth, avoiding the need to reinitiate anew full-bore HOG pathway signaling after every cell division cycle.

During the course of these studies, by using confocal microscopy, we confirmed that the ß-importin family member Nmd5 is required for nuclear uptake of Hog1 following hyperosmotic stress, but we found unexpectedly that catalytic activity of Hog1 is required for its efficient nuclear import, again contrary to previously published reports (20, 46). Moreover, our results suggest that Nmd5 has a function different from the classical view of how such importins are thought to deliver their cargo to the nucleoplasm. We found first that catalytically inactive Hog1—Hog1(K52R), Hog1(D144A), or Hog1-as in the presence of 1-NM-PP1—is defective in entering the nucleus upon a challenge with 1 M sorbitol. Second, and strikingly, all of these mutant enzymes and even normal Hog1 translocate to the NPC in response to hyperosmotic stress and do so even when Nmd5 is absent. Thus, there is an Nmd5-independent mechanism for delivery of the cytosolic pool of Hog1 to the NPC but Nmd5 is needed for passage of Hog1 to the nucleoplasm. Third, upon exposure of cells to 1 M sorbitol, Nmd5 itself accumulates at the NPC and does so even in hog1 mutant cells (Westfall, unpublished), but unlike Hog1 itself, Nmd5 does not seem to enter the nucleus per se, even in wild-type cells.

Collectively, these results suggest that nuclear entry of Hog1 occurs via a previously unsuspected multistep process. First, Hog1 and Nmd5 are transported to the NPC by independent (but as-yet-unknown) mechanisms when cells are challenged with hyperosmotic stress. Once at the NPC, Nmd5 somehow assists in the passage of Hog1 through the NPC because nuclear entry does not occur when Nmd5 is absent. However, the catalytic activity of Hog1 is required for its passage. One possibility is that, normally, Nmd5 at the cytoplasmic face of the NPC forms a complex with Hog1 but must hand off this cargo to another carrier (either the FG repeat nucleoporins that line the NPC channel or an as-yet-uncharacterized factor) because Nmd5 itself does not seem to enter the nucleus (even in wild-type cells), and perhaps Hog1 kinase activity is required for its release from Nmd5. Conversely, Hog1 kinase activity may be required for its productive association with Nmd5 and thus for its subsequent import, and the concentration of Nmd5 at the NPC may merely reflect its steady-state distribution and its high affinity for the FG repeat nucleoporins, as observed for other karyopherins (43). Yet another possibility is that dissociation of the complex between Hog1 and its transporter occurs at the nuclear face of the NPC and requires its kinase activity. Studies of the physical association between purified Nmd5 and purified Hog1-as in the presence and absence of 1-NM-PP1 may help distinguish between these possibilities. In any event, our results document previously unappreciated complexities in the nuclear import of a stress-activated MAPK. The facts that Hog1 activity is required for its nuclear import and that the phosphotyrosine-specific phosphoprotein phosphatase primarily responsible for Hog1 deactivation (Ptp2) is located in the nucleus are sufficient to explain why the rate of dephosphorylation of Hog1 was greatly retarded by any circumstance that impaired its catalytic function.

. . . .

	ACKNOWLEDGMENTS

 
This work was supported by Kirschstein NIH-NRSA postdoctoral fellowship GM68343 (to P.J.W.), by NIH research grant GM21841 (to J.T.), and by facilities provided by the Berkeley campus Cancer Research Laboratory.

Early stages of this study were conducted under the auspices of the late Ira Herskowitz in the Department of Biochemistry and Biophysics of the School of Medicine at the University of California, San Francisco, and we gratefully and respectfully acknowledge his contributions to this work. We thank Kevan Shokat and Chao Zhang for 1-NM-PP1, Roger Tsien for the mCherry derivative of DsRed, Kurt Thorn for plasmids, and Hiten Madhani, Wendall Lim, Sean O'Rourke, Karsten Weis, and all members of the Thorner laboratory for helpful advice.

	FOOTNOTES

 
* Corresponding author. Mailing address: Department of Molecular and Cell Biology, Room 16, Barker Hall, University of California, Berkeley, Berkeley, CA 94720-3202. Phone: (510) 642-2558. Fax: (510) 642-6420. E-mail: jthorner{at}berkeley.edu.

	REFERENCES

Top Abstract Introduction Materials and Methods Results Discussion References

 

1. Albertyn, J., S. Hohmann, J. M. Thevelein, and B. A. Prior. 1994. GPD1, which encodes glycerol-3-phosphate dehydrogenase, is essential for growth under osmotic stress in Saccharomyces cerevisiae, and its expression is regulated by the high-osmolarity glycerol response pathway. Mol. Cell. Biol. 14:4135-4144.[Abstract/Free Full Text]
2. Bao, M. Z., M. A. Schwartz, G. T. Cantin, J. R. Yates III, and H. D. Madhani. 2004. Pheromone-dependent destruction of the Tec1 transcription factor is required for MAP kinase signaling specificity in yeast. Cell 119:991-1000.[CrossRef][Medline]
3. Bardwell, L., J. G. Cook, E. C. Chang, B. R. Cairns, and J. Thorner. 1996. Signaling in the yeast pheromone response pathway: specific and high-affinity interaction of the mitogen-activated protein (MAP) kinases Kss1 and Fus3 with the upstream MAP kinase kinase Ste7. Mol. Cell. Biol. 16:3637-3650.[Abstract]
4. Benjamin, K. R., C. Zhang, K. M. Shokat, and I. Herskowitz. 2003. Control of landmark events in meiosis by the CDK Cdc28 and the meiosis-specific kinase Ime2. Genes Dev. 117:1524-1539.
5. Benton, B. M., W. K. Eng, J. J. Dunn, F. W. Studier, R. Sternglanz, and P. A. Fisher. 1990. Signal-mediated import of bacteriophage T7 RNA polymerase into the Saccharomyces cerevisiae nucleus and specific transcription of target genes. Mol. Cell. Biol. 10:353-360.[Abstract/Free Full Text]
6. Bilsland-Marchesan, E., J. Arino, H. Saito, P. Sunnerhagen, and F. Posas. 2000. Rck2 kinase is a substrate for the osmotic stress-activated mitogen-activated protein kinase Hog1. Mol. Cell. Biol. 20:3887-3895.[Abstract/Free Full Text]
7. Bishop, A. C., O. Buzko, and K. M. Shokat. 2001. Magic bullets for protein kinases. Trends Cell Biol. 11:167-172.[CrossRef][Medline]
8. Bishop, A. C., K. Shah, Y. Liu, L. Witucki, C. Kung, and K. M. Shokat. 1998. Design of allele-specific inhibitors to probe protein kinase signaling. Curr. Biol. 8:257-266.[CrossRef][Medline]
9. Boeke, J. D., J. Trueheart, G. Natsoulis, and G. R. Fink. 1987. 5-Fluoroorotic acid as a selective agent in yeast molecular genetics. Methods Enzymol. 154:164-175.[Medline]
10. Brewster, J. L., T. de Valoir, N. D. Dwyer, E. Winter, and M. C. Gustin. 1993. An osmosensing signal transduction pathway in yeast. Science 259:1760-1763.[Abstract/Free Full Text]
11. Brewster, J. L., and M. C. Gustin. 1994. Positioning of cell growth and division after osmotic stress requires a MAP kinase pathway. Yeast 10:425-439.[CrossRef][Medline]
12. Chou, S., L. Huang, and H. Liu. 2004. Fus3-regulated Tec1 degradation through SCFCdc4 determines MAPK signaling specificity during mating in yeast. Cell 119:981-990.[CrossRef][Medline]
13. Chowdhury, S., K. W. Smith, and M. C. Gustin. 1992. Osmotic stress and the yeast cytoskeleton: phenotype-specific suppression of an actin mutation. J. Cell Biol. 118:561-571.[Abstract/Free Full Text]
14. Cook, J. G., L. Bardwell, and J. Thorner. 1997. Inhibitory and activating functions for MAPK Kss1 in the S. cerevisiae filamentous-growth signalling pathway. Nature 390:85-88.[CrossRef][Medline]
15. Cullen, P. J., W. Sabbagh, Jr., E. Graham, M. M. Irick, E. K. van Olden, C. Neal, J. Delrow, L. Bardwell, and G. F. Sprague, Jr. 2004. A signaling mucin at the head of the Cdc42- and MAPK-dependent filamentous growth pathway in yeast. Genes Dev. 18:1695-1708.[Abstract/Free Full Text]
16. Davenport, K. D., K. E. Williams, B. D. Ullmann, and M. C. Gustin. 1999. Activation of the Saccharomyces cerevisiae filamentation/invasion pathway by osmotic stress in high-osmolarity glycogen pathway mutants. Genetics 153:1091-1103.[Abstract/Free Full Text]
17. Davis, L. I., and G. R. Fink. 1990. The. NUP1 gene encodes an essential component of the yeast nuclear pore complex. Cell 61:965-978.[CrossRef][Medline]
18. de Nadal, E., P. M. Alepuz, and F. Posas. 2002. Dealing with osmostress through MAP kinase activation. EMBO Rep. 3:735-740.[CrossRef][Medline]
19. Elion, E. A. 2001. The Ste5p scaffold. J. Cell Sci. 114:3967-3978.[Medline]
20. Ferrigno, P., F. Posas, D. Koepp, H. Saito, and P. A. Silver. 1998. Regulated nucleo/cytoplasmic exchange of HOG1 MAPK requires the importin beta homologs NMD5 and XPO1. EMBO J. 17:5606-5614.[CrossRef][Medline]
21. Goldsmith, E. J., M. H. Cobb, and C. I. Chang. 2004. Structure of MAPKs. Methods Mol. Biol. 250:127-144.[Medline]
22. Han, J., J. D. Lee, L. Bibbs, and R. J. Ulevitch. 1994. A MAP kinase targeted by endotoxin and hyperosmolarity in mammalian cells. Science 265:808-811.[Abstract/Free Full Text]
23. Hanks, S. K., and T. Hunter. 1995. Protein kinases 6. The eukaryotic protein kinase superfamily: kinase (catalytic) domain structure and classification. FASEB J. 9:576-596.[Abstract]
24. Jacoby, T., H. Flanagan, A. Faykin, A. G. Seto, C. Mattison, and I. Ota. 1997. Two protein-tyrosine phosphatases inactivate the osmotic stress response pathway in yeast by targeting the mitogen-activated protein kinase, Hog1. J. Biol. Chem. 272:17749-17755.[Abstract/Free Full Text]
25. Kim, T. S., H. Y. Kim, J. H. Yoon, and H. S. Kang. 2004. Recruitment of the Swi/Snf complex by Ste12-Tec1 promotes Flo8-Mss11-mediated activation of STA1 expression. Mol. Cell. Biol. 24:9542-9556.[Abstract/Free Full Text]
26. Longtine, M. S., A. McKenzie III, D. J. Demarini, N. G. Shah, A. Wach, A. Brachat, P. Philippsen, and J. R. Pringle. 1998. Additional modules for versatile and economical PCR-based gene deletion and modification in Saccharomyces cerevisiae. Yeast 14:953-961.[CrossRef][Medline]
27. Madhani, H. D., and G. R. Fink. 1997. Combinatorial control required for the specificity of yeast MAPK signaling. Science 275:1314-1317.[Abstract/Free Full Text]
28. Maeda, T., M. Takekawa, and H. Saito. 1995. Activation of yeast PBS2 MAPKK by MAPKKKs or by binding of an SH3-containing osmosensor. Science 269:554-558.[Abstract/Free Full Text]
29. Maeda, T., S. M. Wurgler-Murphy, and H. Saito. 1994. A two-component system that regulates an osmosensing MAP kinase cascade in yeast. Nature 369:242-245.[CrossRef][Medline]
30. Maleri, S., Q. Ge, E. A. Hackett, Y. Wang, H. G. Dohlman, and B. Errede. 2004. Persistent activation by constitutive Ste7 promotes Kss1-mediated invasive growth but fails to support Fus3-dependent mating in yeast. Mol. Cell. Biol. 24:9221-9238.[Abstract/Free Full Text]
31. Mattison, C. P., and I. M. Ota. 2000. Two protein tyrosine phosphatases, Ptp2 and Ptp3, modulate the subcellular localization of the Hog1 MAP kinase in yeast. Genes Dev. 14:1229-1235.[Abstract/Free Full Text]