Eukaryotic Cell
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Eukaryotic Cell, September 2007, p. 1538-1551, Vol. 6, No. 9
1535-9778/07/$08.00+0     doi:10.1128/EC.00169-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

A Cell Number-Counting Factor Regulates Levels of a Novel Protein, SslA, as Part of a Group Size Regulation Mechanism in Dictyostelium{triangledown}

Tong Gao,1 Celine Roisin-Bouffay,1 R. Diane Hatton,1 Lei Tang,2 Debra A. Brock,1 Tiffany DeShazo,2 Laura Olson,2 Wan-Pyo Hong,1 Wonhee Jang,3 Elvia Canseco,2 Deenadayalan Bakthavatsalam,2 and Richard H. Gomer1,2*

Howard Hughes Medical Institute,1 Department of Biochemistry and Cell Biology, MS-140, Rice University, 6100 S. Main Street, Houston, Texas 77005-1892,2 Department of Life Science, Dongguk University, 3-26 Pil-Dong, Chung-gu, Seoul 100-715, Korea3

Received 10 May 2007/ Accepted 13 July 2007

Developing Dictyostelium cells form aggregation streams that break into groups of ~2 x 104 cells. The breakup and subsequent group size are regulated by a secreted multisubunit counting factor (CF). To elucidate how CF regulates group size, we isolated second-site suppressors of smlA–, a transformant that forms small groups due to oversecretion of CF. smlA– sslA1(CR11) cells form roughly wild-type-size groups due to an insertion in the beginning of the coding region of sslA1, one of two highly similar genes encoding a novel protein. The insertion increases levels of SslA. In wild-type cells, the sslA1(CR11) mutation forms abnormally large groups. Reducing SslA levels by antisense causes the formation of smaller groups. The sslA(CR11) mutation does not affect the extracellular accumulation of CF activity or the CF components countin and CF50, suggesting that SslA does not regulate CF secretion. However, CF represses levels of SslA. Wild-type cells starved in the presence of smlA– cells, recombinant countin, or recombinant CF50 form smaller groups, whereas sslA1(CR11) cells appear to be insensitive to the presence of smlA– cells, countin, or CF50, suggesting that the sslA1(CR11) insertion affects CF signal transduction. We previously found that CF reduces intracellular glucose levels. sslA(CR11) does not significantly affect glucose levels, while glucose increases SslA levels. Together, the data suggest that SslA is a novel protein involved in part of a signal transduction pathway regulating group size.

* Corresponding author. Mailing address: Department of Biochemistry and Cell Biology, Rice University, MS-140, 6100 S. Main Street, Houston, TX 77005-1892. Phone: (713) 348-4872. Fax: (713) 348-5154. E-mail: richard{at}rice.edu

{triangledown} Published ahead of print on 27 July 2007.

Eukaryotic Cell, September 2007, p. 1538-1551, Vol. 6, No. 9
1535-9778/07/$08.00+0     doi:10.1128/EC.00169-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.



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