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Eukaryotic Cell doi:10.1128/EC.00193-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Nucleus-Specific Importin Alphas and Nucleoporins Regulate Protein Import and Nuclear Division in the Bi-Nucleate Tetrahymena thermophila

Biology Department, Washington University, St. Louis, MO 63130

^* To whom correspondence should be addressed. Email: dchalker{at}biology2.wustl.edu.

	Abstract

The ciliate Tetrahymena thermophila, having both germ line micronuclei and somatic macronuclei, must possess a specialized nucleo-cytoplasmic transport system to import proteins into the correct nucleus. To understand how Tetrahymena can target proteins to distinct nuclei, we first characterized FG repeat containing nucleoporins and found that micro- and macronuclei utilize unique subsets of these proteins. This finding implicates these proteins in differential permeability of the two nuclei and implies that nuclear pores with discrete specificities are assembled within a single cell. To identify the import machineries that interact with these different pores, we characterized the large families of karyopherin homologs encoded within the genome. Localization studies of 13 putative importin (imp) and 11 imp -like proteins revealed that imp -like proteins are nucleus specific – nine localized to the germ line micronucleus, but that most imp -like proteins localized to both types of nuclei. These data suggest that micronucleus-specific proteins are transported by specific imp adapters. The different imp proteins exhibit substantial sequence divergence and do not appear to be simply redundant in function. Disruption of the IMA10 gene encoding an imp -like protein that accumulates in dividing micronuclei results in nuclear division defects and lethality. Thus, nucleus-specific protein import and nuclear function in Tetrahymena is regulated by diverse, specialized karyopherins.