THE GTP-BOUND FORM OF THE YEAST RAN TC4 HOMOLOG BLOCKS NUCLEAR-PROTEIN IMPORT AND APPEARANCE OF POLY(A)(+) RNA IN THE CYTOPLASM/

Ran/TC4, a Ras-like GTP-binding protein, and its nucleotide exchanger, RCC1, have been implicated in control of protein movement into the nu cleus and cytoplasmic accumulation of mRNA. Saccharomyces cerevisiae c ontains two homologues of the mammalian Ran/TC4, encoded by the GSP1 a nd GSP2 genes. We have constructed yeast strains that overproduce eith er wild-type Gsp1 or a form of Gsp1 with glycine-21 converted to valin e (Gsp1-G21V), which we show stabilizes the GTP-bound form. Cells prod ucing Gsp1-G21V have defects in localization of nuclear proteins; nucl ear proteins accumulate in the cytoplasm following galactose induction of Gsp1-G21V. Similarly, cells Producing Gsp1-G21V retain poly(A)(+) RNA in their nuclei. These findings suggest that hydrolysis of GTP by Ran/TC4 is necessary for proper import of proteins into the nucleus an d appearance of poly(A)(+) RNA in the cytoplasm.