THE YEAST NUCLEAR IMPORT RECEPTOR IS REQUIRED FOR MITOSIS

The nuclear import system is highly conserved among eukaryotes. Here w e report the effects of a conditional mutation in SRP1, which encodes a Saccharomyces cerevisiae homolog of the vertebrate nuclear import re ceptor importin. Importin was isolated as a factor required for the in itial targeting step of a nuclear import substrate to the nuclear enve lope in a mammalian in vitro assay. We show that yeast Srp1 is similar ly required for protein import. In addition, Srp1 is also required for the execution of mitosis: we demonstrate that cells containing a cond itional mutation of SRP1 arrest with a G(2)/M phenotype in a manner an alogous to classic cdc mutants. This defect may be due to the failure of the mutant to degrade the mitotic cyclin Clb2 and other proteins re quired for mitosis. The requirement of a nuclear import receptor for c ell cycle-regulated proteolysis implies that import of cell cycle regu lators into the nucleus is critical for cell cycle progression.