YEAST SRP1P HAS HOMOLOGY TO ARMADILLO PLAKOGLOBIN/BETA-CATENIN AND PARTICIPATES IN APPARENTLY MULTIPLE NUCLEAR FUNCTIONS INCLUDING THE MAINTENANCE OF THE NUCLEOLAR STRUCTURE/

SRP1, a suppressor of certain temperature-sensitive mutations in RNA p olymerase I in Saccharomyces cerevisiae, encodes a protein that is ass ociated with nuclear pores. By using a system of conditional SRP1 expr ession and by isolating temperature-sensitive srp1 mutants, we have de monstrated that Srp1p is essential for maintenance of the crescent-sha ped nucleolar structure, RNA transcription, and the proper functions o f microtubules as inferred from analysis of nuclear division/segregati on and immunofluorescence microscopy of microtubules. Different mutant alleles showed significantly different phenotypes in relation to thes e apparently multiple functional roles of the protein. We have also fo und that eight imperfect 42-amino-acid tandem repeats present in Srp1p are similar to the 42-amino-acid repeats in armadillo/plakoglobin/bet a-catenin proteins present in adhesive junction complexes of higher eu karyotes. We discuss this similarity in connection with the observed p leiotropic effects of srp1 mutations.