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 polymerase I in Saccharomyces cerevisiae, encodes a protein that is associated with nuclear pores. By using a system of conditional SRP1 expression and by isolating temperature-sensitive srp1 mutants, we have demonstrated that Srp1p is essential for maintenance of the crescent-shaped nucleolar structure, RNA transcription, and the proper functions of microtubules as inferred from analysis of nuclear division/segregation and immunofluorescence microscopy of microtubules. Different mutant alleles showed significantly different phenotypes in relation to these apparently multiple functional roles of the protein. We have also found that eight imperfect 42-amino-acid tandem repeats present in Srp1p are similar to the 42-amino-acid repeats in armadillo/plakoglobin/beta-catenin proteins present in adhesive junction complexes of higher eukaryotes. We discuss this similarity in connection with the observed pleiotropic effects of srp1 mutations.