A FUNCTIONAL HOMOLOG OF THE RNA1 GENE-PRODUCT IN SCHIZOSACCHAROMYCES-POMBE - PURIFICATION, BIOCHEMICAL-CHARACTERIZATION, AND IDENTIFICATIONOF A LEUCINE-RICH REPEAT MOTIF

The RNA1 gene from Saccharomyces cerevisiae is defined by the temperat ure-sensitive rna1-1 mutation that interferes with the maturation and/ or nucleocytoplasmic transport of RNA. We describe the purification of a 44-kDa protein from the evolutionary distant fission yeast Schizosa ccharomyces pombe and the cloning and sequence analysis of the corresp onding gene. Although this protein shares only 42% sequence identity w ith the RNA1 gene product, it represents a functional homologue becaus e the expression of the S. pombe gene in S. cerevisiae complements the rna1-1 defect. Disruption in S. pombe of the gene encoding the 44-kDa protein, for which we propose the name S. pombe rna1p, reveals that i t is essential for growth. Our analysis of purified S. pombe rna1p rep resents the first biochemical characterization of an RNA1 gene product and reveals that it is a monomeric protein of globular shape. Cell fr actionation and immunofluorescence microscopy indicate that rna1p is a cytoplasmic protein possibly enriched in the nuclear periphery. We id entify a sequence motif of 29 residues, which is rich in leucine and r epeated eight times both in S. pombe and in S. cerevisiae rna1p. Simil ar leucine-rich repeats present in a series of other proteins, e.g., t he mammalian ribonuclease/angiogenin inhibitor, adenylyl cyclase from S. cerevisiae, the toll protein from Drosophila melanogaster, and the sds22 protein phosphatase regulatory subunit from S. pombe, are though t to be involved in protein-protein interactions. Thus rna1p may act a s a scaffold protein possibly interacting in the nuclear periphery wit h a protein ligand that could be associated with exported RNA.