ALLELE-SPECIFIC SUPPRESSION OF A SACCHAROMYCES-CEREVISIAE PRP20 MUTATION BY OVEREXPRESSION OF A NUCLEAR SERINE THREONINE PROTEIN-KINASE

The yeast PRP20 protein is homologous to the RCC1 protein of higher eu karyotes and is required for mRNA export and maintenance of nuclear st ructure. RCC1/PRP20 act as guanine nucleotide exchange factors for the nuclear Ras-like Ran/GSP1 proteins, In a search for prp20-10 allele-s pecific high-copy-number suppressors, the KSP1 locus, encoding a serin e/threonine protein kinase was isolated, Ksp1p is a nuclear protein th at is not essential for vegetative growth of yeast. Inactivation of th e kinase activity by a mutation affecting the catalytic center of the Ksp1p eliminated the suppressing activity. Based on the isolation of a protein kinase as a high-copy-number suppressor, the phosphorylation of Prp20p was examined. In vivo labeling experiments showed that Prp20 p is a phosphoprotein; however, deletion of the KSP1 kinase did not af fect Prp20p phosphorylation.