SCP160P, A NEW YEAST PROTEIN ASSOCIATED WITH THE NUCLEAR-MEMBRANE ANDTHE ENDOPLASMIC-RETICULUM, IS NECESSARY FOR MAINTENANCE OF EXACT PLOIDY

We have cloned a new gene, SCP160, from Saccharomyces cerevisiae, the deduced amino acid sequence of which does not exhibit overall similari ty to any known yeast protein. A weak resemblance between the C-termin al part of the Scp160 protein and regulatory subunits of cAMP-dependen t protein kinases from eukaryotes as well as the pstB protein of Esche richia coli was observed. The SCP160 gene resides on the left arm of c hromosome X and codes for a polypeptide of molecular weight around 160 kDa. By immunofluorescence microscopy the Scp160 protein appears to b e localized to the nuclear envelope and to the endoplasmic reticulum ( ER). However, no signal sequence or membrane-spanning region exists, s uggesting that the Scp160 protein is attached to the cytoplasmic surfa ce of the ER-nuclear envelope membranes. Disruption of the SCP160 gene is not lethal but results in cells of decreased viability, abnormal m orphology and increased DNA content. This phenotype is not reversible by transformation with a plasmid carrying the wild-type gene. Crosses of SCP160 deletion mutant strains among each other or with unrelated s trains lead to irregular segregation of genetic markers. Taken togethe r the data suggest that the Scp160 protein is required during cell div ision for faithful partitioning of the ER-nuclear envelope membranes w hich in S. cerevisiae enclose the duplicated chromosomes.