MUTATION OF THE RAB6 HOMOLOG OF SACCHAROMYCES-CEREVISIAE, YPT6, INHIBITS BOTH EARLY GOLGI FUNCTION AND RIBOSOME BIOSYNTHESIS

A screen was designed to identify temperature-sensitive mutants of Sac charomyces cerevisiae, whose transcription of both ribosomal RNA and r ibosomal protein genes is repressed at the nonpermissive temperature, The gene from one such mutant was cloned by complementation, The gene encodes a predicted product that is nearly 65% identical to the human GTPase, Rab6, and is likely to be identical to the yeast gene YPT6. It is essential for growth only at elevated temperatures, The mutant str ain is partially defective in the maturation of the vacuolar protein c arboxypeptidase Y, as well as in the secretion of invertase, which acc umulates as a core-glycosylated form characteristic of the endoplasmic reticulum or the cis-Golgi, suggesting that Ypt6p is involved in an e arly step of the secretory pathway, earlier than that reported for the mammalian Rab6., The mutant protein, a truncation at codon 64 of 215, has a stronger phenotype than the null allele of YPT6. Four other mut ants selected for defective ribosome synthesis at the nonpermissive te mperature were also found to have defects in carboxypeptidase Y matura tion, giving emphasis to our previous finding that a functional secret ory pathway is essential for continued ribosome synthesis, Cloning of extragenic suppressors of the ts allele of YPT6 has revealed two addit ional proteins that influence the secretory pathway: Ssd1p, a suppress or of many mutations, and Imh1p, which bears some homology to the C-te rminal portion of the cytoskeletal proteins integrin and myosin.