Disruption of YHC8, a member of the TSR1 gene family, reveals its direct involvement in yeast protein translocation

Genetic studies of Saccharomyces cerevisiae have identified many components acting to deliver specific proteins to their cellular locations. Genome an alysis, however, has indicated that additional genes may also participate i n such protein trafficking. The product of the yeast Yarrowia lipolytica TS R1 gene promotes the signal recognition particle-dependent translocation of secretory proteins through the endoplasmic reticulum. Here we describe the identification of a new gene family of proteins that is well conserved amo ng different yeast species. The TSR1 genes encode polypeptides that share t he same protein domain distribution and, like Tsr1p, may play an important role in the early steps of the signal recognition particle-dependent transl ocation pathway. We have identified five homologues of the TSR1 gene, four of them from the yeast Saccharomyces cerevisiae and the other from Hansenul a polymorpha. We generated a null mutation in the S. cerevisiae YHC8 gene, the closest homologue to Y. lipolytica TSR1, and used different soluble (ca rboxypeptidase Y, LY-factor, invertase) and membrane (dipeptidyl-aminopepti dase) secretory proteins to study its phenotype. A large accumulation of so luble protein precursors was detected in the mutant strain. Immunofluoresce nce experiments show that Yhc8p is localized in the endoplasmic reticulum. We propose that the YHC8 gene is a new and important component of the S. ce revisiae endoplasmic reticulum membrane and that it functions in protein tr anslocation/insertion of secretory proteins through or into this compartmen t.