SSI1 ENCODES A NOVEL HSP70 OF THE SACCHAROMYCES-CEREVISIAE ENDOPLASMIC-RETICULUM

The endoplasmic reticulum (ER) of the budding yeast Saccharomyces cere visiae contains a well-characterized, essential member of the Hsp70 fa mily of molecular chaperones, Kar2p. Kar2p has been shown to be involv ed in the translocation of protein into the ER as well as the proper f olding of proteins in that compartment. We report the characterization of a novel Hsp70 of the ER, Ssi1p. Ssi1p, which shares 24% of the ami no acids of Kar2p, is not essential for growth under normal conditions . However, deletion of SSI1 results in cold sensitivity as well as enh anced resistance to manganese. The localization of Ssi1p to the ER, su ggested by the presence of a conserved S. cerevisiae ER retention sign al at its C terminus, was confirmed by subcellular fractionation, prot ease protection assays, and immunofluorescence. The SSI1 promoter cont ains an element with similarity to the unfolded protein response eleme nt of KAR2. Like KAR2, SSI1 is induced both in the presence of tunicam ycin and in a kar2-159 mutant strain, conditions which lead to an accu mulation of unfolded proteins in the ER. Unlike KAR2, however, SSI1 is not induced by heat shock. Deletion of SSI1 shows a complex pattern o f genetic interactions with various conditional alleles of KAR2, rangi ng from synthetic lethality to synthetic rescue. Interestingly, SSI1 d eletion strains show a partial block in translocation of multiple prot eins into the ER, suggesting that Ssi1p plays a direct role in the tra nslocation process.