Bk. Baxter et al., SSI1 ENCODES A NOVEL HSP70 OF THE SACCHAROMYCES-CEREVISIAE ENDOPLASMIC-RETICULUM, Molecular and cellular biology, 16(11), 1996, pp. 6444-6456
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.