Baxter B K, James P, Evans T, Craig E A
Department of Biomolecular Chemistry, University of Wisconsin, Madison 53706, USA.
Mol Cell Biol. 1996 Nov;16(11):6444-56. doi: 10.1128/MCB.16.11.6444.
The endoplasmic reticulum (ER) of the budding yeast Saccharomyces cerevisiae contains a well-characterized, essential member of the Hsp70 family of molecular chaperones, Kar2p. Kar2p has been shown to be involved in the translocation of proteins into the ER as well as the proper folding of proteins in that compartment. We report the characterization of a novel Hsp70 of the ER, Ssi1p. Ssi1p, which shares 24% of the amino acids of Kar2p, is not essential for growth under normal conditions. However, deletion of SSI1 results in cold sensitivity as well as enhanced resistance to manganese. The localization of Ssi1p to the ER, suggested by the presence of a conserved S. cerevisiae ER retention signal at its C terminus, was confirmed by subcellular fractionation, protease protection assays, and immunofluorescence. The SSI1 promoter contains an element with similarity to the unfolded protein response element of KAR2. Like KAR2, SSI1 is induced both in the presence of tunicamycin and in a kar2-159 mutant strain, conditions which lead to an accumulation of unfolded proteins in the ER. Unlike KAR2, however, SSI1 is not induced by heat shock. Deletion of SSI1 shows a complex pattern of genetic interactions with various conditional alleles of KAR2, ranging from synthetic lethality to synthetic rescue. Interestingly, SSI1 deletion strains show a partial block in translocation of multiple proteins into the ER, suggesting that Ssi1p plays a direct role in the translocation process.
出芽酵母酿酒酵母的内质网（ER）含有热休克蛋白70（Hsp70）分子伴侣家族中一个特征明确的必需成员Kar2p。Kar2p已被证明参与蛋白质向内质网的转运以及该区域内蛋白质的正确折叠。我们报道了一种新型内质网Hsp70即Ssi1p的特征。Ssi1p与Kar2p有24%的氨基酸相同，在正常条件下对生长不是必需的。然而，缺失SSI1会导致冷敏感性以及对锰的抗性增强。Ssi1p定位于内质网，这一点由其C末端存在保守的酿酒酵母内质网滞留信号所提示，并通过亚细胞分级分离、蛋白酶保护试验和免疫荧光得以证实。SSI1启动子含有一个与KAR2的未折叠蛋白反应元件相似的元件。与KAR2一样，SSI1在衣霉素存在时以及在kar2 - 159突变株中均被诱导，这些条件会导致内质网中未折叠蛋白的积累。然而，与KAR2不同的是，SSI1不会被热休克诱导。缺失SSI1显示出与KAR2的各种条件等位基因存在复杂的遗传相互作用模式，从合成致死到合成拯救。有趣的是，缺失SSI1的菌株在多种蛋白质向内质网的转运中表现出部分阻滞，这表明Ssi1p在转运过程中起直接作用。
