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11 Role of hsp70 in Translocation of Proteins across Membranes

Elizabeth A. Craig


A number of functions of heat shock proteins have been postulated, mainly on the basis of results of biochemical experiments. In most cases, however, these proposed roles for hsps have not been supported by physiological or genetic data because strains containing mutations in heat shock genes have not been available. In Saccharomyces cerevisiae there is a large family of HSP70 genes that can be divided into at least four subfamilies based on the structure and function of the genes (see Craig, this volume). Recently, strains containing mutations in these genes have been constructed, allowing a genetic analysis of hsp function. In particular, we have analyzed the phenotypes of strains containing mutations in the subfamily. The SSA genes, SSA1, SSA2, SSA3, and SSA4, compose an essential gene family, in the sense that at least one of the proteins encoded by these genes must be present at relatively high levels for cell growth (Werner-Washburne et al. 1987). An ssa1 ssa2 ssa4 strain is inviable; spores of this genotype do not bud. An ssa1 ssa2 ssa4 strain can be rescued by a plasmid containing SSA1 under the control of the galactose-inducible GAL1 promoter (GAL1p:SSA1). However, in glucose-based media, the GAL1 promoter is repressed and SSA1 production ceases. Because of dilution due to cell division and protein degradation, cellular levels of Ssa1p decrease, allowing analysis of cells lacking protein.

We chose to analyze the effect of Ssa1p depletion on protein translocation for a number of reasons. The results of a number of...

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