Functional Interaction of Cytosolic hsp70 and a DnaJ-Related Protein, Ydj1p, in Protein Translocation In Vivo

Abstract

In order to analyze the in vivo role of the SSA class of cytosolic 70-kDa heat shock proteins (hsps) of Saccharomyces cerevisiae, we isolated a temperature-sensitive mutant of SSA1. The effect of a shift of mutant cells (ssa1^ts ssa2 ssa3 ssa4) from the permissive temperature of 23°C to the nonpermissive temperature of 37°C on the processing of several precursor proteins translocated into the endoplasmic reticulum or mitochondria was assessed. Of three mitochondrial proteins tested, the processing of only one, the β subunit of the F₁F₀ ATPase, was dramatically affected. Of six proteins destined for the endoplasmic reticulum, the translocation of only prepro-α-factor and proteinase A was inhibited. The processing of prepro-α-factor was inhibited within 2 min of the shift to 37°C, suggesting a direct effect of the hsp70 defect on translocation. More than 50% of radiolabeled α-factor accumulated in the precursor form, with the remainder rapidly reaching the mature form. However, the translocation block was complete, as the precursor form could not be chased through the translocation pathway. Since DnaJ-related proteins are known to interact with hsp70s and strains containing conditional mutations in a dnaJ-related gene, YDJ1, are defective in translocation of prepro-α-factor, we looked for a genetic interaction between SSA genes and YDJ1 in vivo. We found that a deletion mutation of YDJ1 was synthetically lethal in a ssa1^ts ssa2 ssa3 ssa4 background. In addition, a strain containing a single functional SSA gene, SSA1, and a deletion of YDJ1 accumulated the precursor form of α-factor. However, no genetic interaction was observed between a YDJ1 mutation and mutations in the SSB genes, which encode a second class of cytosolic hsp70 chaperones. These results are consistent with SSA proteins and Ydj1p acting together in the translocation process.