Rsp5 is an essential E3 ubiquitin ligase in Saccharomyces cerevisiae and is known to ubiquitinate plasma membrane permeases followed by endocytosis and vacuolar degradation. We previously isolated the rsp5 mutant that is hypersensitive to various stresses, suggesting that Rsp5 is involved in degradation of stress-induced abnormal proteins. Here, we analyzed the ability to refold the proteins by stress proteins in the rsp5 mutant. The transcription of stress protein genes in the rsp5 mutant was significantly lower than that in the wild-type strain when exposed to temperature up-shift, ethanol or sorbitol. Interestingly, the amounts of transcription factors Hsf1 and Msn4 were remarkably defective in the rsp5 mutant. These results suggest that expression of stress proteins are mediated by Rsp5 and that Rsp5 primarily regulates post-translational modification of Hsf1 and Msn4.

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Rsp5 is an essential and multi‐functional E3 ubiquitin ligase in Saccharomyces cerevisiae. We previously isolated the Ala401Glu rsp5 mutant that is hypersensitive to various stresses. In rsp5A401E cells, the transcription of the stress protein genes was defective. To understand the mechanism by which Rsp5 regulates the expression of stress proteins, we analyzed the expression and localization of two major transcription factors, Hsf1 and Msn2/4, required for stress protein gene expression in S. cerevisiae. The mRNA levels of HSF1 and MSN2/4 in rsp5A401E cells were slightly lower than those of wild‐type cells. An interesting finding is that the protein levels of HSF1 and Msn2/4 were remarkably defective in rsp5A401E cells after exposure to temperature up‐shift and ethanol, although these proteins are mainly localized in the nucleus under these stress conditions. We also showed that the mRNAs of HSF1 and MSN2/4 were accumulated in the nucleus of rsp5A401E cells after exposure to temperature up‐shift and ethanol, and even under non‐stress conditions, suggesting that Rsp5 is required for the nuclear export of these mRNAs. These results indicate that, in response to environmental stresses, Rsp5 primarily regulates the expression of Hsf1 and Msn2/4 at the post‐transcriptional level and is involved in the repair system of stress‐induced abnormal proteins.

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