MSI3, A MULTICOPY SUPPRESSOR OF MUTANTS HYPERACTIVATED IN THE RAS-CAMP PATHWAY, ENCODES A NOVEL HSP70 PROTEIN OF SACCHAROMYCES-CEREVISIAE

The MSI3 gene was isolated as a multicopy suppressor of the heat shock-sensitive phenotype of the ira1 mutation, which causes hyperactivation of the RAS-cAMP pathway. Overexpression of MSI3 also suppresses the heat shock-sensitive phenotype of the bcy1 mutant. Determination of the DNA sequence of MSI3 revealed that MSI3 can encode a 77.4 kDa protein related to the HSP70 family. The amino acid sequence of Msi3p is about 30% identical to that of the Ssa1p of Saccharomyces cerevisiae. This contrasts with the finding that members of the HSP70 family generally show at least 50% amino acid identity. The consensus nucleotide sequence of the heat shock element (HSE) was found in the upstream region of MSI3. Moreover, the steady-state levels of the MSI3 mRNA and protein were increased upon heat shock. These results indicate that the MSI3 gene encodes a novel HSP70-like heat shock protein. Disruption of the MSI3 gene was associated with a temperature sensitive growth phenotype but unexpectedly, thermotolerance was enhanced in the disruptant.