MAS5, A YEAST HOMOLOG OF DNAJ INVOLVED IN MITOCHONDRIAL PROTEIN IMPORT

The nuclear mas5 mutation causes temperature-sensitive growth and defects in mitochondrial protein import at the nonpermissive temperature in the yeast Saccharomyces cerevisiae. The MAS5 gene was isolated by complementation of the mutant phenotypes, and integrative transformation demonstrated that the complementing fragment encoded the authentic MAS5 gene. The deduced protein sequence of the cloned gene revealed a polypeptide of 410 amino acids which is homologous to Escherichia coli DnaJ and the yeast DnaJ log SCJ1. Northern (RNA blot) analysis revealed that MAS5 is a heat shock gene whose expression increases moderately at elevated temperatures. Cells with a deletion mutation in MAS5 grew slowly at 23-degrees-C and were inviable at 37-degrees-C, demonstrating that MAS5 is essential for growth at increased temperatures. The deletion mutant also displayed a modest import defect at 23-degrees-C and a substantial import defect at 37-degrees-C. These results indicate a role for a DnaJ cognate protein in mitochondrial protein import.