MDJ1P, A NOVEL CHAPERONE OF THE DNAJ FAMILY, IS INVOLVED IN MITOCHONDRIAL BIOGENESIS AND PROTEIN-FOLDING

Mdj1p, a novel member of the DnaJ family, is a heat shock protein that is associated with the inner membrane of mitochondria of Saccharomyce s cerevisiae. Disruption of the MDJ1 gene resulted in a petite phenoty pe, loss of mitochondrial DNA, and inviability at 37 degrees C. Import of precursor proteins was not affected by a lack of Mdj1p, but foldin g of newly imported proteins was markedly impaired. The efficiency of refolding of a tester protein, dihydrofolate reductase, was significan tly reduced in mitochondria lacking Mdj1p after incubation at elevated temperature. We conclude that Mdj1p is an important mitochondrial cha perone that participates in the folding of newly imported proteins and in the protection of proteins against heat denaturation and aggregati on.