Mitochondrial GrpE modulates the function of matrix Hsp70 in translocation and maturation of preproteins

Mitochondrial GrpE (Mge1p) is a mitochondrial cochaperone essential for viability of the yeast Saccharomyces cerevisiae. To study the role of Mge1p in the biogenesis of mitochondrial proteins, we isolated a conditional mutant allele of MGE1 which conferred a temperature-sensitive growth phenotype and led to the accumulation of mitochondrial preproteins after shifting of the cells to the restrictive temperature. The mutant Mge1 protein was impaired in its interaction with the matrix heat shock protein mt-Hsp70. The mutant mitochondria showed a delayed membrane translocation of preproteins, and the maturation of imported proteins was impaired, as evidenced by the retarded second proteolytic processing of a preprotein in the matrix. Moreover, the aggregation of imported proteins was decreased in the mutant mitochondria. The mutant Mge1p differentially modulated the interaction of mt-Hsp70 with preproteins compared with the wild type, resulting in decreased binding to preproteins in membrane transit and enhanced binding to fully imported proteins. We conclude that the interaction of Mge1p with mt-Hsp70 promotes the progress of the Hsp70 reaction cycle, which is essential for import and maturation of mitochondrial proteins.