A MITOCHONDRIAL HOMOLOG OF BACTERIAL GRPE INTERACTS WITH MITOCHONDRIAL HSP70 AND IS ESSENTIAL FOR VIABILITY

Mitochondrial hsp70 (mhsp70) is located in the matrix and an essential component of the mitochondrial protein import system. To study the fu nction of mhsp70 and to identify possible partner proteins we construc ted a yeast strain in which all mhsp70 molecules carry a C-terminal he xa-histidine tag. The tagged mhsp70 appears to be functional in vivo. When an ATP depleted mitochondrial extract was incubated with a nickel -derivatized affinity resin, the resin bound not only mhsp70, but also a 23 kDa protein. This protein was dissociated from mhsp70 by ATP. AD P and GTP were much less effective in promoting dissociation whereas C TP and TTP were inactive. We cloned the gene encoding the 23 kDa prote in. This gene, termed GRPE, encodes a 228 residue protein, whose seque nce closely resembles that of the bacterial GrpE protein. Microsequenc ing the purified 23 kDa protein established it as the product of the y east GRPE gene. Yeast GrpEp is made as a precursor that is cleaved upo n import into isolated mitochondria. GrpEp is essential for viability. We suggest that this protein interacts with mhsp70 in a manner analog ous to that of GrpE with DnaK of E.coli.