DYNAMIC INTERACTION BETWEEN ISP45 AND MITOCHONDRIAL HSP70 IN THE PROTEIN IMPORT SYSTEM OF THE YEAST MITOCHONDRIAL INNER MEMBRANE

The protein import system of the yeast mitochondrial inner membrane in cludes at least three membrane proteins that presumably form a transme mbrane channel as well as several chaperone proteins that mediate the import and refolding of precursor proteins. We show that one of the me mbrane proteins, Isp45, spans the mitochondrial inner membrane yet is extracted from this membrane at high pH. Solubilization of mitochondri a with a nonionic detergent releases lsp45 as a complex with the chape rones mitochondrial hsp70 (mhsp70) and GrpEp. Both chaperones reversib ly dissociate from Isp45 upon addition of ATP or adenosine 5'-[gamma-t hio]triphosphate, suggesting that dissociation requires the binding of ATP. Control experiments indicate that the interaction between mhsp70 and Isp45 occurs in the intact mitochondria. We propose that Isp45 li nes the inside of a proteinaceous channel across the inner membrane an d that it is the membrane anchor for an ATP-driven ''import motor'' co mposed of mhsp70 and GrpEp. This arrangement is reminiscent of the pro tein transport systems of the yeast endoplasmic reticulum and the bact erial plasma membrane.