Ng. Kronidou et al., DYNAMIC INTERACTION BETWEEN ISP45 AND MITOCHONDRIAL HSP70 IN THE PROTEIN IMPORT SYSTEM OF THE YEAST MITOCHONDRIAL INNER MEMBRANE, Proceedings of the National Academy of Sciences of the United Statesof America, 91(26), 1994, pp. 12818-12822
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.