CYCLOPHILIN CATALYZES PROTEIN-FOLDING IN YEAST MITOCHONDRIA

Cyclophilins are a family of ubiquitous proteins that are the intracel lular target of the immunosuppressant drug cyclosporin A. Although cyc lophilins catalyze peptidylprolyl cis-trans isomerization in vitro, it has remained open whether they also perform this function in vivo. He re we show that Cpr3p, a cyclophilin in the matrix of yeast mitochondr ia, accelerates the refolding of a fusion protein that was synthesized in a reticulocyte lysate and imported into the matrix of isolated yea st mitochondria. The fusion protein consisted of the matrix-targeting sequence of subunit 9 of F1F0-ATPase fused to mouse dihydrofolate redu ctase. Refolding of the dihydrofolate reductase moiety in the matrix w as monitored by acquisition of resistance to proteinase K. The rate of refolding was reduced by a factor of 2-6 by 2.5 mu M cyclosporin A. T his reduced rate of folding was also observed with mitochondria lackin g Cpr3p. In these mitochondria, protein folding was insensitive to cyc losporin A. The rate of protein import was not affected by cyclosporin A or by deletion of Cpr3p.