Mitochondrial import driving forces: Enhanced trapping by matrix Hsp70 stimulates translocation and reduces the membrane potential dependence of loosely folded preproteins

The mitochondrial heat shock protein Hsp70 (mtHsp70) is essential for drivi ng translocation of preproteins into the matrix. Two models, trapping and p ulling by mtHsp70, are discussed, but positive evidence for either model ha s not been found so far. We have analyzed a mutant mtHsp70, Ssc1-2, that sh ows a reduced interaction with the membrane anchor Tim44, but an enhanced t rapping of preproteins. Unexpectedly, at a low inner membrane potential, ss c1-2 mitochondria imported loosely folded preproteins more efficiently than wild-type mitochondria. The import of a tightly folded preprotein, however , was not increased in ssc1-2 mitochondria. Thus, enhanced trapping by mtHs p70 stimulates the import of loosely folded preproteins and reduces the dep endence on the import-driving activity of the membrane potential, directly demonstrating that trapping is one of the molecular mechanisms of mtHsp70 a ction.