MSF, a novel cytoplasmic chaperone which functions in precursor targeting to mitochondria.

Mitochondrial import stimulation factor (MSF) unfolds wheat germ lysate synthesized aggregated mitochondrial precursor proteins and stimulates their mitochondrial import in an ATP dependent manner. Here we analysed the function of MSF mainly by utilizing chemically pure adrenodoxin precursor (pAd). MSF bound to the unfolded pAd and prevented it from losing import competence and also restored the import competence of the aggregated pAd dependent on ATP hydrolysis. The import incompetent aggregated mitochondrial precursors induced the ATPase activity of MSF and the activity was strongly inhibited by isolated mitochondrial outer membrane (OM) but not by trypsin treated outer membrane (tOM). The precursor induced ATPase activity of N-ethylmaleimide (NEM)-treated MSF was not inhibited by OM. In this context, the MSF-precursor complex specifically bound to OM and binding was abolished both by the treatment of OM with trypsin and by the treatment of MSF with NEM. These results show that MSF is a novel cytoplasmic chaperone protein with a mitochondrial precursor-targeting function.