Precursor Proteins Are Transported into Mitochondria in the Absence of Proteolytic Cleavage of the Additional Sequences*

Many nuclear-coded mitochondrial proteins are syn-thesized as larger precursor polypeptides that re pro-teolytically processed during import into the mitochon-drion. This processing appears to be catalyzed by a soluble, metal-dependent protease localized in the mi-tochondrial matrix. In this report we employ an in vitro system to investigate the role of processing in protein import. Intact Neurospora crassa mitochon-dria were incubated with radiolabeled precursors in the presence of the chelator o-phenanthroline. Under these conditions, the processing of the precursors of the &subunit of Fl-ATPase (FIP) and subunit 9 of the FoF1-ATPase was strongly inhibited. Protease-map-ping studies indicated that import of the precursor proteins into the mitochondria continued in the absenc