Import of proteins into mitochondria. Partial purification of a matrix-located protease involved in cleavage of mitochondrial precursor polypeptides.

Most mitochondrial proteins are synthesized in the cytoplasm as larger precursor polypeptides which are imported into the organelle in an energy-dependent step. The proteolytic conversion of these precursors to their mature size involves a neutral matrix-located protease which has been purified 100-fold from yeast mitochondria. It cleaves the precursors to several imported proteins of the matrix, the mitochondrial inner membrane and the intermembrane space, but is inactive against all mature mitochondrial proteins or against all nonmitochondrial proteins tested so far. As shown in the subsequent report (Cerletti, N., Böhni, P. C., and Suda, K. (1983) J. Biol. Chem. 258, 4944-4949), processing of the cytochrome c oxidase subunit V precursor with the partially purified protease yielded the correct mature NH2 terminus. Precursors to cytochrome b2 and cytochrome c1 (which are imported into the intermembrane space and the outer face of the inner membrane, respectively) are cleaved to intermediate forms which can also be detected as transient forms in vivo. The protease activity has a pH optimum of 7.5 and is inhibited by 1,10-phenanthroline, EDTA, or nucleoside triphosphates, but not by serine-protease inhibitors or by small peptide inhibitors. Its activity can be restored after chelation by excess Co2+ or Zn2+. The enzyme is coded in the nucleus and is, thus, imported into mitochondria.