A purified precursor polypeptide requires a cytosolic protein fraction for import into mitochondria.

The beta-subunit of mitochondrial ATPase is coded by a nuclear gene, synthesized outside the mitochondria as a larger precursor and imported into mitochondria. The beta-subunit precursor was purified from yeast, both as a homogeneous, unlabeled polypeptide and in radiochemically pure form. Both precursor preparations were cleaved to the mature beta-subunit by partially purified processing protease from the mitochondrial matrix. However, import of the radiochemically pure precursor into isolated yeast mitochondria required a cytosolic fraction from yeast or reticulocytes. The cytosolic factor was non-dialyzable and trypsin-sensitive; its apparent mol. wt. was approximately 40 000 as judged by gel filtration. Import of some proteins into mitochondria thus requires proteins of the 'soluble' cytoplasm.