Mitochondrial membrane biogenesis: identification of a precursor to yeast cytochrome c oxidase subunit II, an integral polypeptide.

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Abstract
Many of the polypeptides made on endogenous ribosomes inside of yeast mitochondria are hydrophobic "integral polypeptides" which are subunits of at least 3 oligomeric enzyme complexes (cytochrome c oxidase, rutamycin-sensitive ATPase, and coenzyme QH2-cytochrome c reductase) of the inner mitochondrial membrane. In order to elucidate the pathway(s) followed by these polypeptides into the inner membrane an in vitro mitochondrial translation system from yeast was used. By inhibiting this system with aurintricarboxylic acid, a transient precursor to subunit II of cytochrome c oxidase was demonstrated and accumulated. This precursor, designated II'', is approximately 1500 daltons larger than mature subunit II and most likely is a form of subunit II with an NH2-terminal extension. Although this precursor appears to be processed cotranslationally under normal conditions, it does associate in unprocessed form with mitochondrial membranes when allowed to accumulate in the presence of aurintricarboxylic acid, and it can be processes postranslationally upon removal of the drug. None of the other mitochondrial translation products made in this system exhibits larger precursors. At least 1 mitochondrial translation product apparently has a transient "leader sequence" and is inserted into the inner mitochondrial membrane and processed cotranslationally. However, other pathways may be followed by the other translation products.