Two nonidentical forms of subunit V are functional in yeast cytochrome c oxidase.

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Abstract
In Saccharomyces cerevisiae, the inner mitochondrial membrane protein cytochrome c oxidase is composed of 9 polypeptide subunits. Of these subunits, 6 (IV, V, VI, VII, VIIa, VIII) are encoded by mitochondrial DNA. Two nonidentical subunit V polypeptides, which are encoded by separate genes within the yeast genome are reported. One gene, COX5a, encodes the polypeptide Va, normally found in preparations of holocytochrome c oxidase. The other gene, COX5b, encodes the polypeptide Vb, which cross-reacts with anti-subunit Va antiserum and restores respiratory competency and cytochrome oxidase activity in transformants of cox5a structural gene mutants. This polypeptide also copurifies with the holoenzyme prepared from these transformants. COX5b is expressed in vegetatively growing yeast cells and the Vb polypeptide can be detected in mitochondria from strain JM28, a cox5a mutant. This mutant has 15%-20% residual cytochrome oxidase activity and it respires at 10%-15% the wild-type rate. By disrupting the COX5b gene in this strain, this residual activity is directly attributable to the presence of a chromosomal copy of the COX5b gene. Va or Vb can function as cytochrome oxidase subunits in yeast and Vb may be used under some specific, as yet undefined, physiological conditions.