COX15 CODES FOR A MITOCHONDRIAL PROTEIN ESSENTIAL FOR THE ASSEMBLY OFYEAST CYTOCHROME-OXIDASE

The respiratory defect of Saccharomyces cerevisiae mutants assigned to complementation group G4 of a pet strain collection stems from their failure to synthesize cytochrome oxidase. The mutations do not affect expression of either the mitochondrially or nuclearly encoded subunits of the enzyme, The cytochrome oxidase deficiency also does not appear to be related to mitochondrial copper metabolism or heme a biosynthes is, These data suggest that the mutants are likely to be impaired in a ssembly of the enzyme, A gene designated COX15 has been cloned by tran sformation of mutants from complementation group G4. This gene is iden tical to reading frame YER141w on chromosome 5, To facilitate further studies, Cox15p has been expressed as a biotinylated protein, Biotinyl ated Cox15p fully restores cytochrome oxidase in cox15 mutants, indica ting that the carboxyl-terminal sequence with biotin does not affect i ts function. Cox15p is a constituent of the mitochondrial inner membra ne and, because of its resistance to proteolysis, probably is largely embedded in the phospholipid bilayer of the membrane, The present stud ies further emphasize the complexity of cytochrome oxidase assembly an d report a new constituent of mitochondria involved in this process. T he existence of COX15 homologs in Schizosaccharomyces pombe and Caenor habditis elegans suggests that it may be widely distributed in eucaryo tic organisms.