Cytochrome c oxidase assembly in primates is sensitive to small evolutionary variations in amino acid sequence

Respiring mitochondria require many interactions between nuclear and mitoch ondrial genomes. Although mitochondrial DNA (mtDNA) from the gorilla and th e chimpanzee are able to restore oxidative phosphorylation in a human cell, mtDNAs from more distant primate species are functionally incompatible wit h human nuclear genes. Using microcell-mediated chromosome and mitochondria transfer, we introduced and maintained a functional orangutan mtDNA in a h uman nuclear background. However, partial oxidative phosphorylation functio n was restored only in the presence of most orangutan chromosomes, suggesti ng that human oxidative phosphorylation-related nuclear-coded genes are not able to replace many orangutan ones. The respiratory capacity of these hyb rids was decreased by 65%-80%, and cytochrome c oxidase (COX) activity was decreased by 85%-95%. The function of other respiratory complexes was not s ignificantly altered. The translation of mtDNA-coded COX subunits was norma l, but their steady-state levels were similar to 10% of normal ones. Nuclea r-coded COX subunits were loosely associated with mitochondrial membranes, a characteristic of COX assembly-defective mutants. Our results suggest tha t many human nuclear-coded genes not only cannot replace the orangutan coun terparts, but also exert a specific interference at the level of COX assemb ly. This cellular model underscores the precision of COX assembly in mammal s and sheds light on the nature of nuclear-mtDNA coevolutionary constraints .