Mitochondrial versus nuclear gene expression and membrane protein assembly: the case of subunit 2 of yeast cytochrome c oxidase.

Deletion of the yeast mitochondrial gene COX2 , encoding subunit 2 (mtCox2) of cytochrome c oxidase (C c O), results in a respiratory-incompetent Δcox2 strain. For a cytosol-synthesized Cox2 to restore respiratory growth, it must carry the W56R mutation (cCox2 ^W56R ). Nevertheless, only a fraction of cCox2 ^W56R is matured in mitochondria, allowing ∼60% steady-state accumulation of C c O. This can be attributed either to the point mutation or to an inefficient biogenesis of cCox2 ^W56R . We generated a strain expressing the mutant protein mtCox2 ^W56R inside mitochondria which should follow the canonical biogenesis of mitochondria-encoded Cox2. This strain exhibited growth rates, C c O steady-state levels, and C c O activity similar to those of the wild type; therefore, the efficiency of Cox2 biogenesis is the limiting step for successful allotopic expression. Upon coexpression of cCox2 ^W56R and mtCox2, each protein assembled into C c O independently from its genetic origin, resulting in a mixed population of C c O with most complexes containing the mtCox2 version. Notably, the presence of the mtCox2 enhances cCox2 ^W56R incorporation. We provide proof of principle that an allotopically expressed Cox2 may complement a phenotype due to a mutant mitochondrial COX2 gene. These results are relevant to developing a rational design of genes for allotopic expression intended to treat human mitochondrial diseases.