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

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