Oxa1p acts as a general membrane insertion machinery for proteins encoded by mitochondrial DNA

Oxa1p is a member of the conserved Oxa1/YidC/Alb3 protein family involved in the membrane insertion of proteins. Oxa1p has been shown previously to directly facilitate the export of the N-terminal domains of membrane proteins across the inner membrane to the intermembrane space of mitochondria. Here we report on a general role of Oxa1p in the membrane insertion of proteins. (i) The function of Oxa1p is not limited to the insertion of membrane proteins that undergo N-terminal tail export; rather, it also extends to the insertion of other polytopic proteins such as the mitochondrially encoded Cox1p and Cox3p proteins. These are proteins whose N-termini are retained in the mitochondrial matrix. (ii) Oxa1p interacts directly with these substrates prior to completion of their synthesis. (iii) The interaction of Oxa1p with its substrates is particularly strong when nascent polypeptide chains are inserted into the inner membrane, suggesting a direct function of Oxa1p in co-translational insertion from the matrix. Taken together, we conclude that the Oxa1 complex represents a general membrane protein insertion machinery in the inner membrane of mitochondria.