Integral membrane proteins in the mitochondrial outer membrane of Saccharomyces cerevisiae

Mitochondria evolved from a bacterial endosymbiont ancestor in which the integral outer membrane proteins would have been β-barrel structured within the plane of the membrane. Initial proteomics on the outer membrane from yeast mitochondria suggest that while most of the protein components are integral in the membrane, most of these mitochondrial proteins behave as if they have α-helical transmembrane domains, rather than β-barrels. These proteins are usually predicted to have a single α-helical transmembrane segment at either the N- or C-terminus, however, more complex topologies are also seen. We purified the novel outer membrane protein Om14 and show it is encoded in the gene YBR230c. Protein sequencing revealed an intron is spliced from the transcript, and both transcription from the YBR230c gene and steady-state level of the Om14 protein is dramatically less in cells grown on glucose than in cells grown on nonfermentable carbon sources. Hydropathy predictions together with data from limited protease digestion show three α-helical transmembrane segments in Om14. The α-helical outer membrane proteins provide functions derived after the endosymbiotic event, and require the translocase in the outer mitochondrial membrane complex for insertion into the outer membrane.