MTCH2 is a mitochondrial outer membrane protein insertase

In the mitochondrial outer membrane, α-helical transmembrane proteins play critical roles in cytoplasmic-mitochondrial communication. Using genome-wide CRISPR screens, we identified mitochondrial carrier homolog 2 (MTCH2), and its paralog MTCH1, and showed that it is required for insertion of biophysically diverse tail-anchored (TA), signal-anchored, and multipass proteins, but not outer membrane β-barrel proteins. Purified MTCH2 was sufficient to mediate insertion into reconstituted proteoliposomes. Functional and mutational studies suggested that MTCH2 has evolved from a solute carrier transporter. MTCH2 uses membrane-embedded hydrophilic residues to function as a gatekeeper for the outer membrane, controlling mislocalization of TAs into the endoplasmic reticulum and modulating the sensitivity of leukemia cells to apoptosis. Our identification of MTCH2 as an insertase provides a mechanistic explanation for the diverse phenotypes and disease states associated with MTCH2 dysfunction. The essential roles of mitochondria in metabolism and signaling depend on a functionally and structurally diverse class of alpha-helical proteins embedded in the outer mitochondrial membrane. Guna et al . identified the mitochondrial outer membrane protein MTCH2 (mitochondrial carrier homolog 2) and found that it is both necessary and sufficient for the insertion of mitochondrial alpha-helical proteins. MTCH2 is the defining member of a broadly conserved class of insertases that exploit a diverged ancestral solute transporter fold to mediate membrane protein insertion. MTCH2’s role as a gatekeeper for outer mitochondrial membrane biogenesis rationalizes its pleotropic phenotypes and association with human disease. —SMH Mitochondrial carrier homolog 2 mediates the insertion of diverse alpha-helical proteins into the mitochondrial outer membrane.