Alternative function for the mitochondrial SAM complex in biogenesis of α-helical TOM proteins

The mitochondrial outer membrane contains two preprotein translocases: the general translocase of outer membrane (TOM) and the β-barrel–specific sorting and assembly machinery (SAM). TOM functions as the central entry gate for nuclear-encoded proteins. The channel-forming Tom40 is a β-barrel protein, whereas all Tom receptors and small Tom proteins are membrane anchored by a transmembrane α-helical segment in their N- or C-terminal portion. Synthesis of Tom precursors takes place in the cytosol, and their import occurs via preexisting TOM complexes. The precursor of Tom40 is then transferred to SAM for membrane insertion and assembly. Unexpectedly, we find that the biogenesis of α-helical Tom proteins with a membrane anchor in the C-terminal portion is SAM dependent. Each SAM protein is necessary for efficient membrane integration of the receptor Tom22, whereas assembly of the small Tom proteins depends on Sam37. Thus, the substrate specificity of SAM is not restricted to β-barrel proteins but also includes the majority of α-helical Tom proteins.