The Mitochondrial Receptor Complex: A Central Role of MOM22 in Mediating Preprotein Transfer from Receptors to the General Insertion Pore

The receptor complex in the mitochondrial outer membrane, which consists of at least seven different proteins, is responsible for the recognition and translocation of cytosolically synthesized preproteins. Two of its subunits, MOM19 and MOM72, function as surface receptors for preproteins. Four other subunits (MOM38, MOM30, MOM8, and MOM7) have been suggested to constitute the general insertion pore (GIP). Here we report on the structure and function of MOM22. MOM22 is anchored in the outer membrane by a single transmembrane segment. The highly negatively charged N-terminal domain is exposed to the cytosol and the C-terminal domain to the intermembrane space. MOM22 appears to be a central component of the receptor complex, required for the transfer of preproteins from the receptors to the GIP. We speculate that the negatively charged domain of MOM22 is involved in the transfer of positively charged signal sequences of preproteins.