Biogenesis of the mitochondrial receptor complex. Two receptors are required for binding of MOM38 to the outer membrane surface.

Targeting of preproteins to mitochondria is mediated by the receptor complex in the outer membrane that contains two import receptors and the general insertion pore with MOM38 (38-kDa mitochondrial outer membrane protein) as major constituent. As all components of the receptor complex have to be imported from the cytosol themselves, the specificity of their targeting is fundamental for the correct assembly of mitochondria. None of the receptors is involved in its own import; the precursor of the main receptor MOM19 is even targeted without any surface receptor but directly assembles with MOM38. We report that import of the precursor of MOM38 strictly depended on surface receptors. The import followed a new highly selective mechanism in that both receptors together were needed for the specific binding of the preprotein to the outer membrane surface, which was followed by its assembly into the receptor complex. These findings suggest that targeting of the mitochondrial targeting components involves a complex system of mutual specificity control, ensuring a selective assembly of the components into preexisting import sites.