Coupling of Mitochondrial Import and Export Translocases by Receptor-Mediated Supercomplex Formation

The mitochondrial outer membrane harbors two protein translocases that are essential for cell viability: the translocase of the outer mitochondrial membrane (TOM) and the sorting and assembly machinery (SAM). The precursors of beta-barrel proteins use both translocases-TOM for import to the intermembrane space and SAM for export into the outer membrane. It is unknown if the translocases cooperate and where the beta-barrel of newly imported proteins is formed. We established a position-specific assay for monitoring beta-barrel formation in vivo and in organello and demonstrated that the beta-barrel was formed and membrane inserted while the precursor was bound to SAM. beta-barrel formation was inhibited by SAM mutants and, unexpectedly, by mutants of the central import receptor, Tom22. We show that the cytosolic domain of Tom22 links TOM and SAM into a supercomplex, facilitating precursor transfer on the intermembrane space side. Our study reveals receptor-mediated coupling of import and export translocases as a means of precursor channeling.