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Summary
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 ¦Â-barrel proteins use both translocases¡ªTOM for import to the intermembrane space and SAM for export into the outer membrane. It is unknown if the translocases cooperate and where the ¦Â-barrel of newly imported proteins is formed. We established a position-specific assay for monitoring ¦Â-barrel formation in?vivo and in organello and demonstrated that the ¦Â-barrel was formed and membrane inserted while the precursor was bound to SAM. ¦Â-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.
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