The morphology proteins Mdm12/Mmm1 function in the major β-barrel assembly pathway of mitochondria

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Abstract
The β‐barrel proteins of mitochondria are synthesized on cytosolic ribosomes. The proteins are imported by the translocase of the outer membrane (TOM) and the sorting and assembly machinery (SAM). It has been assumed that the SAMcore complex with the subunits Sam35, Sam37 and Sam50 represents the last import stage common to all β‐barrel proteins, followed by splitting in a Tom40‐specific route and a route for other β‐barrel proteins. We have identified new components of the β‐barrel assembly machinery and show that the major β‐barrel pathway extends beyond SAMcore. Mdm12/Mmm1 function after SAMcore yet before splitting of the major pathway. Mdm12/Mmm1 have been known for their role in maintenance of mitochondrial morphology but we reveal assembly of β‐barrel proteins as their primary function. Moreover, Mdm10, which functions in the Tom40‐specific route, can associate with SAMcore as well as Mdm12/Mmm1 to form distinct assembly complexes, indicating a dynamic exchange between the machineries governing mitochondrial β‐barrel assembly. We conclude that assembly of mitochondrial β‐barrel proteins represents a major function of the morphology proteins Mdm12/Mmm1.