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Summary: The mitochondrial outer membrane contains integral proteins with α-helical membrane anchors or a transmembrane β-barrel. The translocase of the outer membrane (TOM) cooperates with the sorting and assembly machinery (SAM) in the import of β-barrel proteins, whereas the mitochondrial import (MIM) complex inserts precursors of multi-spanning α-helical proteins. Single-spanning proteins constitute more than half of the integral outer membrane proteins; however, their biogenesis is poorly understood. We report that the yeast MIM complex promotes the insertion of proteins with N-terminal (signal-anchored) or C-terminal (tail-anchored) membrane anchors. The MIM complex exists in three dynamic populations. MIM interacts with TOM to accept precursor proteins from the receptor Tom70. Free MIM complexes insert single-spanning proteins that are imported in a Tom70-independent manner. Finally, coupling of MIM and SAM promotes early assembly steps of TOM subunits. We conclude that the MIM complex is a major and versatile protein translocase of the mitochondrial outer membrane. : Doan et al. report that the mitochondrial import (MIM) complex constitutes the major import site for single-spanning and multi-spanning outer membrane proteins. MIM exists in three forms and dynamically cooperates with outer membrane protein translocases to import different types of precursor proteins. Keywords: MIM complex, mitochondria, outer membrane, protein assembly, protein sorting, protein translocase, SAM complex, TOM complex

Summary: Mitochondria possess elaborate machineries for the import of proteins from the cytosol. Cytosolic factors like Hsp70 chaperones and their co-chaperones, the J-proteins, guide proteins to the mitochondrial surface. The translocase of the mitochondrial outer membrane (TOM) forms the entry gate for preproteins. How the proteins are delivered to mitochondrial preprotein receptors is poorly understood. We identify the cytosolic J-protein Xdj1 as a specific interaction partner of the central receptor Tom22. Tom22 recruits Xdj1 to the mitochondrial surface to promote import of preproteins and assembly of the TOM complex. Additionally, we find that the receptor Tom70 binds a different cytosolic J-protein, Djp1. Our findings suggest that cytosolic J-proteins target distinct TOM receptors and promote the biogenesis of mitochondrial proteins. : Opaliński et al. report that mitochondrial protein import receptors selectively recognize J-protein co-chaperones of the cytosol. The co-chaperones bind hydrophobic precursor proteins and assist in transferring them to the receptors of the mitochondrial protein entry gate. Keywords: mitochondria, TOM complex, protein targeting, J-proteins, cytosolic chaperones