The Mitochondrial Import Complex MIM Functions as Main Translocase for α-Helical Outer Membrane Proteins

The mitochondrial outer membrane contains integral proteins with alpha-helical membrane anchors or a transmembrane beta-barrel. The translocase of the outer membrane (TOM) cooperates with the sorting and assembly machinery (SAM) in the import of beta-barrel proteins, whereas the mitochondrial import (MIM) complex inserts precursors of multi-spanning alpha-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.