Transport of proteins across or into the mitochondrial outer membrane

Abstract

Mitochondria are surrounded by two biological membranes. The outer mitochondrial membrane contains two major translocators, the TOM40 (TOM) and TOB/SAM complexes for protein translocation across and/or insertion into the outer membrane. The TOM40 complex functions as an entry gate for most mitochondrial proteins, and the TOB/SAM complex as a specialized insertion machinery for beta-barrel membrane proteins. In order to handle loosely folded or unfolded precursor polypeptides, those translocators cooperate with chaperones in the cytosol and intermembrane space, and also exhibit chaperone-like functions on their own. Several alpha-helical membrane proteins take 'non-standard' routes to be inserted into the outer membrane. Here we review the current view on a remarkable variety of mechanisms of protein transport taking place at the mitochondrial outer membrane.