Molecular machineries and pathways of mitochondrial protein transport

Mitochondria contain about 1,000-1,500 different proteins, most of which are encoded by the nuclear genome and synthesized in the cytosol, although a handful are specified by the mitochondrial DNA and translated within mitochondria. The coordinated transport of nucleus-encoded proteins into mitochondria, followed by their proper folding, assembly and/or integration into mitochondrial membranes, is central to mitochondrial biogenesis. In this Review, we describe the pathways and machineries for protein transport across and insertion into the inner and outer mitochondrial membranes, as well as the targeting and sorting signals, and energy requirements for these processes. These machineries include the TOM and SAM complexes in the outer membrane and the TIM complexes in the inner membrane, and some components in the intermembrane space. We emphasize recent developments in our understanding of the protein structures of the transport machineries and discuss mechanisms for the shift of protein localization and correction of mislocalization.