New developments in mitochondrial assembly.

The mitochondrion has developed an elaborate translocation system for the import of nuclear-coded proteins and the export of proteins coded on the mitochondrial genome. Precursor proteins contain targeting and sorting information to reach the mitochondrion, whereas the translocons recognize the information and direct the precursor to the correct compartment. The outer membrane contains the TOM (translocase of the outer membrane) complex for translocation and the SAM (sorting and assembly machinery) complex for assembly of outer membrane proteins with complex topologies. At the inner membrane, the TIM23 (translocase of the inner membrane) mediates the import of mitochondrial proteins with a typical N-terminal targeting sequence, and the TIM22 complex mediates the import of polytopic inner membrane proteins. Based on its prokaryotic origin, the inner membrane also contains several components that mediate the export and assembly of proteins from within the matrix. Together the translocation and assembly complexes coordinate assembly of the mitochondrion.