Inner membrane protein import complex subunit Tim54
Mitochondrial function depends on the import of hundreds of different proteins synthesized in the cytosol. Protein import is a multi-step pathway which includes the binding of precursor proteins to surface receptors, translocation of the precursor across one or both mitochondrial membranes, and folding and assembly of the imported protein inside the mitochondrion. Most precursor proteins carry amino-terminal targeting signals, called pre-sequences, and are imported into mitochondria via import complexes located in both the outer and the inner membrane (IM). The IM complex, TIM, is made up of at least two proteins which mediate translocation of proteins into the matrix by removing their signal peptide and another pair of proteins, Tim54 and Tim22, that insert the polytopic proteins, that carry internal targetting information, into the inner membrane.