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Most mitochondrial proteins are synthesized as precursor proteins on cytosofic polysomes and are subsequently imported into mitochondria. Many precursors carry amino-terminal presequences which contain information for their targeting to mitochondria. In several cases, targeting and sorting information is also contained in non-amino-terminal portions of the precursor protein. Nucleoside triphosphates are required to keep precursors in an import-competent (unfolded) conformation. The precursors bind to specific receptor proteins on the mitochondrial surface and interact with a general insertion protein (GIP) in thc outer membrane. The initial interaction of the precursor with the inner membrane requires the mitochondrial membrane potential (A6) and occurs at contact sites between outer and inner membranes. Completion of translocation into the inner membrane or matrix is independent of Ark. The presequences …