THE PROTEIN IMPORT APPARATUS OF THE MITOCHONDRIAL OUTER-MEMBRANE

The majority of proteins within mitochondria are synthesized on cytoso lic ribosomes and imported into the organelles. Protein complexes in t he mitochondrial outer membrane harbour both the receptors that recogn ize these preproteins, and a translocation pore. These ''receptor comp lexes'' are the entry points for most preproteins, which are subsequen tly targeted to their final submitochondrial locations. The outer memb rane complexes cooperate with the import machinery of the inner membra ne to target preproteins to the inner membrane itself, the matrix, or, in some cases, to the intermembrane space. In isolated outer membrane s, these complexes are capable of accurately importing preproteins des tined for the outer membrane. Our current understanding of the composi tion, function, and biogenesis of these outer membrane receptor comple xes is the focus of this article.