Insertion into the mitochondrial inner membrane of a polytopic protein, the nuclear-encoded Oxa1p

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Abstract
Oxa1p, a nuclear‐encoded protein of the mitochondrial inner membrane with five predicted transmembrane (TM) segments is synthesized as a precursor (pOxa1p) with an N‐terminal presequence. It becomes imported in a process requiring the membrane potential, matrix ATP, mt‐Hsp70 and the mitochondrial processing peptidase (MPP). After processing, the negatively charged N‐terminus of Oxa1p (∼90 amino acid residues) is translocated back across the inner membrane into the intermembrane space and thereby attains its native Nout–Cin orientation. This export event is dependent on the membrane potential. Chimeric preproteins containing N‐terminal stretches of increasing lengths of Oxa1p fused on mouse dehydrofolate reductase (DHFR) were imported into isolated mitochondria. In each case, their DHFR moieties crossed the inner membrane into the matrix. Thus Oxa1p apparently does not contain a stop transfer signal. Instead the TM segments are inserted into the membrane from the matrix side in a pairwise fashion. The sorting pathway of pOxa1p is suggested to combine the pathways of general import into the matrix with a bacterial‐type export process. We postulate that at least two different sorting pathways exist in mitochondria for polytopic inner membrane proteins, the evolutionarily novel pathway for members of the ADP/ATP carrier family and a conserved Oxa1p‐type pathway.