Import of cytochrome c heme lyase into mitochondria: a novel pathway into the intermembrane space.

Abstract

Cytochrome c heme lyase (CCHL) catalyses the covalent attachment of the heme group to apocytochrome c during its import into mitochondria. The enzyme is membrane‐associated and is located within the intermembrane space. The precursor of CCHL synthesized in vitro was efficiently translocated into isolated mitochondria from Neurospora crassa. The imported CCHL, like the native protein, was correctly localized to the intermembrane space, where it was membrane‐bound. As with the majority of mitochondrial precursor proteins, CCHL uses the MOM19‐GIP receptor complex in the outer membrane for import. In contrast to proteins taking the general import route, CCHL was imported independently of both ATP‐hydrolysis and an electrochemical potential as external energy sources. CCHL which lacks a cleavable signal sequence apparently does not traverse the inner membrane to reach the intermembrane space; rather, it translocates through the outer membrane only. Thus, CCHL represents an example of a novel, ‘non‐conservative’ import pathway into the intermembrane space, thereby also showing that the import apparatus in the outer membrane acts separately from the import machinery in the inner membrane.

Import of cytochrome b2 to the mitochondrial intermembrane space: The tightly folded heme‐binding domain makes import dependent upon matrix ATP

“…Some precursors will remain unfolded and aggregation prone as long as they are in the cytosol and thus will be associated with chaperones until translocation occurs (Chirico et al, 1988;Deshaies et al, 1988b;Murakami et al, 1988;Zimmermann et al, 1988;Waegemann et ai., 1990). Other precursors may be released from the chaperones in a partially folded form that is both resistant to aggregation and competent for translocation (Pilon et al, 1992;Lill et al, 1992;Miller & Cumsky, 1993). Still other precursors will be capable of partial or complete folding in the cytosol, but folding will be slow because association with the chaperones is relatively long-lived.…”

Mitochondrial protein import: Mechanisms, components and energetics

“…Transport of CCHL occurs directly across the outer membrane and requires the receptor-GIP complex but is not dependent on a membrane potential. At the same time, extramitochondrial ATP does not appear to be necessary, implying that the CCHL precursor does not bind to cytosolic heat-shock proteins [48]. Possibly, the driving force for import is the folding/assembly reaction which occurs in the intermembrane space.…”

Mitochondrial protein import: Specific recognition and membrane translocation of preproteins

“…Evidence for an independently acting translocation machinery in the outer membrane came from import studies with the cytochrome c heme lyase (CCHL), a peripheral protein of the inner membrane (Lill et al, 1992b) which catalyzes the attachment of the heine group to apocytochrome c in the intermembrane space or at the trans side of the outer membrane (Nicholson, Hergersberg & Neupert, 1988;C. Hergersberg and W. Neupert, unpublished).…”