Translocation to the periplasm and signal sequence cleavage of preapocytochrome c depend on sec and lep, but not on the ccm gene products.
Post-translational maturation of soluble cytochrome c includes translocation of the precursor polypeptide and heme through the cytoplasmic membrane, proteolytic cleavage of the signal sequence, and covalent attachment of heme. Specific genes for cytochrome c maturation (ccmABCDEFGH in Escherichia coli) are required for holocytochrome c formation, among them genes encoding an ABC transporter (ccmABC). We investigated the requirements of apocytochrome translocation to the periplasm and characterized specific intermediates of the cytochrome c maturation pathway. Apocytochrome precursor was present in the membrane fraction. Translocation of the polypeptide was independent of ccm gene products, but dependent on a functional secretion machinery, as shown by accumulation of preapocytochrome c in the membranes of secA and secY mutants. After translocation, cleavage of the signal sequence allowed the release of apocytochrome into the periplasm, where heme was bound in a ccm-dependent manner. By contrast, non-cleaved holocytochrome c containing covalently bound heme accumulated in the membranes of a lepB mutant, which indicated that signal sequence cleavage and heme attachment are independent steps in the cytochrome c maturation pathway.
Thöny-Meyer
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L
Mikrobiologisches Institut, Eidgenössische Technische Hochschule, ETH-Zentrum, Zürich, Switzerland. lthoeny@micro.biol.ethz.ch
Künzler
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P
D018528:ATP-Binding Cassette Transporters / Q000235:genetics; D001059:Apoproteins / Q000378:metabolism*; D003574:Cytochrome c Group / Q000235:genetics* / Q000378:metabolism*; D045304:Cytochromes c; D003593:Cytoplasm / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism*; D029721:Drosophila Proteins; D004926:Escherichia coli; D006418:Heme / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D008565:Membrane Proteins; D011499:Protein Processing, Post-Translational; D021382:Protein Sorting Signals / Q000378:metabolism; D012697:Serine Endopeptidases / Q000378:metabolism*; D014157:Transcription Factors / Q000378:metabolism*; D014178:Translocation, Genetic
D018528:ATP-Binding Cassette Transporters; D001059:Apoproteins; D003574:Cytochrome c Group; D004268:DNA-Binding Proteins; D029721:Drosophila Proteins; D008565:Membrane Proteins; D021382:Protein Sorting Signals; D014157:Transcription Factors; C089197:sc protein, Drosophila; D006418:Heme; D045304:Cytochromes c; D012697:Serine Endopeptidases; C026515:type I signal peptidase