Heme transfer to the heme chaperone CcmE during cytochrome c maturation requires the CcmC protein, which may function independently of the ABC-transporter CcmAB.
Cytochrome c maturation in Escherichia coli requires the ccm operon, which encodes eight membrane proteins (CcmABCDEFGH). CcmE is a periplasmic heme chaperone that binds heme covalently and transfers it onto apocytochrome c in the presence of CcmF, CcmG, and CcmH. In this work we addressed the functions of the ccmABCD gene products with respect to holo-CcmE formation and the subsequent ligation of heme to apocytochrome c. In the absence of the ccmABCD genes, heme is not bound to CcmE. We report that CcmC is functionally uncoupled from the ABC transporter subunits CcmA and CcmB, because it is the only Ccm protein that is strictly required for heme transfer and attachment to CcmE. Site-directed mutagenesis of conserved histidines inactivates the CcmC protein, which is in agreement with the hypothesis that this protein interacts directly with heme. We also present evidence that questions the role of CcmAB as a heme exporter; yet, the transported substrate remains unknown. CcmD was found to be involved in stabilizing the heme chaperone CcmE in the membrane. We propose a heme-trafficking pathway as part of a substantially revised model for cytochrome c maturation in E. coli.
Schulz
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Mikrobiologisches Institut, Eidgenössische Technische Hochschule, Schmelzbergstrasse 7, CH 8092-Zürich, Switzerland.
Fabianek
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RA
Pellicioli
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EC
Hennecke
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Thöny-Meyer
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D018528:ATP-Binding Cassette Transporters / Q000235:genetics / Q000378:metabolism*; D019943:Amino Acid Substitution; D001425:Bacterial Outer Membrane Proteins / Q000378:metabolism*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002876:Chromosomes, Bacterial; D003574:Cytochrome c Group / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism*; D029968:Escherichia coli Proteins; D017353:Gene Deletion; D005838:Genotype; D006418:Heme / Q000378:metabolism*; D006420:Hemeproteins / Q000378:metabolism*; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism; D016297:Mutagenesis, Site-Directed; D011994:Recombinant Proteins / Q000378:metabolism
D018528:ATP-Binding Cassette Transporters; D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; C492058:CcmC protein, E coli; C116246:CcmC protein, bacteria; C115384:CcmE protein, E coli; D003574:Cytochrome c Group; D029968:Escherichia coli Proteins; D006420:Hemeproteins; D008565:Membrane Proteins; D011994:Recombinant Proteins; D006418:Heme