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 c haperone that binds heme covalently and transfers it onto apocytochrome c i n the presence of CcmF, CcmG, and CcmH. In this work we addressed the funct ions of the ccmABCD gene products with respect to holo-CcmE formation and t he subsequent ligation of heme to apocytochrome c. In the absence of the cc mABCD 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 t he only Ccm protein that is strictly required for heme transfer and attachm ent to CcmE. Site-directed mutagenesis of conserved histidines inactivates the CcmC protein, which is in agreement with the hypothesis that this prote in interacts directly with heme. We also present evidence that questions th e role of CcmAB as a heme exporter; yet, the transported substrate remains unknown. CcmD was found to be involved in stabilizing the heme chaperone Cc mE in the membrane. We propose a heme trafficking pathway as part of a subs tantially revised model for cytochrome c maturation in E. coli.