FIG. 13.

Cytochrome c-type biogenesis through the CcmA to CcmH and CydDC pathways. CcmA and CcmB (CcmA₂B₂) may form an ABC transporter for the transport of an as yet undiscovered molecule, and CcmC was proposed to export heme (represented as a cross with a dot) into the periplasm. Alternatively, a CcmA₂BC complex may be involved in the export of heme. After transport, heme is transferred to the heme chaperone CcmE. CcmE is proposed to shuttle between CcmC and CcmF, and the protein CcmD has been shown to facilitate the interaction between CcmC and CcmE. Heme is transferred from CcmE to CcmF, which, in association with CcmG and CcmH, forms a heme-lyase complex. CcmG and CcmH maintain the apocytochrome (apo-cytc; green oval) in a reduced state. CcmG interacts with a periplasmic domain of the DsbD protein, which transfers electrons from the cytoplasm to the periplasm. Export of reductants such as cysteine or glutathione by the CydDC ABC transporter would help to maintain periplasmic redox homeostasis. This figure is based on data summarized in references 75 and 480.