3.A.1.107.3
CcmABCD exporter; CcmD (69aas, 1TMS) is required for the release of CcmE (which binds heme in the periplasm) from CcmABC. CcmC (9.B.14.2.3) is required for the transfer of heme to CcmE in the periplasm (Richard-Fogal et al., 2008) In the presence of heme, CcmC and CcmE form a stable complex (Richard-Fogal & Kranz, 2010) as do CcmE and CcmF (San Francisco and Kranz 2014). The cytochrome c maturation system I, consisting of eight membrane/periplasmic proteins (CcmABCDEFGH), results in the attachment of heme to cysteine residues of cytochrome c proteins. Since all c-type cytochromes are periplasmic, heme is first transported to a periplasmic heme chaperone, CcmE. A large membrane complex, CcmABCD has been proposed to carry out this transport and linkage to CcmE. Li et al. 2022 described high resolution cryo-EM structures of CcmABCD in an unbound form, in complex with inhibitor AMP-PNP, and in complex with ATP and heme. The ATP-binding site in CcmA and the heme-binding site in CcmC were identified. They proposed a model of heme trafficking, heme transfer to CcmE, and ATP-dependent release of holoCcmE from CcmABCD. CcmABCD represents an ABC transporter complex using the energy of ATP hydrolysis for the transfer of heme from one binding partner (CcmC) to another (CcmE) (Li et al. 2022).