Table 1.
Functions of the proteins (and variants) involved in cytochrome c biogenesis System I. All the Ccm proteins and DsbD are essential for cytochrome c maturation in E. coli. The involvement of DsbA has not been resolved. Note that, in E. coli and some other organisms, the CcmH protein contains two domains that are found as two separate proteins in many other organisms. Thus, the N-terminus of E. coli CcmH is known as CcmH elsewhere but the C-terminus of E. coli CcmH (C-CcmH) is known as CcmI when it occurs elsewhere. Although loss of the N-terminal region of E. coli CcmH results in loss of c-type cytochromes, the C-terminal domain is dispensable [26,28]. There are contradicting reports of the function and topology of CcmD [34,35]. The structures of DsbA and the soluble domains of CcmE, CcmG, N-CcmH and DsbD have been solved. The structures of the other proteins are unknown. –, no known variants.
| Protein | Functions/features | Known variants |
|---|---|---|
| CcmA | Cytoplasmic ATP hydrolysis subunit of ABC protein that includes CcmB | – |
| CcmB | Membrane subunit of ABC protein involved in heme-handling by CcmE via an unknown mechanism | – |
| CcmC | Transmembrane protein providing heme to CcmE | – |
| CcmD | Small transmembrane protein that facilitates the interaction of other Ccm proteins | – |
| CcmE | Heme chaperone that binds heme covalently (to His in E. coli) | Covalent binding to Cys in System I* |
| CcmF | Large heme-containing transmembrane protein proposed to deliver reductant to heme on CcmE and to function in heme transfer from CcmE to apocytochrome | NfrE (required for biogenesis of NrfA nitrite reductase) |
| CcmG | Periplasmic thiol-oxidoreductase considered to be involved in the reduction of the cysteines in the -CXXCH- motif of apocytochromes | Absent, or equivalent protein not detected, in plants and System I* |
| N-CcmH | Periplasmic thiol-oxidoreductase considered to be involved in the reduction of the cysteines in the -CXXCH- motif of apocytochromes | NrfF (for biogenesis of NrfA nitrite reductase). Absent from System I* in some cases |
| C-CcmH/CcmI | TPR-motif-containing protein considered to facilitate interaction with apocytochrome c | NrfG (for biogenesis of NrfA nitrite reductase) |
| DsbD | Transmembrane protein with two soluble periplasmic domains that transfers reductant from the cytoplasm to the periplasm | CcdA (a transmembrane reductant-transferring protein) found in some organisms |
| DsbA | Periplasmic thiol-oxidase functioning in disulfide bond formation | – |