InterPro domain: IPR003557

Within mitochondria and bacteria, a family of related proteins is involved in the assembly of periplasmic c-type cytochromes: these include CycK [ 1 ], CcmF [ 2 , 3 ], NrfE [ 4 ] and CcbS [ 5 ]. These proteins may play a role in guidance of apocytochromes and haem groups for their covalent linkage by the cytochrome-c-haem lyase. Members of the family are probably integral membrane proteins, with up to 16 predicted transmembrane (TM) helices.

The gene products of the hel and ccl loci have been shown to be required specifically for the biogenesis of c-type cytochromes in the Gram-negative photosynthetic bacterium Rhodobacter capsulatus [ 6 ]. Genetic and molecular analyses show that the hel locus contains at least 4 genes, helA, helB, helC and orf52. HelA is similar to the ABC transporters and helA, helB, and helC are proposed to encode an export complex [ 7 ]. It is believed that the hel-encoded proteins are required for the export of haem to the periplasm, where it is subsequently ligated to the c-type apocytochromes [ 7 ]. However, while CcmB and CcmC have the potential to interact with CcmA, the 3 gene products probably associating to form a complex with (CcmA)2-CcmB-CcmC stoichiometry, the substrate for the putative CcmABC-transporter is probably neither haem nor c-type apocytochromes [ 7 ]. Hydropathy analysis suggests the presence of 6 TM domains.

1. Characterization of the cycHJKL genes involved in cytochrome c biogenesis and symbiotic nitrogen fixation in Rhizobium leguminosarum. J. Bacteriol. 177, 4927-34
2. Escherichia coli genes required for cytochrome c maturation. J. Bacteriol. 177, 4321-6
3. The Paracoccus denitrificans ccmA, B and C genes: cloning and sequencing, and analysis of the potential of their products to form a haem or apo- c-type cytochrome transporter. Microbiology (Reading, Engl.) 143 ( Pt 2), 563-76
4. A seven-gene operon essential for formate-dependent nitrite reduction to ammonia by enteric bacteria. Mol. Microbiol. 12, 153-63
5. A plant mitochondrial gene encodes a protein involved in cytochrome c biogenesis. Mol. Gen. Genet. 239, 49-57
6. Bacterial cytochromes c biogenesis. Genes Dev. 6, 268-83