Disease relevance of ECs3086

• Here, an Escherichia coli hemoprotein, CcmE, was shown to bind heme in the bacterial periplasm by way of a single covalent bond to a histidine [1].
• The cytochrome c maturation protein CcmE is an essential membrane-anchored heme chaperone involved in the post-translational covalent attachment of heme to c-type cytochromes in Gram-negative bacteria such as Escherichia coli [2].
• By expressing CcmE (CycJ) from Bradyrhizobium japonicum in E. coli we demonstrated that heme is bound covalently to this protein at a strictly conserved histidine residue [3].
• The CxxxY-type of CcmE is, surprisingly, also found in some bacterial genomes (including Desulfovibrio species) [4].
• The most notable adaptation relative to the well-studied apparatus from proteobacteria and plants is a novel form of the heme chaperone CcmE, lacking the highly conserved histidine that covalently binds heme and is essential for function in Escherichia coli [4].

High impact information on ECs3086

• The CcmE protein of the c-type cytochrome biogenesis system: unusual in vitro heme incorporation into apo-CcmE and transfer from holo-CcmE to apocytochrome [5].
• Previous in vitro studies have shown that CcmE can bind heme both covalently (via a histidine residue) and non-covalently [2].
• The heme chaperone CcmE is a novel protein that binds heme covalently via a histidine residue as part of its essential function in the process of cytochrome c biogenesis in many bacteria as well as plant mitochondria [6].
• Interaction of heme with variants of the heme chaperone CcmE carrying active site mutations and a cleavable N-terminal His tag [7].
• A strain carrying a CcmCE fusion had a similar phenotype, suggesting that CcmC is important not only for heme transfer to CcmE but also for its delivery to cytochrome c. Co-immunoprecipitation of CcmC with CcmF was not detectable although CcmE co-precipitated individually with CcmC and CcmF [8].

Biological context of ECs3086

• Significantly, CcmH is absent from all of the complete archaeal genomes we have studied, and also from most of the bacterial genomes that have CxxxY-type CcmE [4].
• We previously reported that a 17.5-kDa haem-binding polypeptide accumulates in Escherichia coli K-12 mutants defective in an essential gene for cytochrome c assembly, ccmF, and speculated that this polypeptide is either CcmE or CcmG [9].

Anatomical context of ECs3086

• The haem-containing polypeptide, which is associated with the cytoplasmic membrane, has now been identified by N-terminal sequencing to be CcmE [9].

Associations of ECs3086 with chemical compounds

• These results have implications for the mechanism of heme attachment to the histidine of CcmE [7].
• Mutation of the hydrophobic amino acids F37, F103, L127, and Y134 to alanine affected CcmE more than mutation of charged and polar residues [10].
• However, haem binding to CcmE is transient and is succeeded by a further transfer to apocytochrome c. Both haem binding to and release from CcmE involve integral membrane proteins, CcmC and CcmF respectively, which carry a conserved tryptophan-rich motif in a periplasmic domain [11].

Analytical, diagnostic and therapeutic context of ECs3086

• In this work we addressed the functions of the ccmABCD gene products with respect to holo-CcmE formation and the subsequent ligation of heme to apocytochrome c. In the absence of the ccmABCD genes, heme is not bound to CcmE [12].
• Co-immunoprecipitation experiments revealed that CcmF interacts directly with the heme donor CcmE and with CcmH but not with apocytochrome c. We propose that CcmFH forms a bacterial heme lyase complex for the transfer of heme from CcmE to apocytochrome c [13].
• Examination of a number of site-directed mutants of E. coli CcmE by resonance Raman spectroscopy has identified ligands of the heme iron and provided insight into the initial steps of heme binding by CcmE before it binds the heme covalently [2].

References