Comparing the substrate specificities of cytochrome c biogenesis Systems I and II

Abstract

c-Type cytochromes require specific post-translational protein systems, which vary in different organisms, for the characteristic covalent attachment of heme to the cytochrome polypeptide. Cytochrome c biogenesis System II, found in chloroplasts and many bacteria, comprises four subunits, two of which (ResB and ResC) are the minimal functional unit. The ycf5 gene from Helicobacter pylori encodes a fusion of ResB and ResC. Heterologous expression of ResBC in Escherichia coli lacking its own biogenesis machinery allowed us to investigate the substrate specificity of System II. ResBC is able to attach heme to monoheme c-type cytochromes c 550 from Paracoccus denitrificans and c 552 from Hydrogenobacter thermophilus, both normally matured by System I. The production of holocytochrome is enhanced by the addition of exogenous reductant. Single-cysteine variants of these cytochromes were not efficiently matured by System II, but System I was able to produce detectable amounts of AXXCH variants; this adds to evidence that there is no obligate requirement for a disulfide-bonded intermediate for the latter c-type cytochrome biogenesis system. In addition, System II was able to mature an AXXAH-containing variant into a b-type cytochrome, with implications for both heme supply to the periplasm and substrate recognition by System II

Cytochrome c biogenesis System I: An intricate process catalyzed by a maturase supercomplex?

“…CcdA and CcsX are required for thioreduction of apocyt c thiol groups, whereas CcsAB complex transports heme b to the p side of the membrane, recognizes the apocyt c substrates and forms the thioether bonds between the apocyt c and heme b . Substrate specificity of Ccs is not yet fully established, but it may be as broad as the Ccm-System I [32, 33]. …”

Molecular Basis Behind Inability of Mitochondrial Holocytochrome c Synthase to Mature Bacterial Cytochromes

“…In prokaryotes, many c-type cytochromes exist in addition to monoheme proteins (17)(18)(19)(20). These are all matured by Systems I and II, consistent with the proposal that only the CXXCH motif is recognized (21)(22)(23)(24). By contrast, the eukaryotic HCCS appears to require a more extended recognition domain than just the CXXCH motif for cytochrome maturation, specifically sequence features or residues located in the cyt c N terminus (see Fig.…”

c-Type Cytochrome Biogenesis Can Occur via a Natural Ccm System Lacking CcmH, CcmG, and the Heme-binding Histidine of CcmE

“…This also contrasts with the situation in E. coli where CcmG has been shown to be an essential member of the endogenous Ccm system (21). However, and similarly, it has been shown that expression of c-type cytochrome biogenesis System II in a ⌬ccm E. coli strain leads to cytochrome c production without CcmG (or ResA, the analogous protein from System II) (32,44). Normally, CcmG transfers reductant from DsbD to the rest of cytochrome c assembly apparatus and/or the apocytochrome.…”