Structural Basis and Kinetics of DsbD-Dependent Cytochrome c Maturation

Scs system links copper and redox homeostasis in bacterial pathogens

The bacterial envelope is an essential compartment involved in metabolism and metabolites transport, virulence, and stress defense. Its roles become more evident when homeostasis is challenged during host-pathogen interactions. In particular, the presence of free radical groups and excess copper in the periplasm causes noxious reactions, such as sulfhydryl group oxidation leading to enzymatic inactivation and protein denaturation. In response to this, canonical and accessory oxidoreductase systems are induced, performing quality control of thiol groups, and therefore contributing to restoring homeostasis and preserving survival under these conditions. Here, we examine recent advances in the characterization of the Dsb-like, Salmonella-specific Scs system. This system includes the ScsC/ScsB pair of Cu+-binding proteins with thiol-oxidoreductase activity, an alternative ScsB-partner, the membrane-linked ScsD, and a likely associated protein, ScsA, with a role in peroxide resistance. We discuss the acquisition of the scsABCD locus and its integration into a global regulatory pathway directing envelope response to Cu stress during the evolution of pathogens that also harbor the canonical Dsb systems. The evidence suggests that the canonical Dsb systems cannot satisfy the extra demands that the host-pathogen interface imposes to preserve functional thiol groups. This resulted in the acquisition of the Scs system by Salmonella. We propose that the ScsABCD complex evolved to connect Cu and redox stress responses in this pathogen as well as in other bacterial pathogens.

NapB Restores cytochrome c biosynthesis in bacterial dsbD-deficient mutants

Cytochromes c (cyts c), essential for respiration and photosynthesis in eukaryotes, confer bacteria respiratory versatility for survival and growth in natural environments. In bacteria having a cyt c maturation (CCM) system, DsbD is required to mediate electron transport from the cytoplasm to CcmG of the Ccm apparatus. Here with cyt c-rich Shewanella oneidensis as the research model, we identify NapB, a cyt c per se, that suppresses the CCM defect of a dsbD mutant during anaerobiosis, when NapB is produced at elevated levels, a result of activation by cAMP-Crp. Data are then presented to suggest that NapB reduces CcmG, leading to the suppression. We further show that NapB proteins capable of rescuing CCM in the dsbD mutant form a small distinct clade. The study sheds light on multifunctionality of cyts c, and more importantly, unravels a self-salvation strategy through which bacteria have evolved to better adjust to the natural world.

The DsbD protein is normally required for cytochrome c maturation (Ccm) in bacteria. With cytochrome c-rich Shewanella oneidensis as the research model, NapB, the small subunit of the nitrate reductase which is a cytochrome c per se, was found to suppress the Ccm defect resulting from DsbD loss under anaerobic conditions.

Functional and structural characterization of PII-like protein CutA does not support involvement in heavy metal tolerance and hints at a small-molecule carrying/signaling role

The PII-like protein CutA is annotated as being involved in Cu²⁺ tolerance, based on analysis of Escherichia coli mutants. However, the precise cellular function of CutA remains unclear. Our bioinformatic analysis reveals that CutA proteins are universally distributed across all domains of life. Based on sequence-based clustering, we chose representative cyanobacterial CutA proteins for physiological, biochemical, and structural characterization and examined their involvement in heavy metal tolerance, by generating CutA mutants in filamentous Nostoc sp. and in unicellular Synechococcus elongatus. However, we were unable to find any involvement of cyanobacterial CutA in metal tolerance under various conditions. This prompted us to re-examine experimentally the role of CutA in protecting E. coli from Cu²⁺ . Since we found no effect on copper tolerance, we conclude that CutA plays a different role that is not involved in metal protection. We resolved high-resolution CutA structures from Nostoc and S. elongatus. Similarly to their counterpart from E. coli and to canonical PII proteins, cyanobacterial CutA proteins are trimeric in solution and in crystal structure; however, no binding affinity for small signaling molecules or for Cu²⁺ could be detected. The clefts between the CutA subunits, corresponding to the binding pockets of PII proteins, are formed by conserved aromatic and charged residues, suggesting a conserved binding/signaling function for CutA. In fact, we find binding of organic Bis-Tris/MES molecules in CutA crystal structures, revealing a strong tendency of these pockets to accommodate cargo. This highlights the need to search for the potential physiological ligands and for their signaling functions upon binding to CutA. DATABASES: Structural data are available in Protein Data Bank (PDB) under the accession numbers 6GDU, 6GDV, 6GDW, 6GDX, 6T76, and 6T7E.

Local frustration determines loop opening during the catalytic cycle of an oxidoreductase

Local structural frustration, the existence of mutually exclusive competing interactions, may explain why some proteins are dynamic while others are rigid. Frustration is thought to underpin biomolecular recognition and the flexibility of protein-binding sites. Here, we show how a small chemical modification, the oxidation of two cysteine thiols to a disulfide bond, during the catalytic cycle of the N-terminal domain of the key bacterial oxidoreductase DsbD (nDsbD), introduces frustration ultimately influencing protein function. In oxidized nDsbD, local frustration disrupts the packing of the protective cap-loop region against the active site allowing loop opening. By contrast, in reduced nDsbD the cap loop is rigid, always protecting the active-site thiols from the oxidizing environment of the periplasm. Our results point toward an intricate coupling between the dynamics of the active-site cysteines and of the cap loop which modulates the association reactions of nDsbD with its partners resulting in optimized protein function.

Disulfide Bond Formation in the Periplasm of Escherichia coli

The formation of disulfide bonds is critical to the folding of many extracytoplasmic proteins in all domains of life. With the discovery in the early 1990s that disulfide bond formation is catalyzed by enzymes, the field of oxidative folding of proteins was born. Escherichia coli played a central role as a model organism for the elucidation of the disulfide bond-forming machinery. Since then, many of the enzymatic players and their mechanisms of forming, breaking, and shuffling disulfide bonds have become understood in greater detail. This article summarizes the discoveries of the past 3 decades, focusing on disulfide bond formation in the periplasm of the model prokaryotic host E. coli.

Molecular Mechanisms of the Methionine Sulfoxide Reductase System from Neisseria meningitidis

Neisseria meningitidis, an obligate pathogenic bacterium in humans, has acquired different defense mechanisms to detect and fight the oxidative stress generated by the host’s defense during infection. A notable example of such a mechanism is the PilB reducing system, which repairs oxidatively-damaged methionine residues. This review will focus on the catalytic mechanism of the two methionine sulfoxide reductase (MSR) domains of PilB, which represent model enzymes for catalysis of the reduction of a sulfoxide function by thiols through sulfenic acid chemistry. The mechanism of recycling of these MSR domains by various “Trx-like” disulfide oxidoreductases will also be discussed.

Structural and biochemical insights into the disulfide reductase mechanism of DsbD, an essential enzyme for neisserial pathogens

The worldwide incidence of neisserial infections, particularly gonococcal infections, is increasingly associated with antibiotic-resistant strains. In particular, extensively drug-resistant Neisseria gonorrhoeae strains that are resistant to third-generation cephalosporins are a major public health concern. There is a pressing clinical need to identify new targets for the development of antibiotics effective against Neisseria-specific processes. In this study, we report that the bacterial disulfide reductase DsbD is highly prevalent and conserved among Neisseria spp. and that this enzyme is essential for survival of N. gonorrhoeae DsbD is a membrane-bound protein that consists of two periplasmic domains, n-DsbD and c-DsbD, which flank the transmembrane domain t-DsbD. In this work, we show that the two functionally essential periplasmic domains of Neisseria DsbD catalyze electron transfer reactions through unidirectional interdomain interactions, from reduced c-DsbD to oxidized n-DsbD, and that this process is not dictated by their redox potentials. Structural characterization of the Neisseria n- and c-DsbD domains in both redox states provides evidence that steric hindrance reduces interactions between the two periplasmic domains when n-DsbD is reduced, thereby preventing a futile redox cycle. Finally, we propose a conserved mechanism of electron transfer for DsbD and define the residues involved in domain-domain recognition. Inhibitors of the interaction of the two DsbD domains have the potential to be developed as anti-neisserial agents.

Impact of selected amino acids of HP0377 (Helicobacter pylori thiol oxidoreductase) on its functioning as a CcmG (cytochrome c maturation) protein and Dsb (disulfide bond) isomerase

Helicobacter pylori HP0377 is a thiol oxidoreductase, a member of the CcmG family involved in cytochrome biogenesis, as previously shown by in vitro experiments. In this report, we document that HP0377 also acts in vivo in the cytochrome assembly process in Bacillus subtilis, where it complements the lack of ResA. However, unlike other characterized proteins in this family, HP0377 is a dithiol reductase and isomerase. We elucidated how the amino acid composition of its active site modulates its functionality. We demonstrated that cis-proline (P156) is involved in its interaction with the redox partner (CcdA), as a P156T HP0377 variant is inactive in vivo and is present in the oxidized form in B. subtilis. Furthermore, we showed that engineering the HP0377 active motif by changing CSYC motif into CSYS or SSYC, clearly diminishes two activities (reduction and isomerization) of the protein. Whereas HP0377CSYA is inactive in reduction as well as in isomerization, HP0377CSYS retains reductive activity. Also, replacement of F95 by Q decreases its ability to regenerate scRNase and does not influence the reductive activity of HP0377CSYS towards apocytochrome c. HP0377 is also distinguished from other CcmGs as it forms a 2:1 complex with apocytochrome c. Phylogenetic analyses showed that, although HP0377 is capable of complementing ResA in Bacillus subtilis, its thioredoxin domain has a different origin, presumably common to DsbC.

Production, biophysical characterization and initial crystallization studies of the N- and C-terminal domains of DsbD, an essential enzyme in Neisseria meningitidis

The membrane protein DsbD is a reductase that acts as an electron hub, translocating reducing equivalents from cytoplasmic thioredoxin to a number of periplasmic substrates involved in oxidative protein folding, cytochrome c maturation and oxidative stress defence. DsbD is a multi-domain protein consisting of a transmembrane domain (t-DsbD) flanked by two periplasmic domains (n-DsbD and c-DsbD). Previous studies have shown that DsbD is required for the survival of the obligate human pathogen Neisseria meningitidis. To help understand the structural and functional aspects of N. meningitidis DsbD, the two periplasmic domains which are required for electron transfer are being studied. Here, the expression, purification and biophysical properties of n-NmDsbD and c-NmDsbD are described. The crystallization and crystallographic analysis of n-NmDsbD and c-NmDsbD are also described in both redox states, which differ only in the presence or absence of a disulfide bond but which crystallized in completely different conditions. Crystals of n-NmDsbDOx, n-NmDsbDRed, c-NmDsbDOx and c-NmDsbDRed diffracted to 2.3, 1.6, 2.3 and 1.7 Å resolution and belonged to space groups P213, P321, P41 and P1211, respectively.

Reexamining the Function of Glutathione in Oxidative Protein Folding and Secretion

SIGNIFICANCE

Disturbance of glutathione (GSH) metabolism is a hallmark of numerous diseases, yet GSH functions are poorly understood. One key to this question is to consider its functional compartmentation. GSH is present in the endoplasmic reticulum (ER), where it competes with substrates for oxidation by the oxidative folding machinery, composed in eukaryotes of the thiol oxidase Ero1 and proteins from the disulfide isomerase family (protein disulfide isomerase). Yet, whether GSH is required for proper ER oxidative protein folding is a highly debated question. Recent Advances: Oxidative protein folding has been thoroughly dissected over the past decades, and its actors and their mode of action elucidated. Genetically encoded GSH probes have recently provided an access to subcellular redox metabolism, including the ER.

CRITICAL ISSUES

Of the few often-contradictory models of the role of GSH in the ER, the most popular suggest it serves as reducing power. Yet, as a reductant, GSH also activates Ero1, which questions how GSH can nevertheless support protein reduction. Hence, whether GSH operates in the ER as a reductant, an oxidant, or just as a "blank" compound mirroring ER/periplasm redox activity is a highly debated question, which is further stimulated by the puzzling occurrence of GSH in the Escherichia coli periplasmic "secretory" compartment, aside from the Dsb thiol-reducing and oxidase pathways.

FUTURE DIRECTIONS

Addressing the mechanisms controlling GSH traffic in and out of the ER/periplasm and its recycling will help address GSH function in secretion. In addition, as thioredoxin reductase was recently implicated in ER oxidative protein folding, the relative contribution of each of these two reducing pathways should now be addressed. Antioxid. Redox Signal. 27, 1178-1199.

An overview on molecular chaperones enhancing solubility of expressed recombinant proteins with correct folding

The majority of research topics declared that most of the recombinant proteins have been expressed by Escherichia coli in basic investigations. But the majority of high expressed proteins formed as inactive recombinant proteins that are called inclusion body. To overcome this problem, several methods have been used including suitable promoter, environmental factors, ladder tag to secretion of proteins into the periplasm, gene protein optimization, chemical chaperones and molecular chaperones sets. Co-expression of the interest protein with molecular chaperones is one of the common methods The chaperones are a group of proteins, which are involved in making correct folding of recombinant proteins. Chaperones are divided two groups including; cytoplasmic and periplasmic chaperones. Moreover, periplasmic chaperones and proteases can be manipulated to increase the yields of secreted proteins. In this article, we attempted to review cytoplasmic chaperones such as Hsp families and periplasmic chaperones including; generic chaperones, specialized chaperones, PPIases, and proteins involved in disulfide bond formation.

The thioreduction component CcmG confers efficiency and the heme ligation component CcmH ensures stereo-specificity during cytochrome c maturation

In many Gram-negative bacteria, including Rhodobacter capsulatus, cytochrome c maturation (Ccm) is carried out by a membrane-integral machinery composed of nine proteins (CcmA to I). During this process, the periplasmic thiol-disulfide oxidoreductase DsbA is thought to catalyze the formation of a disulfide bond between the Cys residues at the apocytochrome c heme-binding site (CXXCH). Subsequently, a Ccm-specific thioreductive pathway involving CcmG and CcmH reduces this disulfide bond to allow covalent heme ligation. Currently, the sequence of thioredox reactions occurring between these components and apocytochrome c and the identity of their active Cys residues are unknown. In this work, we first investigated protein-protein interactions among the apocytochrome c, CcmG, and the heme-ligation components CcmF, CcmH, and CcmI. We found that they all interact with each other, forming a CcmFGHI-apocytochrome c complex. Using purified wild-type CcmG, CcmH, and apocytochrome c, as well as their respective Cys mutant variants, we determined the rates of thiol-disulfide exchange reactions between selected pairs of Cys residues from these proteins. We established that CcmG can efficiently reduce the disulfide bond of apocytochrome c and also resolve a mixed disulfide bond formed between apocytochrome c and CcmH. We further show that Cys-45 of CcmH and Cys-34 of apocytochrome c are most likely to form this mixed disulfide bond, which is consistent with the stereo-specificity of the heme-apocytochrome c ligation reaction. We conclude that CcmG confers efficiency, and CcmH ensures stereo-specificity during Ccm and present a comprehensive model for thioreduction reactions that lead to heme-apocytochrome c ligation.

Biochemical and functional characterization of a periplasmic disulfide oxidoreductase from Neisseria meningitidis essential for meningococcal viability

TlpAs (thioredoxin-like proteins) are bacterial thioredoxin-like periplasmic disulfide oxidoreductases generally involved in cytochrome c maturation (Ccm) process. They contain a characteristic CXXC active site motif involved in disulfide exchange reaction. In the human pathogenic Neisseria meningitidis species, no TlpA has been characterized so far. In the present study, using an in silico analysis, we identified a putative periplasmic TlpA, called TlpA2. Biochemical and kinetic characterizations of the soluble form of TlpA2, tTlpA2 (truncated TlpA2), were performed. A reduction potential of -0.230 V at pH 7 was calculated, suggesting that TlpA2 acts as a reductant in the oxidative environment of the periplasm. Using a second-order reactive probe, high pKapp (apparent pKa) values were determined for the two cysteines of the SCXXC motif. The tTlpA2 was shown to be efficiently reduced by the N-terminal domain of the DsbD, whereas tTlpA2 reduced a mimetic peptide of cytochrome c' with a catalytic efficiency similar to that observed with other disulfide oxidoreductase like ResA. Moreover, the corresponding gene tlpA2 was shown to be essential for the pathogen viability and able to partially complement a Bordetella pertussis CcsX mutant. Together, these data support an essential role of TlpA2 in the Ccm process in N. meningitidis.

Legionella pneumophila utilizes a single-player disulfide-bond oxidoreductase system to manage disulfide bond formation and isomerization

Legionella pneumophila uses a single homodimeric disulfide bond (DSB) oxidoreductase DsbA2 to catalyze extracytoplasmic protein folding and to correct DSB errors through protein-disulfide isomerase (PDI) activity. In Escherichia coli, these functions are separated to avoid futile cycling. In L. pneumophila, DsbA2 is maintained as a mixture of disulfides (S-S) and free thiols (SH), but when expressed in E. coli, only the SH form is observed. We provide evidence to suggest that structural differences in DsbB oxidases (LpDsbB1 and LpDsbB2) and DsbD reductases (LpDsbD1 and LpDsbD2) (compared with E. coli) permit bifunctional activities without creating a futile cycle. LpdsbB1 and LpdsbB2 partially complemented an EcdsbB mutant while neither LpdsbD1 nor LpdsbD2 complemented an EcdsbD mutant unless DsbA2 was also expressed. When the dsb genes of E. coli were replaced with those of L. pneumophila, motility was restored and DsbA2 was present as a mixture of redox forms. A dominant-negative approach to interfere with DsbA2 function in L. pneumophila determined that DSB oxidase activity was necessary for intracellular multiplication and assembly/function of the Dot/Icm Type IVb secretion system. Our studies show that a single-player system may escape the futile cycle trap by limiting transfer of reducing equivalents from LpDsbDs to DsbA2.

The Vibrio cholerae Cpx envelope stress response senses and mediates adaptation to low iron

The Cpx pathway, a two-component system that employs the sensor histidine kinase CpxA and the response regulator CpxR, regulates crucial envelope stress responses across bacterial species and affects antibiotic resistance. To characterize the CpxR regulon in Vibrio cholerae, the transcriptional profile of the pandemic V. cholerae El Tor C6706 strain was examined upon overexpression of cpxR. Our data show that the Cpx regulon of V. cholerae is enriched in genes encoding membrane-localized and transport proteins, including a large number of genes known or predicted to be iron regulated. Activation of the Cpx pathway further led to the expression of TolC, the major outer membrane pore, and of components of two RND efflux systems in V. cholerae. We show that iron chelation, toxic compounds, or deletion of specific RND efflux components leads to Cpx pathway activation. Furthermore, mutations that eliminate the Cpx response or members of its regulon result in growth phenotypes in the presence of these inducers that, together with Cpx pathway activation, are partially suppressed by iron. Cumulatively, our results suggest that a major function of the Cpx response in V. cholerae is to mediate adaptation to envelope perturbations caused by toxic compounds and the depletion of iron.

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

Cytochromes c are ubiquitous heme proteins that are found in most living organisms and are essential for various energy production pathways as well as other cellular processes. Their biosynthesis relies on a complex post-translational process, called cytochrome c biogenesis, responsible for the formation of stereo-specific thioether bonds between the vinyl groups of heme b (protoporphyrin IX-Fe) and the thiol groups of apocytochromes c heme-binding site (C1XXC2H) cysteine residues. In some organisms this process involves up to nine (CcmABCDEFGHI) membrane proteins working together to achieve heme ligation, designated the Cytochrome c maturation (Ccm)-System I. Here, we review recent findings related to the Ccm-System I found in bacteria, archaea and plant mitochondria, with an emphasis on protein interactions between the Ccm components and their substrates (apocytochrome c and heme). We discuss the possibility that the Ccm proteins may form a multi subunit supercomplex (dubbed "Ccm machine"), and based on the currently available data, we present an updated version of a mechanistic model for Ccm. This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.

An extended active-site motif controls the reactivity of the thioredoxin fold

Proteins belonging to the thioredoxin (Trx) superfamily are abundant in all organisms. They share the same structural features, arranged in a seemingly simple fold, but they perform a multitude of functions in oxidative protein folding and electron transfer pathways. We use the C-terminal domain of the unique transmembrane reductant conductor DsbD as a model for an in-depth analysis of the factors controlling the reactivity of the Trx fold. We employ NMR spectroscopy, x-ray crystallography, mutagenesis, in vivo functional experiments applied to DsbD, and a comparative sequence analysis of Trx-fold proteins to determine the effect of residues in the vicinity of the active site on the ionization of the key nucleophilic cysteine of the -CXXC- motif. We show that the function and reactivity of Trx-fold proteins depend critically on the electrostatic features imposed by an extended active-site motif.

Disulfide bond formation in the bacterial periplasm: major achievements and challenges ahead

SIGNIFICANCE

The discovery of the oxidoreductase disulfide bond protein A (DsbA) in 1991 opened the way to the unraveling of the pathways of disulfide bond formation in the periplasm of Escherichia coli and other Gram-negative bacteria. Correct oxidative protein folding in the E. coli envelope depends on both the DsbA/DsbB pathway, which catalyzes disulfide bond formation, and the DsbC/DsbD pathway, which catalyzes disulfide bond isomerization.

RECENT ADVANCES

Recent data have revealed an unsuspected link between the oxidative protein-folding pathways and the defense mechanisms against oxidative stress. Moreover, bacterial disulfide-bond-forming systems that differ from those at play in E. coli have been discovered.

CRITICAL ISSUES

In this review, we discuss fundamental questions that remain unsolved, such as what is the mechanism employed by DsbD to catalyze the transfer of reducing equivalents across the membrane and how do the oxidative protein-folding catalysts DsbA and DsbC cooperate with the periplasmic chaperones in the folding of secreted proteins.

FUTURE DIRECTIONS

Understanding the mechanism of DsbD will require solving the structure of the membranous domain of this protein. Another challenge of the coming years will be to put the knowledge of the disulfide formation machineries into the global cellular context to unravel the interplay between protein-folding catalysts and chaperones. Also, a thorough characterization of the disulfide bond formation machineries at work in pathogenic bacteria is necessary to design antimicrobial drugs targeting the folding pathway of virulence factors stabilized by disulfide bonds.

Thioredoxin-like proteins in F and other plasmid systems

Bacterial conjugation is the process by which a conjugative plasmid transfers from donor to recipient bacterium. During this process, single-stranded plasmid DNA is actively and specifically transported from the cytoplasm of the donor, through a large membrane-spanning assembly known as the pore complex, and into the cytoplasm of the recipient. In Gram negative bacteria, construction of the pore requires localization of a subset of structural and catalytically active proteins to the bacterial periplasm. Unlike the cytoplasm, the periplasm contains proteins that promote disulfide bond formation within or between cysteine-containing proteins. To ensure proper protein folding and assembly, bacteria employ periplasmic redox systems for thiol oxidation, disulfide bond/sulfenic acid reduction, and disulfide bond isomerization. Recent data suggest that plasmid-based proteins belonging to the disulfide bond formation family play an integral role in the conjugative process by serving as mediators in folding and/or assembly of pore complex proteins. Here we report the identification of 165 thioredoxin-like family members across 89 different plasmid systems. Using phylogenetic analysis, all but nine family members were categorized into thioredoxin-like subfamilies. In addition, we discuss the diversity, conservation, and putative roles of thioredoxin-like proteins in plasmid systems, which include homologs of DsbA, DsbB, DsbC, DsbD, DsbG, and CcmG from Escherichia coli, TlpA from Bradyrhizobium japonicum, Com1 from Coxiella burnetii, as well as TrbB and TraF from plasmid F, and the absolute conservation of a disulfide isomerase in plasmids containing homologs of the transfer proteins TraH, TraN, and TraU.

Many roles of the bacterial envelope reducing pathways

SIGNIFICANCE

The cell envelope of aerobic bacteria is an oxidizing environment in which most cysteine residues are involved in disulfide bonds. However, reducing redox pathways are also present in this cellular compartment where they provide electrons to a variety of cellular processes. The membrane protein DsbD plays a central role in these pathways by functioning as an electron hub that dispatches electrons received from the cytoplasmic thioredoxin system to periplasmic oxidoreductases.

RECENT ADVANCES

Recent data have revealed that DsbD provides reducing equivalents to a large array of periplasmic redox proteins. Those proteins use the reducing power received from DsbD to correct non-native disulfides, mature c-type cytochromes, protect cysteines on secreted proteins from irreversible oxidation, reduce methionine sulfoxides, and scavenge reactive oxygen species such as hydrogen peroxide.

CRITICAL ISSUES

Despite the prominent role played by DsbD, we have a poor understanding of how this protein transfers electrons across the inner membrane. Another critical issue will be to grasp the full physiological significance of the new reducing pathways that have been identified in the cell envelope such as the peroxide reduction pathway.

FUTURE DIRECTIONS

A detailed understanding of DsbD's mechanism will require solving the structure of this intriguing protein. Moreover, bioinformatic, biochemical, and genetic approaches need to be combined for a better comprehension of the broad spectrum of periplasmic reducing systems present in bacteria, which will likely lead to the discovery of novel pathways.

Functional analysis of paralogous thiol-disulfide oxidoreductases in Streptococcus gordonii

Disulfide bonds are important for the stability of many extracellular proteins, including bacterial virulence factors. Formation of these bonds is catalyzed by thiol-disulfide oxidoreductases (TDORs). Little is known about their formation in Gram-positive bacteria, particularly among facultative anaerobic Firmicutes, such as streptococci. To investigate disulfide bond formation in Streptococcus gordonii, we identified five putative TDORs from the sequenced genome. Each of the putative TDOR genes was insertionally inactivated with an erythromycin resistance cassette, and the mutants were analyzed for autolysis, extracellular DNA release, biofilm formation, bacteriocin production, and genetic competence. This analysis revealed a single TDOR, SdbA, which exhibited a pleiotropic mutant phenotype. Using an in silico analysis approach, we identified the major autolysin AtlS as a natural substrate of SdbA and showed that SdbA is critical to the formation of a disulfide bond that is required for autolytic activity. Analysis by BLAST search revealed homologs to SdbA in other Gram-positive species. This study provides the first in vivo evidence of an oxidoreductase, SdbA, that affects multiple phenotypes in a Gram-positive bacterium. SdbA shows low sequence homology to previously identified oxidoreductases, suggesting that it may belong to a different class of enzymes. Our results demonstrate that SdbA is required for disulfide bond formation in S. gordonii and indicate that this enzyme may represent a novel type of oxidoreductase in Gram-positive bacteria.

(1)H, (13)C and (15)N backbone and side-chain resonance assignments of reduced CcmG from Escherichia coli

CcmG is a periplasmic, membrane-anchored protein widely distributed in a variety of species. In Escherichia coli, the CcmG protein always acts as a weak reductant in the electron transport chain during cytochrome c maturation (Ccm). Here we report (1)H, (15)N and (13)C backbone and side-chain resonance assignments of the reduced CcmG protein (residues 19-185, renumbered as 1-167) from E. coli. This work lays the essential basis for the further structural and functional analysis of reduced CcmG.

Genomic island genes in a coastal marine Synechococcus strain confer enhanced tolerance to copper and oxidative stress

Highly variable regions called genomic islands are found in the genomes of marine picocyanobacteria, and have been predicted to be involved in niche adaptation and the ecological success of these microbes. These picocyanobacteria are typically highly sensitive to copper stress and thus, increased copper tolerance could confer a selective advantage under some conditions seen in the marine environment. Through targeted gene inactivation of genomic island genes that were known to be upregulated in response to copper stress in Synechococcus sp. strain CC9311, we found two genes (sync_1495 and sync_1217) conferred tolerance to both methyl viologen and copper stress in culture. The prevalence of one gene, sync_1495, was then investigated in natural samples, and had a predictable temporal variability in abundance at a coastal monitoring site with higher abundance in winter months. Together, this shows that genomic island genes can confer an adaptive advantage to specific stresses in marine Synechococcus, and may help structure their population diversity.

Protein Machineries Involved in the Attachment of Heme to Cytochrome c: Protein Structures and Molecular Mechanisms

Cytochromes c (Cyt c) are ubiquitous heme-containing proteins, mainly involved in electron transfer processes, whose structure and functions have been and still are intensely studied. Surprisingly, our understanding of the molecular mechanism whereby the heme group is covalently attached to the apoprotein (apoCyt) in the cell is still largely unknown. This posttranslational process, known as Cyt c biogenesis or Cyt c maturation, ensures the stereospecific formation of the thioether bonds between the heme vinyl groups and the cysteine thiols of the apoCyt heme binding motif. To accomplish this task, prokaryotic and eukaryotic cells have evolved distinctive protein machineries composed of different proteins. In this review, the structural and functional properties of the main maturation apparatuses found in gram-negative and gram-positive bacteria and in the mitochondria of eukaryotic cells will be presented, dissecting the Cyt c maturation process into three functional steps: (i) heme translocation and delivery, (ii) apoCyt thioreductive pathway, and (iii) apoCyt chaperoning and heme ligation. Moreover, current hypotheses and open questions about the molecular mechanisms of each of the three steps will be discussed, with special attention to System I, the maturation apparatus found in gram-negative bacteria.

1H, 13C and 15N resonance assignments for the oxidized and reduced states of the N-terminal domain of DsbD from Escherichia coli

Viability and pathogenicity of Gram-negative bacteria is linked to the cytochrome c maturation and the oxidative protein folding systems in the periplasm. The transmembrane reductant conductor DsbD is a unique protein which provides the necessary reducing power to both systems through thiol-disulfide exchange reactions in a complex network of protein-protein interactions. The N-terminal domain of DsbD (nDsbD) is the delivery point of the reducing power originating from cytoplasmic thioredoxin to a variety of periplasmic partners. Here we report (1)H, (13)C and (15)N assignments for resonances of nDsbD in its oxidized and reduced states. These assignments provide the starting point for detailed investigations of the interactions of nDsbD with its protein partners.

Biochemical properties and catalytic domain structure of the CcmH protein from Escherichia coli

In the Gram-negative bacterium of Escherichia coli, eight genes organized as a ccm operon (ccmABCDEFGH) are involved in the maturation of c-type cytochromes. The proteins encoded by the last three genes ccmFGH are believed to form a lyase complex functioning in the reduction of apocytochrome c and haem attachment. Among them, CcmH is a membrane-associated protein; its N-terminus is a catalytic domain with the active CXXC motif and the C-terminus is predicted as a TPR-like domain with unknown function. By using SCAM (scanning cysteine accessibility mutagenesis) and Gaussia luciferase fusion assays, we provide experimental evidence for the entire topological structure of E. coli CcmH. The mature CcmH is a periplasm-resident oxidoreductase anchored to the inner membrane by two transmembrane segments. Both N- and C-terminal domains are located and function in the periplasmic compartment. Moreover, the N-terminal domain forms a monomer in solution, while the C-terminal domain is a compact fold with helical structures. The NMR solution structure of the catalytic domain in reduced form exhibits mainly a three-helix bundle, providing further information for the redox mechanism. The redox potential suggests that CcmH exhibits a strong reductase that may function in the last step of reduction of apocytochrome c for haem attachment.

Mechanism of the prokaryotic transmembrane disulfide reduction pathway and its in vitro reconstitution from purified components

Making your (Dsb) connection: the redox pathway bringing reducing equivalents from bacterial cytoplasm, across the inner membrane, to the three reductive Dsb pathways in the otherwise oxidizing periplasm (see scheme; TR=thioredoxin reductase, Trx=thioredoxin) is reconstituted from purified components. Transfer of reducing equivalents across the membrane is demonstrated and underlying mechanistic details are revealed.

The interplay between the disulfide bond formation pathway and cytochrome c maturation in Escherichia coli

Heme attachment to c-type cytochromes in bacteria requires cysteine thiols in the CXXCH motif of the protein. The involvement of the periplasmic disulfide generation system in this process remains unclear. We undertake a systematic evaluation of the role of DsbA and DsbD in cytochrome c biogenesis in Escherichia coli and show unequivocally that DsbA is not essential for holocytochrome production under aerobic or anaerobic conditions. We also prove that DsbD is important but not essential for maturation of c-type cytochromes. We discuss the findings in the context of a model in which heme attachment to, and oxidation of, the apocytochrome are competing processes.

A new family of membrane electron transporters and its substrates, including a new cell envelope peroxiredoxin, reveal a broadened reductive capacity of the oxidative bacterial cell envelope

The Escherichia coli membrane protein DsbD functions as an electron hub that dispatches electrons received from the cytoplasmic thioredoxin system to periplasmic oxidoreductases involved in protein disulfide isomerization, cytochrome c biogenesis, and sulfenic acid reduction. Here, we describe a new class of DsbD proteins, named ScsB, whose members are found in proteobacteria and Chlamydia. ScsB has a domain organization similar to that of DsbD, but its amino-terminal domain differs significantly. In DsbD, this domain directly interacts with substrates to reduce them, which suggests that ScsB acts on a different array of substrates. Using Caulobacter crescentus as a model organism, we searched for the substrates of ScsB. We discovered that ScsB provides electrons to the first peroxide reduction pathway identified in the bacterial cell envelope. The reduction pathway comprises a thioredoxin-like protein, TlpA, and a peroxiredoxin, PprX. We show that PprX is a thiol-dependent peroxidase that efficiently reduces both hydrogen peroxide and organic peroxides. Moreover, we identified two additional proteins that depend on ScsB for reduction, a peroxiredoxin-like protein, PrxL, and a novel protein disulfide isomerase, ScsC. Altogether, our results reveal that the array of proteins involved in reductive pathways in the oxidative cell envelope is significantly broader than was previously thought. Moreover, the identification of a new periplasmic peroxiredoxin indicates that in some bacteria, it is important to directly scavenge peroxides in the cell envelope even before they reach the cytoplasm.

IMPORTANCE

Peroxides are reactive oxygen species (ROS) that damage cellular components such as lipids, proteins, and nucleic acids. The presence of protection mechanisms against ROS is essential for cell survival. Bacteria express cytoplasmic catalases and thiol-dependent peroxidases to directly scavenge harmful peroxides. We report the identification of a peroxide reduction pathway active in the periplasm of Caulobacter crescentus, which reveals that, in some bacteria, it is important to directly scavenge peroxides in the cell envelope even before they reach the cytoplasm. The electrons required for peroxide reduction are delivered to this pathway by ScsB, a new type of membrane electron transporter. We also identified two additional likely ScsB substrates, including a novel protein disulfide isomerase. Our results reveal that the array of proteins involved in reductive pathways in the oxidative environment of the cell envelope is significantly broader than was previously thought.

How proteins form disulfide bonds

The identification of protein disulfide isomerase, almost 50 years ago, opened the way to the study of oxidative protein folding. Oxidative protein folding refers to the composite process by which a protein recovers both its native structure and its native disulfide bonds. Pathways that form disulfide bonds have now been unraveled in the bacterial periplasm (disulfide bond protein A [DsbA], DsbB, DsbC, DsbG, and DsbD), the endoplasmic reticulum (protein disulfide isomerase and Ero1), and the mitochondrial intermembrane space (Mia40 and Erv1). This review summarizes the current knowledge on disulfide bond formation in both prokaryotes and eukaryotes and highlights the major problems that remain to be solved.

Oxidation state-dependent protein-protein interactions in disulfide cascades

Bacterial growth and pathogenicity depend on the correct formation of disulfide bonds, a process controlled by the Dsb system in the periplasm of Gram-negative bacteria. Proteins with a thioredoxin fold play a central role in this process. A general feature of thiol-disulfide exchange reactions is the need to avoid a long lived product complex between protein partners. We use a multidisciplinary approach, involving NMR, x-ray crystallography, surface plasmon resonance, mutagenesis, and in vivo experiments, to investigate the interaction between the two soluble domains of the transmembrane reductant conductor DsbD. Our results show oxidation state-dependent affinities between these two domains. These observations have implications for the interactions of the ubiquitous thioredoxin-like proteins with their substrates, provide insight into the key role played by a unique redox partner with an immunoglobulin fold, and are of general importance for oxidative protein-folding pathways in all organisms.

Structural plasticity of the thioredoxin recognition site of yeast methionine S-sulfoxide reductase Mxr1

The methionine S-sulfoxide reductase MsrA catalyzes the reduction of methionine sulfoxide, a ubiquitous reaction depending on the thioredoxin system. To investigate interactions between MsrA and thioredoxin (Trx), we determined the crystal structures of yeast MsrA/Mxr1 in their reduced, oxidized, and Trx2-complexed forms, at 2.03, 1.90, and 2.70 Å, respectively. Comparative structure analysis revealed significant conformational changes of the three loops, which form a plastic "cushion" to harbor the electron donor Trx2. The flexible C-terminal loop enabled Mxr1 to access the methionine sulfoxide on various protein substrates. Moreover, the plasticity of the Trx binding site on Mxr1 provides structural insights into the recognition of diverse substrates by a universal catalytic motif of Trx.

Redox processes controlling the biogenesis of c-type cytochromes

In mitochondria, two mono heme c-type cytochromes are essential electron shuttles of the respiratory chain. They are characterized by the covalent attachment of their heme C to a CXXCH motif in the apoproteins. This post-translational modification occurs in the intermembrane space compartment. Dedicated assembly pathways have evolved to achieve this chemical reaction that requires a strict reducing environment. In mitochondria, two unrelated machineries operate, the rather simple System III in yeast and animals and System I in plants and some protozoans. System I is also found in bacteria and shares some common features with System II that operates in bacteria and plastids. This review aims at presenting how different systems control the chemical requirements for the heme ligation in the compartments where cytochrome c maturation takes place. A special emphasis will be given on the redox processes that are required for the heme attachment reaction onto apocytochromes c.

Structural and functional characterization of CcmG from Pseudomonas aeruginosa, a key component of the bacterial cytochrome c maturation apparatus

The cytochrome c maturation process is carried out in the bacterial periplasm, where some specialized thiol-disulfide oxidoreductases work in close synergy for the correct reduction of oxidized apocytochrome before covalent heme attachment. We present a structural and functional characterization of the soluble periplasmic domain of CcmG from the opportunistic pathogen P. aeruginosa (Pa-CcmG), a component of the protein machinery involved in cyt c maturation in gram-negative bacteria. X-ray crystallography reveals that Pa-CcmG is a TRX-like protein; high-resolution crystal structures show that the oxidized and the reduced forms of the enzyme are identical except for the active-site disulfide. The standard redox potential was calculated to be E(0') = -0.213 V at pH 7.0; the pK(a) of the active site thiols were pK(a) = 6.13 +/- 0.05 for the N-terminal Cys74 and pK(a) = 10.5 +/- 0.17 for the C-terminal Cys77. Experiments were carried out to characterize and isolate the mixed disulfide complex between Pa-CcmG and Pa-CcmH (the other redox active component of System I in P. aeruginosa). Our data indicate that the target disulfide of this TRX-like protein is not the intramolecular disulfide of oxidized Pa-CcmH, but the intermolecular disulfide formed between Cys28 of Pa-CcmH and DTNB used for the in vitro experiments. This observation suggests that, in vivo, the physiological substrate of Pa-CcmG may be the mixed-disulfide complex between Pa-CcmH and apo-cyt.

Cytochrome c biogenesis: the Ccm system

Cytochromes of c-type contain covalently attached hemes that are formed via thioether bonds between the vinyls of heme b and cysteines within C(1)XXC(2)H motifs of apocytochromes. In diverse organisms this post-translational modification relies on membrane-associated specific biogenesis proteins, referred to as cytochrome c maturation (Ccm) systems. A highly complex version of these systems, Ccm or System I, is found in Gram-negative bacteria, archaea and plant mitochondria. We describe emerging functional interactions between the Ccm components categorized into three conserved modules, and present a mechanistic view of the molecular basis of ubiquitous vinyl-2 approximately Cys(1) and vinyl-4 approximately Cys(2) heme b-apocytochrome thioether bonds in c-type cytochromes.

Formation of the complex between DsbD and PilB N-terminal domains from Neisseria meningitidis necessitates an adaptability of nDsbD

DsbD transmembrane protein dispatches electrons to periplasmic Trx/DsbE-like partners via specific interactions with its N-terminal domain, nDsbD. In the present study, PilB N-terminal domain (NterPilB) is shown to efficiently accept electrons coming from nDsbD from Neisseria meningitidis. Using an NMR-driven docking approach, we have modeled the structure of a mixed disulfide complex between NterPilB and nDsbD. We show the needed opening of nDsbD cap-loop whereas NterPilB FLHE loop does not seem essential in the formation and stabilization of the complex. Relaxation analysis performed on backbone amide groups highlights a kind of dynamics transfer from nDsbD cap-loop on NterPilB alpha1 helix, suggesting that a mobility contribution is required not only for the formation of the mixed disulfide complex, but also for its disruption. Taking into account previous X-ray data on covalent complexes involving nDsbD, a cartoon of interactions between Trx-like partners and nDsbD is proposed that illustrates the adaptability of nDsbD.

Protein engineering of the quaternary sulfiredoxin.peroxiredoxin enzyme.substrate complex reveals the molecular basis for cysteine sulfinic acid phosphorylation

Oxidative stress can damage the active site cysteine of the antioxidant enzyme peroxiredoxin (Prx) to the sulfinic acid form, Prx-SO(2)(-). This modification leads to inactivation. Sulfiredoxin (Srx) utilizes a unique ATP-Mg(2+)-dependent mechanism to repair the Prx molecule. Using selective protein engineering that involves disulfide bond formation and site-directed mutagenesis, a mimic of the enzyme.substrate complex has been trapped. Here, we present the 2.1 A crystal structure of human Srx in complex with PrxI, ATP, and Mg(2+). The Cys(52) sulfinic acid moiety was substituted by mutating this residue to Asp, leading to a replacement of the sulfur atom with a carbon atom. Because the Srx reaction cannot occur, the structural changes in the Prx active site that lead to the attack on ATP may be visualized. The local unfolding of the helix containing C52D resulted in the packing of Phe(50) in PrxI within a hydrophobic pocket of Srx. Importantly, this structural rearrangement positioned one of the oxygen atoms of Asp(52) within 4.3 A of the gamma-phosphate of ATP bound to Srx. These observations support a mechanism where phosphorylation of Prx-SO(2)(-) is the first chemical step.

Crystal structure and biophysical properties of Bacillus subtilis BdbD. An oxidizing thiol:disulfide oxidoreductase containing a novel metal site

BdbD is a thiol:disulfide oxidoreductase (TDOR) from Bacillus subtilis that functions to introduce disulfide bonds in substrate proteins/peptides on the outside of the cytoplasmic membrane and, as such, plays a key role in disulfide bond management. Here we demonstrate that the protein is membrane-associated in B. subtilis and present the crystal structure of the soluble part of the protein lacking its membrane anchor. This reveals that BdbD is similar in structure to Escherichia coli DsbA, with a thioredoxin-like domain with an inserted helical domain. A major difference, however, is the presence in BdbD of a metal site, fully occupied by Ca(2+), at an inter-domain position some 14 A away from the CXXC active site. The midpoint reduction potential of soluble BdbD was determined as -75 mV versus normal hydrogen electrode, and the active site N-terminal cysteine thiol was shown to have a low pK(a), consistent with BdbD being an oxidizing TDOR. Equilibrium unfolding studies revealed that the oxidizing power of the protein is based on the instability introduced by the disulfide bond in the oxidized form. The crystal structure of Ca(2+)-depleted BdbD showed that the protein remained folded, with only minor conformational changes. However, the reduced form of Ca(2+)-depleted BdbD was significantly less stable than reduced Ca(2+)-containing protein, and the midpoint reduction potential was shifted by approximately -20 mV, suggesting that Ca(2+) functions to boost the oxidizing power of the protein. Finally, we demonstrate that electron exchange does not occur between BdbD and B. subtilis ResA, a low potential extra-cytoplasmic TDOR.

Genetic analysis of activation of the Vibrio cholerae Cpx pathway

The Cpx two-component system is thought to mediate envelope stress responses in many gram-negative bacteria and has been implicated in the pathogenicity of several enteric pathogens. While cues that activate the Escherichia coli Cpx system have been identified, the nature of the molecular signals that stimulate this pathway is not well understood. Here, we investigated stimuli that trigger this system in Vibrio cholerae, a facultative pathogen that adapts to various niches during its life cycle. In contrast to E. coli, there was no basal activity of the V. cholerae Cpx pathway under standard laboratory conditions. Furthermore, several known stimuli of the E. coli pathway did not induce expression of this system in V. cholerae. There were no defects in intestinal growth in V. cholerae cpx mutants, arguing against the idea that this pathway promotes V. cholerae adaptation to conditions in the mammalian host. We discovered that chloride ions activate the V. cholerae Cpx pathway, raising the possibility that this signal transduction system provides a means for V. cholerae to sense and respond to alterations in salinity. We used a genetic approach to screen for mutants in which the Cpx pathway is activated. We found that mutations in genes whose products are required for periplasmic disulfide bond isomerization result in activation of the Cpx pathway, suggesting that periplasmic accumulation of proteins with aberrant disulfide bonds triggers the V. cholerae Cpx pathway.

Solution structure and backbone dynamics of the cysteine 103 to serine mutant of the N-terminal domain of DsbD from Neisseria meningitidis

The DsbD protein is essential for electron transfer from the cytoplasm to the periplasm of Gram-negative bacteria. Its N-terminal domain dispatches electrons coming from cytoplasmic thioredoxin (Trx), via its central transmembrane and C-terminal domains, to its periplasmic partners: DsbC, DsbE/CcmG, and DsbG. Previous structural studies described the latter proteins as Trx-like folds possessing a characteristic C-X-X-C motif able to generate a disulfide bond upon oxidation. The Escherichia coli nDsbD displays an immunoglobulin-like fold in which two cysteine residues (Cys103 and Cys109) allow a disulfide bond exchange with its biological partners.We have determined the structure in solution and the backbone dynamics of the C103S mutant of the N-terminal domain of DsbD from Neisseria meningitidis. Our results highlight significant structural changes concerning the beta-sheets and the local topology of the active site compared with the oxidized form of the E. coli nDsbD. The structure reveals a "cap loop" covering the active site, similar to the oxidized E. coli nDsbD X-ray structure. However, regions featuring enhanced mobility were observed both near to and distant from the active site, revealing a capacity of structural adjustments in the active site and in putative interaction areas with nDsbD biological partners. Results are discussed in terms of functional consequences.

Control of periplasmic interdomain thiol:disulfide exchange in the transmembrane oxidoreductase DsbD

The bacterial protein DsbD transfers reductant from the cytoplasm to the otherwise oxidizing environment of the periplasm. This reducing power is required for several essential pathways, including disulfide bond formation and cytochrome c maturation. DsbD includes a transmembrane domain (tmDsbD) flanked by two globular periplasmic domains (nDsbD/cDsbD); each contains a cysteine pair involved in electron transfer via a disulfide exchange cascade. The final step in the cascade involves reduction of the Cys(103)-Cys(109) disulfide of nDsbD by Cys(461) of cDsbD. Here we show that a complex between the globular periplasmic domains is trapped in vivo only when both are linked by tmDsbD. We have found previously ( Mavridou, D. A., Stevens, J. M., Ferguson, S. J., & Redfield, C. (2007) J. Mol. Biol. 370, 643-658 ) that the attacking cysteine (Cys(461)) in isolated cDsbD has a high pK(a) value (10.5) that makes this thiol relatively unreactive toward the target disulfide in nDsbD. Here we show using NMR that active-site pK(a) values change significantly when cDsbD forms a complex with nDsbD. This modulation of pK(a) values is critical for the specificity and function of cDsbD. Uncomplexed cDsbD is a poor nucleophile, allowing it to avoid nonspecific reoxidation; however, in complex with nDsbD, the nucleophilicity of cDsbD increases permitting reductant transfer. The observation of significant changes in active-site pK(a) values upon complex formation has wider implications for understanding reactivity in thiol:disulfide oxidoreductases.

Cytochrome c Biogenesis

Escherichia coli employs several c-type cytochromes, which are found in the periplasm or on the periplasmic side of the cytoplasmic membrane; they are used for respiration under different growth conditions. All E. colic-type cytochromes are multiheme cytochromes; E. coli does not have a monoheme cytochrome c of the kind found in mitochondria. The attachment of heme to cytochromes c occurs in the periplasm, and so the apoprotein must be transported across the cytoplasmic membrane; this step is mediated by the Sec system, which transports unfolded proteins across the membrane. The protein CcmE has been found to bind heme covalently via a single bond and then transfer the heme to apocytochromes. It should be mentioned that far less complex systems for cytochrome c biogenesis exist in other organisms and that enterobacteria do not function as a representative model system for the process in general, although plant mitochondria use the Ccm system found in E. coli. The variety and distribution of cytochromes and their biogenesis systems reflect their significance and centrality in cellular bioenergetics, though the necessity for and origin of the diverse biogenesis systems are enigmatic.

Helix swapping leads to dimerization of the N-terminal domain of the c-type cytochrome maturation protein CcmH from Escherichia coli

In the process of cytochrome c maturation, heme groups are covalently attached to reduced cysteines of specific heme-binding motifs (CXXCH) in an apocytochrome c sequence. In Escherichia coli, the CcmH protein maintains apo-protein cysteines in a reduced state prior to heme attachment. We have purified and biophysically, as well as structurally characterized the soluble, N-terminal domain of E. coli CcmH that carries the functionally relevant LRCXXC-motif. In contrast to a recently presented structure of the homologous domain from Pseudomonas aeruginosa, the E. coli protein forms a tightly interlinked dimer by swapping its N-terminal helix between two monomers. We propose that an altered environment of the functional motif may help to discern between the two redox partners CcmG and apocytochrome c.