Assembly of chloroplast cytochromes b and c

A new mechanistic understanding of light-limitation in the seagrass Zostera muelleri

In this study we investigated the effect of light-limitation (∼20 μmol photons m^-2 s^-1) on the southern hemisphere seagrass, Zostera muelleri. RNA sequencing, chlorophyll fluorometry and HPLC techniques were used to investigate how the leaf-specific transcriptome drives changes in photosynthesis and photo-pigments in Z. muelleri over 6 days. 1593 (7.51%) genes were differentially expressed on day 2 and 1481 (6.98%) genes were differentially expressed on day 6 of the experiment. Differential gene expression correlated with significant decreases in rETR_Max, I_k, an increase in Yi (initial photosynthetic quantum yield of photosystem II), and significant changes in pigment composition. Regulation of carbohydrate metabolism was observed along with evidence that abscisic acid may serve a role in the low-light response of this seagrass. This study provides a novel understanding of how Z. muelleri responds to light-limitation in the marine water column and provides potential molecular markers for future conservation monitoring efforts.

Photosynthetic complex stoichiometry dynamics in higher plants: biogenesis, function, and turnover of ATP synthase and the cytochrome b6f complex

During plant development and in response to fluctuating environmental conditions, large changes in leaf assimilation capacity and in the metabolic consumption of ATP and NADPH produced by the photosynthetic apparatus can occur. To minimize cytotoxic side reactions, such as the production of reactive oxygen species, photosynthetic electron transport needs to be adjusted to the metabolic demand. The cytochrome b6f complex and chloroplast ATP synthase form the predominant sites of photosynthetic flux control. Accordingly, both respond strongly to changing environmental conditions and metabolic states. Usually, their contents are strictly co-regulated. Thereby, the capacity for proton influx into the lumen, which is controlled by electron flux through the cytochrome b6f complex, is balanced with proton efflux through ATP synthase, which drives ATP synthesis. We discuss the environmental, systemic, and metabolic signals triggering the stoichiometry adjustments of ATP synthase and the cytochrome b6f complex. The contribution of transcriptional and post-transcriptional regulation of subunit synthesis, and the importance of auxiliary proteins required for complex assembly in achieving the stoichiometry adjustments is described. Finally, current knowledge on the stability and turnover of both complexes is summarized.

CYB5D2 requires heme-binding to regulate HeLa cell growth and confer survival from chemotherapeutic agents

The cytochrome b5 domain containing 2 (CYB5D2; Neuferricin) protein has been reported to bind heme, however, the critical residues responsible for heme-binding are undefined. Furthermore, the relationship between heme-binding and CYB5D2-mediated intracellular functions remains unknown. Previous studies examining heme-binding in two cytochrome b5 heme-binding domain-containing proteins, damage-associated protein 1 (Dap1; Saccharomyces cerevisiae) and human progesterone receptor membrane component 1 (PGRMC1), have revealed that conserved tyrosine (Y) 73, Y79, aspartic acid (D) 86, and Y127 residues present in human CYB5D2 may be involved in heme-binding. CYB5D2 binds to type b heme, however, only the substitution of glycine (G) at D86 (D86G) within its cytochrome b5 heme-binding (cyt-b5) domain abolished its heme-binding ability. Both CYB5D2 and CYB5D2(D86G) localize to the endoplasmic reticulum. Ectopic CYB5D2 expression inhibited cell proliferation and anchorage-independent colony growth of HeLa cells. Conversely, CYB5D2 knockdown and ectopic CYB5D2(D86G) expression increased cell proliferation and colony growth. As PGRMC1 has been reported to regulate the expression and activities of cytochrome P450 proteins (CYPs), we examined the role of CYB5D2 in regulating the activities of CYPs involved in sterol synthesis (CYP51A1) and drug metabolism (CYP3A4). CYB5D2 co-localizes with cytochrome P450 reductase (CYPOR), while CYB5D2 knockdown reduced lanosterol demethylase (CYP51A1) levels and rendered HeLa cells sensitive to mevalonate. Additionally, knockdown of CYB5D2 reduced CYP3A4 activity. Lastly, CYB5D2 expression conferred HeLa cell survival from chemotherapeutic agents (paclitaxel, cisplatin and doxorubicin), with its ability to promote survival being dependent on its heme-binding ability. Taken together, this study provides evidence that heme-binding is critical for CYB5D2 in regulating HeLa cell growth and survival, with endogenous CYB5D2 being required to modulate CYP activities.

Regulatory factors for the assembly of thylakoid membrane protein complexes

Major multi-protein photosynthetic complexes, located in thylakoid membranes, are responsible for the capture of light and its conversion into chemical energy in oxygenic photosynthetic organisms. Although the structures and functions of these photosynthetic complexes have been explored, the molecular mechanisms underlying their assembly remain elusive. In this review, we summarize current knowledge of the regulatory components involved in the assembly of thylakoid membrane protein complexes in photosynthetic organisms. Many of the known regulatory factors are conserved between prokaryotes and eukaryotes, whereas others appear to be newly evolved or to have expanded predominantly in eukaryotes. Their specific features and fundamental differences in cyanobacteria, green algae and land plants are discussed.

Cytochrome c assembly

Cytochromes c are central proteins in energy transduction processes by virtue of their functions in electron transfer in respiration and photosynthesis. They have heme covalently attached to a characteristic CXXCH motif via protein-catalyzed post-translational modification reactions. Several systems with diverse constituent proteins have been identified in different organisms and are required to perform the heme attachment and associated functions. The necessary steps are translocation of the apocytochrome polypeptide to the site of heme attachment, transport and provision of heme to the appropriate compartment, reduction and chaperoning of the apocytochrome, and finally, formation of the thioether bonds between heme and two cysteines in the cytochrome. Here we summarize the established classical models for these processes and present recent progress in our understanding of the individual steps within the different cytochrome c biogenesis systems.

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.

The requirement for carotenoids in the assembly and function of the photosynthetic complexes in Chlamydomonas reinhardtii

We have investigated the importance of carotenoids on the accumulation and function of the photosynthetic apparatus using a mutant of the green alga Chlamydomonas reinhardtii lacking carotenoids. The FN68 mutant is deficient in phytoene synthase, the first enzyme of the carotenoid biosynthesis pathway, and therefore is unable to synthesize any carotenes and xanthophylls. We find that FN68 is unable to accumulate the light-harvesting complexes associated with both photosystems as well as the RC subunits of photosystem II. The accumulation of the cytochrome b₆f complex is also strongly reduced to a level approximately 10% that of the wild type. However, the residual fraction of assembled cytochrome b₆f complexes exhibits single-turnover electron transfer kinetics comparable to those observed in the wild-type strain. Surprisingly, photosystem I is assembled to significant levels in the absence of carotenoids in FN68 and possesses functional properties that are very similar to those of the wild-type complex.

DAC is involved in the accumulation of the cytochrome b6/f complex in Arabidopsis

The biogenesis and assembly of photosynthetic multisubunit protein complexes is assisted by a series of nucleus-encoded auxiliary protein factors. In this study, we characterize the dac mutant of Arabidopsis (Arabidopsis thaliana), which shows a severe defect in the accumulation of the cytochrome b(6)/f complex, and provide evidence suggesting that the efficiency of cytochrome b(6)/f complex assembly is affected in the mutant. DAC is a thylakoid membrane protein with two predicted transmembrane domains that is conserved from cyanobacteria to vascular plants. Yeast (Saccharomyces cerevisiae) two-hybrid and coimmunoprecipitation analyses revealed a specific interaction between DAC and PetD, a subunit of the cytochrome b(6)/f complex. However, DAC was found not to be an intrinsic component of the cytochrome b(6)/f complex. In vivo chloroplast protein labeling experiments showed that the labeling rates of the PetD and cytochrome f proteins were greatly reduced, whereas that of the cytochrome b(6) protein remained normal in the dac mutant. DAC appears to be a novel factor involved in the assembly/stabilization of the cytochrome b(6)/f complex, possibly through interaction with the PetD protein.

A novel component of the disulfide-reducing pathway required for cytochrome c assembly in plastids

In plastids, the conversion of energy in the form of light to ATP requires key electron shuttles, the c-type cytochromes, which are defined by the covalent attachment of heme to a CXXCH motif. Plastid c-type cytochrome biogenesis occurs in the thylakoid lumen and requires a system for transmembrane transfer of reductants. Previously, CCDA and CCS5/HCF164, found in all plastid-containing organisms, have been proposed as two components of the disulfide-reducing pathway. In this work, we identify a small novel protein, CCS4, as a third component in this pathway. CCS4 was genetically identified in the green alga Chlamydomonas reinhardtii on the basis of the rescue of the ccs4 mutant, which is blocked in the synthesis of holoforms of plastid c-type cytochromes, namely cytochromes f and c(6). Although CCS4 does not display sequence motifs suggestive of redox or heme-binding function, biochemical and genetic complementation experiments suggest a role in the disulfide-reducing pathway required for heme attachment to apoforms of cytochromes c. Exogenous thiols partially rescue the growth phenotype of the ccs4 mutant concomitant with recovery of holocytochrome f accumulation, as does expression of an ectopic copy of the CCDA gene, encoding a trans-thylakoid transporter of reducing equivalents. We suggest that CCS4 might function to stabilize CCDA or regulate its activity.

c-type cytochrome assembly in Saccharomyces cerevisiae: a key residue for apocytochrome c1/lyase interaction

The electron transport chains in the membranes of bacteria and organelles generate proton-motive force essential for ATP production. The c-type cytochromes, defined by the covalent attachment of heme to a CXXCH motif, are key electron carriers in these energy-transducing membranes. In mitochondria, cytochromes c and c(1) are assembled by the cytochrome c heme lyases (CCHL and CC(1)HL) and by Cyc2p, a putative redox protein. A cytochrome c(1) mutant with a CAPCH heme-binding site instead of the wild-type CAACH is strictly dependent upon Cyc2p for assembly. In this context, we found that overexpression of CC(1)HL, as well as mutations of the proline in the CAPCH site to H, L, S, or T residues, can bypass the absence of Cyc2p. The P mutation was postulated to shift the CXXCH motif to an oxidized form, which must be reduced in a Cyc2p-dependent reaction before heme ligation. However, measurement of the redox midpoint potential of apocytochrome c(1) indicates that neither the P nor the T residues impact the thermodynamic propensity of the CXXCH motif to occur in a disulfide vs. dithiol form. We show instead that the identity of the second intervening residue in the CXXCH motif is key in determining the CCHL-dependent vs. CC(1)HL-dependent assembly of holocytochrome c(1). We also provide evidence that Cyc2p is dedicated to the CCHL pathway and is not required for the CC(1)HL-dependent assembly of cytochrome c(1).

CCS5, a thioredoxin-like protein involved in the assembly of plastid c-type cytochromes

The c-type cytochromes are metalloproteins with a heme molecule covalently linked to the sulfhydryls of a CXXCH heme-binding site. In plastids, at least six assembly factors are required for heme attachment to the apo-forms of cytochrome f and cytochrome c(6) in the thylakoid lumen. CCS5, controlling plastid cytochrome c assembly, was identified through insertional mutagenesis in the unicellular green alga Chlamydomonas reinhardtii. The complementing gene encodes a protein with similarity to Arabidopsis thaliana HCF164, which is a thylakoid membrane-anchored protein with a lumen-facing thioredoxin-like domain. HCF164 is required for cytochrome b(6)f biogenesis, but its activity and site of action in the assembly process has so far remained undeciphered. We show that CCS5 is a component of a trans-thylakoid redox pathway and operates by reducing the CXXCH heme-binding site of apocytochrome c prior to the heme ligation reaction. The proposal is based on the following findings: 1) the ccs5 mutant is rescued by exogenous thiols; 2) CCS5 interacts with apocytochrome f and c(6) in a yeast two-hybrid assay; and 3) recombinant CCS5 is able to reduce a disulfide in the CXXCH heme-binding site of apocytochrome f.

A novel pathway of cytochrome c biogenesis is involved in the assembly of the cytochrome b6f complex in arabidopsis chloroplasts

We recently characterized a novel heme biogenesis pathway required for heme c(i)' covalent binding to cytochrome b6 in Chlamydomonas named system IV or CCB (cofactor assembly, complex C (b6f), subunit B (PetB)). To find out whether this CCB pathway also operates in higher plants and extend the knowledge of the c-type cytochrome biogenesis, we studied Arabidopsis insertion mutants in the orthologs of the CCB genes. The ccb1, ccb2, and ccb4 mutants show a phenotype characterized by a deficiency in the accumulation of the subunits of the cytochrome b6f complex and lack covalent heme binding to cytochrome b6. These mutants were functionally complemented with the corresponding wild type cDNAs. Using fluorescent protein reporters, we demonstrated that the CCB1, CCB2, CCB3, and CCB4 proteins are targeted to the chloroplast compartment of Arabidopsis. We have extended our study to the YGGT family, to which CCB3 belongs, by studying insertion mutants of two additional members of this family for which no mutants were previously characterized, and we showed that they are not functionally involved in the CCB system. Thus, we demonstrate the ubiquity of the CCB proteins in chloroplast heme c(i)' binding.

Insight into the haem d1 biosynthesis pathway in heliobacteria through bioinformatics analysis

Haem d(1) is a unique tetrapyrrole molecule that serves as a prosthetic group of cytochrome cd(1), which reduces nitrite to nitric oxide during the process of denitrification. Very little information is available regarding the biosynthesis of haem d(1). The extreme difficulty in studying the haem d(1) biosynthetic pathway can be partly attributed to the lack of a theoretical basis for experimental investigation. We report here a gene cluster encoding enzymes involved in the biosynthesis of haem d(1) in two heliobacterial species, Heliobacillus mobilis and Heliophilum fasciatum. The gene organization of the cluster is conserved between the two species, and contains a complete set of genes that lead to the biosynthesis of uroporphyrinogen III and genes thought to be involved in the late steps of haem d(1) biosynthesis. Detailed bioinformatics analysis of some of the proteins encoded in the gene cluster revealed important clues to the precise biochemical roles of the proteins in the biosynthesis of haem d(1), as well as the membrane transport and insertion of haem d(1) into an apocytochrome during the maturation of cytochrome cd(1).

A chloroplast cyclophilin functions in the assembly and maintenance of photosystem II in Arabidopsis thaliana

Photosynthetic light reactions rely on the proper function of large protein complexes (including photosystems I and II) that reside in the thylakoid membrane. Although their composition, structure, and function are known, the repertoire of assembly and maintenance factors is still being determined. Here we show that an immunophilin of the cyclophilin type, CYP38, plays a critical role in the assembly and maintenance of photosystem II (PSII) supercomplexes (SCs) in Arabidopsis. Mutant plants with the CYP38 gene interrupted by T-DNA insertion showed stunted growth and were hypersensitive to high light. Leaf chlorophyll fluorescence analysis and thylakoid membrane composition indicated that cyp38 mutant plants had defects in PSII SCs. Sucrose supplementation enabled the rescue of the mutant phenotype under low-light conditions, but failed to mitigate hypersensitivity to high-light stress. Protein radiolabeling assays showed that, although individual thylakoid proteins were synthesized equally in mutant and wild type, the assembly of the PSII SC was impaired in the mutant. In addition, the D1 and D2 components of the mutant PSII had a short half-life under high-light stress. The results provide evidence that CYP38 is necessary for the assembly and stabilization of PSII.

Insights into porphyrin chemistry provided by the microperoxidases, the haempeptides derived from cytochrome c

The water-soluble haem-containing peptides obtained by proteolytic digestion of cytochrome c, the microperoxidases, have been used to explore aspects of the chemistry of iron porphyrins, and as mimics for some reactions catalysed by the haemoproteins, including the reactions catalysed by the peroxidases and the cytochromes P450. The preparation of the microperoxidases, their physical and chemical properties including their electronic structure, the kinetics and thermodynamics of their reactions with ligands, electrochemical studies and examples of their uses as haemoproteins mimics, is reviewed.

HCF164 receives reducing equivalents from stromal thioredoxin across the thylakoid membrane and mediates reduction of target proteins in the thylakoid lumen

HCF164 is a membrane-anchored thioredoxin-like protein known to be indispensable for assembly of cytochrome b6 f in the thylakoid membranes. In this study, we report the finding that chloroplast stroma m-type thioredoxin is the source of reducing equivalents for reduction of HCF164 in the thylakoid lumen, providing strong evidence that higher plant chloroplasts possess a trans-membrane reducing equivalent transfer system similar to that found in bacteria. To probe the function of HCF164 in the lumen, a screen to identify the reducing equivalent acceptor proteins of HCF164 was carried out by using a resin-immobilized HCF164 single cysteine mutant, leading to the isolation of putative target thylakoid proteins. Among the newly identified target proteins, the reduction of the PSI-N subunit of photosystem I by HCF164 was confirmed both in vitro and in isolated thylakoids. Two components of the cytochrome b6 f complex, the cytochrome f and Rieske FeS proteins, were also identified as novel potential target proteins. The data presented here suggest that HCF164 serves as an important transducer of reducing equivalents to proteins in the thylakoid lumen.

Cyc2p, a Membrane-bound Flavoprotein Involved in the Maturation of Mitochondrial c-Type Cytochromes

Mitochondrial apocytochrome c and c1 are converted to their holoforms in the intermembrane space by attachment of heme to the cysteines of the CXXCH motif through the activity of assembly factors cytochrome c heme lyase and cytochrome c1 heme lyase (CCHL and CC1HL). The maintenance of apocytochrome sulfhydryls and heme substrates in a reduced state is critical for the ligation of heme. Factors that control the redox chemistry of the heme attachment reaction to apocytochrome c are known in bacteria and plastids but not in mitochondria. We have explored the function of Cyc2p, a candidate redox cytochrome c assembly component in yeast mitochondria. We show that Cyc2p is required for the activity of CCHL toward apocytochrome c and c1 and becomes essential for the heme attachment to apocytochrome c1 carrying a CAPCH instead of CAACH heme binding site. A redox function for Cyc2p in the heme lyase reaction is suggested from 1) the presence of a noncovalently bound FAD molecule in the C-terminal domain of Cyc2p, 2) the localization of Cyc2p in the inner membrane with the FAD binding domain exposed to the intermembrane space, and 3) the ability of recombinant Cyc2p to carry the NADPH-dependent reduction of ferricyanide. We postulate that, in vivo, Cyc2p interacts with CCHL and is involved in the reduction of heme prior to its ligation to apocytochrome c by CCHL.

Maturation of the unusual single-cysteine (XXXCH) mitochondrial c-type cytochromes found in trypanosomatids must occur through a novel biogenesis pathway

The c-type cytochromes are characterized by the covalent attachment of haem to the polypeptide via thioether bonds formed from haem vinyl groups and, normally, the thiols of two cysteines in a CXXCH motif. Intriguingly, the mitochondrial cytochromes c and c1 from two euglenids and the Trypanosomatidae contain only a single cysteine within the haem-binding motif (XXXCH). There are three known distinct pathways by which c-type cytochromes are matured post-translationally in different organisms. The absence of genes encoding any of these c-type cytochrome biogenesis machineries is established here by analysis of six trypanosomatid genomes, and correlates with the presence of single-cysteine cytochromes c and c1. In contrast, we have identified a comprehensive catalogue of proteins required for a typical mitochondrial oxidative phosphorylation apparatus. Neither spontaneous nor catalysed maturation of the single-cysteine Trypanosoma brucei cytochrome c occurred in Escherichia coli. However, a CXXCH variant was matured by the E. coli cytochrome c maturation machinery, confirming the proposed requirement of the latter for two cysteines in the haem-binding motif and indicating that T. brucei cytochrome c can accommodate a second cysteine in a CXXCH motif. The single-cysteine haem attachment conserved in cytochromes c and c1 of the trypanosomatids is suggested to be related to their cytochrome c maturation machinery, and the environment in the mitochondrial intermembrane space. Our genomic and biochemical studies provide very persuasive evidence that the trypanosomatid mitochondrial cytochromes c are matured by a novel biogenesis system.

Redox regulation: a broadening horizon

Initially discovered in the context of photosynthesis, regulation by change in the redox state of thiol groups (S-S <--> 2SH) is now known to occur throughout biology. Several systems, each linking a hydrogen donor to an intermediary disulfide protein, act to effect changes that alter the activity of target proteins: the ferredoxin/thioredoxin system, comprised of reduced ferredoxin, a thioredoxin, and the enzyme, ferredoxin-thioredoxin reductase; the NADP/thioredoxin system, including NADPH, a thioredoxin, and NADP-thioredoxin reductase; and the glutathione/glutaredoxin system, composed of reduced glutathione and a glutaredoxin. A related disulfide protein, protein disulfide isomerase (PDI) acts in protein assembly. Regulation linked to plastoquinone and signaling induced by reactive oxygen species (ROS) and other agents are also being actively investigated. Progress made on these systems has linked redox to the regulation of an increasing number of processes not only in plants, but in other types of organisms as well. Research in areas currently under exploration promises to provide a fuller understanding of the role redox plays in cellular processes, and to further the application of this knowledge to technology and medicine.

A Homolog of Prokaryotic Thiol Disulfide Transporter CcdA Is Required for the Assembly of the Cytochrome bf Complex in Arabidopsis Chloroplasts

The c-type cytochromes are defined by the occurrence of heme covalently linked to the polypeptide via thioether bonds between heme and the cysteine sulfhydryls in the CXXCH motif of apocytochrome. Maintenance of apocytochrome sulfhydryls in a reduced state is a prerequisite for covalent ligation of heme to the CXXCH motif. In bacteria, a thiol disulfide transporter and a thioredoxin are two components in a thio-reduction pathway involved in c-type cytochrome assembly. We have identified in photosynthetic eukaryotes nucleus-encoded homologs of a prokaryotic thiol disulfide transporter, CcdA, which all display an N-terminal extension with respect to their bacterial counterparts. The extension of Arabidopsis CCDA functions as a targeting sequence, suggesting a plastid site of action for CCDA in eukaryotes. Using PhoA and LacZ as topological reporters, we established that Arabidopsis CCDA is a polytopic protein with within-membrane strictly conserved cysteine residues. Insertional mutants in the Arabidopsis CCDA gene were identified, and loss-of-function alleles were shown to impair photosynthesis because of a defect in cytochrome b(6)f accumulation, which we attribute to a block in the maturation of holocytochrome f, whose heme binding domain resides in the thylakoid lumen. We postulate that plastid cytochrome c maturation requires CCDA, thioredoxin HCF164, and other molecules in a membrane-associated trans-thylakoid thiol-reducing pathway.

Tracking the function of the cytochrome c6-like protein in higher plants

The contention that plastocyanin is the only mobile electron donor to photosystem I in higher plants was recently shaken by the discovery of a cytochrome c(6)-like protein in Arabidopsis and other flowering plants. However, the genetic and biochemical data presented in support of the idea that the cytochrome c(6) homologue can replace plastocyanin have now been challenged by two complementary studies. This re-opens the debate on the real function(s) of cytochrome c in the chloroplasts of higher plants.

Overlapping specificities of the mitochondrial cytochrome c and c1 heme lyases

Heme attachment to the apoforms of fungal mitochondrial cytochrome c and c1 requires the activity of cytochrome c and c1 heme lyases (CCHL and CC1HL), which are enzymes with distinct substrate specificity. However, the presence of a single heme lyase in higher eukaryotes is suggestive of broader substrate specificity. Here, we demonstrate that yeast CCHL is active toward the non-cognate substrate apocytochrome c1, i.e. CCHL promotes low levels of apocytochrome c1 conversion to its holoform in the absence of CC1HL. Moreover, that the single human heme lyase also displays a broader cytochrome specificity is evident from its ability to substitute for both yeast CCHL and CC1HL. Multicopy and genetic suppressors of the absence of CC1HL were isolated and their analysis revealed that the activity of CCHL toward cytochrome c1 can be enhanced by: 1) reducing the abundance of the cognate substrate apocytochrome c, 2) increasing the accumulation of CCHL, 3) modifying the substrate-enzyme interaction through point mutations in CCHL or cytochrome c1, or 4) overexpressing Cyc2p, a protein known previously only as a mitochondrial biogenesis factor. Based on the functional interaction of Cyc2p with CCHL and the presence of a putative FAD-binding site in the protein, we hypothesize that Cyc2p controls the redox chemistry of the heme lyase reaction.

Functional analysis of a divergent system II protein, Ccs1, involved in c-type cytochrome biogenesis

The Ccs1 gene, encoding a highly divergent novel component of a system II type c-type cytochrome biogenesis pathway, is encoded by the previously defined CCS1 locus in Chlamydomonas reinhardtii. phoA and lacZalpha bacterial topological reporters were used to deduce a topological model of the Synechocystis sp. 6803 Ccs1 homologue, CcsB. CcsB, and therefore by analogy Ccs1, possesses a large soluble lumenal domain at its C terminus that is tethered in the thylakoid membrane by three closely spaced transmembrane domains in the N-terminal portion of the protein. Molecular analysis of ccs1 alleles reveals that the entire C-terminal soluble domain is essential for Ccs1 function and that a stromal loop appears to be important in vivo, at least for maintenance of Ccs1. Site-directed mutational analysis reveals that a single histidine (His(274)) within the last transmembrane domain, preceding the large lumenal domain, is required for c-type cytochrome assembly, whereas an invariant cysteine residue (Cys(199)) is shown to be non-essential. Ccs1 is proposed to interact with other Ccs components based on its reduced accumulation in ccs2, ccs3, ccs4, and ccsA strains.

Essential histidine and tryptophan residues in CcsA, a system II polytopic cytochrome c biogenesis protein

Three distinct systems (I, II, and III) for catalysis of heme attachment to c-type apocytochromes are known. The CcsA and Ccs1 proteins are required in system II for the assembly of bacterial and plastid cytochromes c. A tryptophan-rich signature motif (WWD), also occurring in CcmC and CcmF found in system I, and three histidinyl residues, all strictly conserved in CcsA suggest a function in heme handling. Topological analysis of plastid CcsA in bacteria using the PhoA and LacZalpha reporters placed the WWD motif, the conserved residues His(212) and His(347) on the lumen side of the membrane, whereas His(309) was assigned a location on the stromal side. Functional analysis of CcsA through site-directed mutagenesis enabled the designation of the initiation codon of the ccsA gene and established the functional importance of the WWD signature motif and the absolute requirement of all three histidines for the assembly of plastid c-type cytochromes. In a ccsA mutant, a 200-kDa Ccs1-containing complex is absent from solubilized thylakoid membranes, suggesting that CcsA operates together with Ccs1. We propose a model where the WWD motif and histidine residues function in relaying heme from stroma to lumen and we postulate the existence of a cytochrome c assembly machinery containing CcsA, Ccs1 and additional components.