Abstract
In bacteria and energy generating organelles, c-type cytochromes are a class of universal electron carriers with a heme cofactor covalently linked via one or two thioether bonds to a heme binding site. The covalent attachment of heme to apocytochromes is a catalyzed process, taking place via three evolutionarily distinct assembly pathways (Systems I, II, III). System II was discovered in the green alga Chlamydomonas reinhardtii through the genetic analysis of the ccs mutants (cytochrome csynthesis), which display a block in the apo- to holo- form conversion of cytochrome f and c6, the thylakoid lumen resident c-type cytochromes functioning in photosynthesis. Here we show that the gene corresponding to the CCS2 locus encodes a 1,719 amino acid polypeptide and identify the molecular lesions in the ccs2-1 to ccs2-5 alleles. The CCS2 protein displays seven degenerate amino acid repeats, which are variations of the octatricopeptide-repeat motif (OPR) recently recognized in several nuclear-encoded proteins controlling the maturation, stability, or translation of chloroplast transcripts. A plastid site of action for CCS2 is inferred from the finding that GFP fused to the first 100 amino acids of the algal protein localizes to chloroplasts in Nicotiana benthamiana. We discuss the possible functions of CCS2 in the heme attachment reaction.
Highlights
Energy-transducing membranes are specialized membranes in archaea, bacteria, mitochondria, and chloroplasts, which rely on electron carriers to generate the proton gradient necessary for ATP synthesis
While we have proposed that CCS4 regulates the disulfide reducing pathway in the plastid (Gabilly et al, 2011), the gene products for CCS2, CCS3, and CCS6 remain unknown
To gain further insights into plastid cytochrome c assembly, we sought to clone the gene corresponding to the CCS2 locus, defined by the ccs2-1 to -5 alleles (Xie et al, 1998)
Summary
Energy-transducing membranes are specialized membranes in archaea, bacteria, mitochondria, and chloroplasts, which rely on electron carriers to generate the proton gradient necessary for ATP synthesis. In energy-transducing membranes, the c-type cytochromes, generically referred to as cytochromes c, are a class of structurally diverse metalloproteins with one or more covalently linked heme(s) (ferro-protoporphyrin IX) that participate in electron transfer reactions (ThonyMeyer, 1997; Bonnard et al, 2010; Verissimo and Daldal, 2014). Plastid Metalloprotein Assembly cysteine in naturally occurring cytochromes c The histidine in this motif acts as a ligand of ferroheme (Bowman and Bren, 2008; Allen et al, 2009). Heme attachment to the heme-binding site is a catalyzed reaction in vivo and requires heme transport across at least one biological membrane and covalent linkage of ferroheme to free sulfhydryls of the heme binding cysteines in the CXnCH motif (Bonnard et al, 2010; Mavridou et al, 2013)
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