title: CCS5, a thioredoxin-like protein involved in the assembly of plastid c-type cytochromes.
abstract: 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.
authors.affiliation: From the Department of Molecular Genetics and Department of Molecular Cellular Biochemistry and.
authors.forename: Stéphane T
authors.initials: ST
authors.lastname: Gabilly
authors.forename: Beth Welty
authors.initials: BW
authors.lastname: Dreyfuss
authors.forename: Mohamed
authors.initials: M
authors.lastname: Karamoko
authors.forename: Vincent
authors.initials: V
authors.lastname: Corvest
authors.forename: Janette
authors.initials: J
authors.lastname: Kropat
authors.forename: M Dudley
authors.initials: MD
authors.lastname: Page
authors.forename: Sabeeha S
authors.initials: SS
authors.lastname: Merchant
authors.forename: Patrice P
authors.initials: PP
authors.lastname: Hamel
mesh_terms: D000818:Animals; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D001665:Binding Sites; D016825:Chlamydomonas reinhardtii / Q000235:genetics / Q000378:metabolism*; D045304:Cytochromes c / Q000235:genetics / Q000378:metabolism*; D045365:Cytochromes c6 / Q000235:genetics / Q000378:metabolism*; D045348:Cytochromes f / Q000235:genetics / Q000378:metabolism; D006418:Heme / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D010084:Oxidation-Reduction; D015800:Protozoan Proteins / Q000235:genetics / Q000378:metabolism*; D013879:Thioredoxins / Q000235:genetics / Q000378:metabolism*; D020524:Thylakoids / Q000235:genetics / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D045365:Cytochromes c6; D015800:Protozoan Proteins; D006418:Heme; D013879:Thioredoxins; D045304:Cytochromes c; D045348:Cytochromes f