CCS5, a Thioredoxin-like Protein Involved in the Assembly of Plastid c-Type Cytochromes

• DOI: 10.1074/jbc.m109.099069
• Publication Date: 2010-07-14T00:13:20Z
• Authors: Stéphane T. Gabilly; Beth Welty Dreyfuss; Mohamed Karamoko; Vincent Corvest; Janette Kropat; M. Dudley Page; Sabeeha S. Merchant; Patrice P. Hamel
• Keywords: 0301 basic medicine; Binding Sites; Arabidopsis Proteins; Arabidopsis; Protozoan Proteins; Cytochromes c; Heme; Thylakoids; Cytochromes f; 03 medical and health sciences; Cytochromes c6; Thioredoxins; Mutation; Animals; Oxidation-Reduction; Chlamydomonas reinhardtii

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.