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

Lina Lezhneva1,2, Richard Kuras1,2, Genevieve Ephritikhine3,4 and Catherine de Vitry1,2

1CNRS, UMR 7141, Laboratoire de Physiologie Membranaire et Moléculaire du Chloroplaste, Institut de Biologie Physico-Chimique, 13 Rue Pierre et Marie Curie, 75005 Paris, France 2UPMC Univ Paris 06 3CNRS, UPR 2355, Institut des Sciences du Végétal, B ât 22, 1 Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex, France 4Univ Paris-Diderot.

Our studies of Chlamydomonas mutants revealed that the covalent binding of c type hemes localized on the electronegative side of the thylakoid membranes (case of heme ci' covalent binding to cytochrome b6) requires a specific maturation pathway named system IV or CCB (cofactor assembly, complex C (b6f), subunit B (PetB)) implicating at least four membrane proteins (Kuras et al., 2007, Proc Natl Acad Sci USA 104, 9906-9910). 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 T-DNA insertion mutants in the orthologs of the CCB genes. The ccb1, ccb2 and ccb4 plants homozygous for the T-DNA insertion are non phototrophic and 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. No mutants altered in the CCB3 gene were available. Using fluorescent protein reporters we also 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 and we showed that they are not functionally involved in the CCB system. Hence, we confirm the similar role of the CCB proteins in Arabidopsis and Chlamydomonas and thereby establish the generality of the CCB pathway in chloroplasts.

e-mail address of presenting author: Richard.Kuras@ibpc.fr