CRYSTAL-STRUCTURE OF CHLOROPLAST CYTOCHROME-F REVEALS A NOVEL CYTOCHROME FOLD AND UNEXPECTED HEME LIGATION

Background: Cytochrome f is the high potential electron acceptor of th e chloroplast cytochrome b6f complex, and is the electron donor to pla stocyanin. The 285-residue cytochrome f subunit is anchored in the thy lakoid membrane of the chloroplast by a single membrane-spanning segme nt near the carboxyl terminus. A soluble redox-active 252-residue lume n-side polypeptide with native spectroscopic and redox properties, mis sing the membrane anchor and carboxyl terminus, was purified from turn ip chloroplasts for structural studies. Results: The crystal structure of cytochrome f, determined to 2.3 angstrom resolution, has several u nexpected features. The 252-residue polypeptide is organized into one large and one small domain. The larger heme-binding domain is striking ly different from known structures of other c-type cytochromes and has the same fold as the type III domain of the animal protein, fibronect in. Cytochrome fbinds heme with an unprecedented axial heme iron ligan d: the amino terminus of the polypeptide. Conclusion: The first atomic structure of a subunit of either the cytochrome b6f complex or of the related cytochrome bc1 complex has been obtained. The structure of cy tochrome f allows prediction of the approximate docking site of plasto cyanin and should allow systematic studies of the mechanism of intra- and inter-protein electron transfer between the cytochrome heme and pl astocyanin copper, which are approximately isopotential. The unprecede nted axial heme iron ligand also provides information on the sequence of events (i.e. cleavage of signal peptide and ligation of heme) assoc iated with translocation of the cytochrome across the membrane and its subsequent folding.