title: Transmembrane signaling and assembly of the cytochrome b6f-lipidic charge transfer complex.
abstract: Structure-function properties of the cytochrome b6f complex are sufficiently unique compared to those of the cytochrome bc1 complex that b6f should not be considered a trivially modified bc1 complex. A unique property of the dimeric b6f complex is its involvement in transmembrane signaling associated with the p-side oxidation of plastoquinol. Structure analysis of lipid binding sites in the cyanobacterial b6f complex prepared by hydrophobic chromatography shows that the space occupied by the H transmembrane helix in the cytochrome b subunit of the bc1 complex is mostly filled by a lipid in the b6f crystal structure. It is suggested that this space can be filled by the domain of a transmembrane signaling protein. The identification of lipid sites and likely function defines the intra-membrane conserved central core of the b6f complex, consisting of the seven trans-membrane helices of the cytochrome b and subunit IV polypeptides. The other six TM helices, contributed by cytochrome f, the iron-sulfur protein, and the four peripheral single span subunits, define a peripheral less conserved domain of the complex. The distribution of conserved and non-conserved domains of each monomer of the complex, and the position and inferred function of a number of the lipids, suggests a model for the sequential assembly in the membrane of the eight subunits of the b6f complex, in which the assembly is initiated by formation of the cytochrome b6-subunit IV core sub-complex in a monomer unit. Two conformations of the unique lipidic chlorophyll a, defined in crystal structures, are described, and functions of the outlying β-carotene, a possible 'latch' in supercomplex formation, are discussed. This article is part of a Special Issue entitled: Respiratory complex III and related bc complexes.
authors.affiliation: Department of Biological Sciences, Purdue University, West Lafayette, IN 47907, USA.
authors.forename: S
authors.initials: S
authors.lastname: Saif Hasan
authors.forename: Eiki
authors.initials: E
authors.lastname: Yamashita
authors.forename: William A
authors.initials: WA
authors.lastname: Cramer
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D000458:Cyanobacteria / Q000378:metabolism; D045346:Cytochrome b6f Complex / Q000737:chemistry* / Q000378:metabolism; D008563:Membrane Lipids / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D010971:Plastoquinone / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D021122:Protein Subunits / Q000737:chemistry / Q000378:metabolism; D015398:Signal Transduction
chemical_list: D001426:Bacterial Proteins; D008563:Membrane Lipids; D021122:Protein Subunits; C003165:plastoquinol; D045346:Cytochrome b6f Complex; D010971:Plastoquinone
keywords: Assembly; Chl-a; Cyt; Cytochrome; Em7; LHC kinase; PC; PDB; PQ; PS; TMH; Transmembrane signaling; b(6)f; bc(1); chlorophyll-a; cytochrome; midpoint oxidation-reduction potential at pH7; n; p-sides, electrochemically negative and positive side of the membrane; photosystem; plastocyanin; plastoquinone; protein data bank; subIV; subunit IV of the b(6)f complex; transmembrane helix; transmembrane proton electrochemical potential gradient; Δμ̃H+; β-Car; β-carotene