title: Hydrophobic interactions in a cyanobacterial plastocyanin-cytochrome f complex.
abstract: The complex of the photosynthetic redox partners plastocyanin and cytochrome f from the thermophilic cyanobacterium, Phormidium laminosum, was investigated by nuclear magnetic resonance (NMR). Chemical-shift perturbation analysis of amide proton and nitrogen nuclei implicates the hydrophobic patch and, to a lesser extent, the "eastern face" of plastocyanin in the complex interface. Intermolecular pseudocontact shifts observed in the complex of cadmium-substituted plastocyanin and ferric cytochrome f specifically define the site of interaction to be between the hydrophobic patch of plastocyanin and the heme region of cytochrome f. Rigid-body structure calculations using NMR-derived restraints demonstrate that plastocyanin is oriented in a "head-on" fashion, with the long axis of the molecule perpendicular to the heme plane. Remarkably, the structure and affinity of the complex are independent of ionic strength, indicating that there is little electrostatic interaction. Lowering the pH results in limited reorganization of the complex interface, while the binding affinity is unaffected. Therefore, protonation of the exposed copper ligand, His92, plays only a minor role in the complex. In contrast to other electron-transfer complexes, the plastocyanin-cytochrome f complex from P. laminosum is predominantly controlled by hydrophobic interactions. These findings are discussed in the context of the previously characterized angiosperm complex.
authors.affiliation: Gorlaeus Laboratories, Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands.
authors.forename: P B
authors.initials: PB
authors.lastname: Crowley
authors.forename: G
authors.initials: G
authors.lastname: Otting
authors.forename: B G
authors.initials: BG
authors.lastname: Schlarb-Ridley
authors.forename: G W
authors.initials: GW
authors.lastname: Canters
authors.forename: M
authors.initials: M
authors.lastname: Ubbink
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism; D000458:Cyanobacteria / Q000737:chemistry* / Q000378:metabolism; D003580:Cytochromes / Q000737:chemistry* / Q000378:metabolism; D045348:Cytochromes f; D006863:Hydrogen-Ion Concentration; D009587:Nitrogen Isotopes; D019906:Nuclear Magnetic Resonance, Biomolecular / Q000379:methods; D009994:Osmolar Concentration; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000378:metabolism; D010970:Plastocyanin / Q000737:chemistry* / Q000378:metabolism; D011487:Protein Conformation; D013499:Surface Properties
chemical_list: D001426:Bacterial Proteins; D003580:Cytochromes; D009587:Nitrogen Isotopes; D045322:Photosynthetic Reaction Center Complex Proteins; D010970:Plastocyanin; D045348:Cytochromes f