title: Solution structures of the core light-harvesting alpha and beta polypeptides from Rhodospirillum rubrum: implications for the pigment-protein and protein-protein interactions.
abstract: We have determined the solution structures of the core light-harvesting (LH1) alpha and beta-polypeptides from wild-type purple photosynthetic bacterium Rhodospirillum rubrum using multidimensional NMR spectroscopy. The two polypeptides form stable alpha helices in organic solution. The structure of alpha-polypeptide consists of a long helix of 32 amino acid residues over the central transmembrane domain and a short helical segment at the N terminus that is followed by a three-residue loop. Pigment-coordinating histidine residue (His29) in the alpha-polypeptide is located near the middle of the central helix. The structure of beta-polypeptide shows a single helix of 32 amino acid residues in the membrane-spanning region with the pigment-coordinating histidine residue (His38) at a position close to the C-terminal end of the helix. Strong hydrogen bonds have been identified for the backbone amide protons over the central helical regions, indicating a rigid property of the two polypeptides. The overall structures of the R.rubrum LH1 alpha and beta-polypeptides are different from those previously reported for the LH1 beta-polypeptide of Rhodobacter sphaeroides, but are very similar to the structures of the corresponding LH2 alpha and beta-polypeptides determined by X-ray crystallography. A model constructed for the structural subunit (B820) of LH1 complex using the solution structures reveals several important features on the interactions between the LH1 alpha and beta-polypeptides. The significance of the N-terminal regions of the two polypeptides for stabilizing both B820 and LH1 complexes, as clarified by many experiments, may be attributed to the interactions between the short N-terminal helix (Trp2-Gln6) of alpha-polypeptide and a GxxxG motif in the beta-polypeptide.
authors.affiliation: Department of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Aramaki-aza, Aoba, Aoba-ku, Sendai 980-8579, Japan. wang@biophys.che.tohoku.ac.jp
authors.forename: Zheng-Yu
authors.initials: ZY
authors.lastname: Wang
authors.forename: Kazutaka
authors.initials: K
authors.lastname: Gokan
authors.forename: Masayuki
authors.initials: M
authors.lastname: Kobayashi
authors.forename: Tsunenori
authors.initials: T
authors.lastname: Nozawa
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002942:Circular Dichroism; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D008958:Models, Molecular; D008969:Molecular Sequence Data; D019906:Nuclear Magnetic Resonance, Biomolecular; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D021122:Protein Subunits / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D012247:Rhodospirillum rubrum / Q000737:chemistry*
chemical_list: D001426:Bacterial Proteins; D045342:Light-Harvesting Protein Complexes; D021122:Protein Subunits; C094744:light-harvesting complex 1, Rhodospirillum