title: Projection structures of three photosynthetic complexes from Rhodobacter sphaeroides: LH2 at 6 A, LH1 and RC-LH1 at 25 A.
abstract: Three photosynthetic complexes, light-harvesting complex 2 (LH2), light-harvesting complex 1 (LH1), and the reaction centre-light-harvesting complex 1 photounit (RC-LH1), were purified from a single species of a purple bacterium, Rhodobacter sphaeroides, and reconstituted into two-dimensional (2-D) crystals. Vesicular 2-D crystals of LH1 and RC-LH1 were imaged in negative stain and projection maps at 25 A resolution were produced. The rings formed by LH1 have approximately the same mean diameter as the LH1 rings from Rhodospirillum rubrum ( approximately 90 A) and therefore are likely to be composed of 15 to 17 alphabeta subunits. In the projection map calculated from the RC-LH1 2-D crystals, the reaction centre is represented by an additional density in the centre of the ring formed by the LH1 subunits. The marked improvement of shape and fine structure after a rotational pre-alignment of the RC-LH1 unit cells before averaging strongly suggests that the RC is not in a unique orientation within the LH1 rings. Tubular crystals of LH2 showed a high degree of order and allowed calculation of a projection map at 6 A resolution from glucose-embedded specimens. The projection structure shows a ring of nine alphabeta subunits. Variation of the alpha-helical projection densities suggests that the 9-fold symmetry axis is tilted with respect to the membrane normal.
authors.affiliation: Department of Molecular Biology and Biotechnology, University of Sheffield, Firth Court Western Bank, Sheffield, S10 2TN, UK. T.Walz@Sheffield.ac.uk
authors.forename: T
authors.initials: T
authors.lastname: Walz
authors.forename: S J
authors.initials: SJ
authors.lastname: Jamieson
authors.forename: C M
authors.initials: CM
authors.lastname: Bowers
authors.forename: P A
authors.initials: PA
authors.lastname: Bullough
authors.forename: C N
authors.initials: CN
authors.lastname: Hunter
mesh_terms: D001426:Bacterial Proteins; D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism; D003460:Crystallization; D005947:Glucose; D045342:Light-Harvesting Protein Complexes; D008854:Microscopy, Electron; D010713:Phosphatidylcholines; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry / Q000302:isolation & purification / Q000648:ultrastructure*; D017433:Protein Structure, Secondary; D012242:Rhodobacter sphaeroides / Q000737:chemistry*; D012995:Solubility; D013057:Spectrum Analysis
chemical_list: D001426:Bacterial Proteins; D045342:Light-Harvesting Protein Complexes; D010713:Phosphatidylcholines; D045322:Photosynthetic Reaction Center Complex Proteins; C094744:light-harvesting complex 1, Rhodospirillum; C017251:1,2-oleoylphosphatidylcholine; D005947:Glucose