title: The role of the supernumerary subunit of Rhodobacter sphaeroides cytochrome bc1 complex.
abstract: The smallest molecular weight subunit (subunit IV), which contains no redox prosthetic group, is the only supernumerary subunit in the four-subunit Rhodobacter sphaeroides bc1 complex. This subunit is involved in Q binding and the structural integrity of the complex. When the cytochrome bc1 complex is photoaffinity labeled with [3H]azido-Q derivative, radioactivity is found in subunits IV and I (cytochrome b), indicating that these two subunits are responsible for Q binding in the complex. When the subunit IV gene (fbcQ) is deleted from the R. sphaeroides chromosome, the resulting strain (RSdeltaIV) requires a period of adaptation before the start of photosynthetic growth. The cytochrome bc1 complex in adapted RSdeltaIV chromatophores is labile to detergent treatment (60-75% inactivation), and shows a four-fold increase in the Km for Q2H2. The first two changes indicate a structural role of subunit IV; the third change supports its Q-binding function. Tryptophan-79 is important for structural and Q-binding functions of subunit IV. Subunit IV is overexpressed in Escherichia coli as a GST fusion protein using the constructed expression vector, pGEX/IV. Purified recombinant subunit IV is functionally active as it can restore the bc1 complex activity from the three-subunit core complex to the same level as that of wild-type or complement complex. Three regions in the subunit IV sequence, residues 86-109, 77-85, and 41-55, are essential for interaction with the core complex because deleting one of these regions yields a subunit completely or partially unable to restore cytochrome bc1 from the core complex.
authors.affiliation: Department of Biochemistry and Molecular Biology, Oklahoma State University, Stillwater 74074, USA. Linda@okway.okstate.edu
authors.forename: L
authors.initials: L
authors.lastname: Yu
authors.forename: S C
authors.initials: SC
authors.lastname: Tso
authors.forename: S K
authors.initials: SK
authors.lastname: Shenoy
authors.forename: B N
authors.initials: BN
authors.lastname: Quinn
authors.forename: D
authors.initials: D
authors.lastname: Xia
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D002384:Catalysis; D019281:Dimerization; D014450:Electron Transport Complex III / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D017353:Gene Deletion; D005816:Genetic Complementation Test; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D011487:Protein Conformation; D012242:Rhodobacter sphaeroides / Q000201:enzymology* / Q000235:genetics; D013329:Structure-Activity Relationship; D014451:Ubiquinone / Q000031:analogs & derivatives / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D014451:Ubiquinone; D014450:Electron Transport Complex III; C003741:ubiquinol