The Role of the Supernumerary Subunit of Rhodobactersphaeroides Cytochrome bc
1 Complex |
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Authors: | Linda Yu Shih-Chia Tso Sudha K Shenoy Byron N Quinn Di Xia |
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Institution: | (1) Department of Biochemistry and Molecular Biology, OklahomaState University, Stillwater, Oklahoma, 74074;(2) Laboratory of Cell Biology, National Cancer Institute, NationalInstitutes of Health, Bethesda, Maryland, 20892 |
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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 bc
1 complex.This subunit is involved in Q binding and the structural integrity of the complex. When thecytochrome bc
1 complex is photoaffinity labeled with 3H]azido-Q derivative, radioactivity isfound in subunits IV and I (cytochrome b), indicating that these two subunits are responsiblefor Q binding in the complex. When the subunit IV gene (fbcQ) is deleted from the R.sphaeroides chromosome, the resulting strain (RS IV) requires a period of adaptation beforethe start of photosynthetic growth. The cytochrome bc
1 complex in adapted RS IVchromatophores is labile to detergent treatment (60–75% inactivation), and shows a four-fold increasein the K
m for Q2H2. The first two changes indicate a structural role of subunit IV; the thirdchange supports its Q-binding function. Tryptophan-79 is important for structural andQ-binding functions of subunit IV. Subunit IV is overexpressed in Escherichia coli as a GSTfusion protein using the constructed expression vector, pGEX/IV. Purified recombinant subunitIV is functionally active as it can restore the bc
1 complex activity from the three-subunit corecomplex to the same level as that of wild-type or complement complex. Three regions in thesubunit IV sequence, residues 86–109, 77–85, and 41–55, are essential for interaction withthe core complex because deleting one of these regions yields a subunit completely or partiallyunable to restore cytochrome bc
1 from the core complex. |
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Keywords: | nonredox subunit the supernumerary subunit subunit interaction activity restoration |
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