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J Biol Chem, Vol. 275, Issue 20, 15287-15294, May 19, 2000

Subunit IV of Cytochrome bc₁ Complex from Rhodobacter sphaeroides
LOCALIZATION OF REGIONS ESSENTIAL FOR INTERACTION WITH THE THREE-SUBUNIT CORE COMPLEX^*

Shih-Chia Tso, Sudha K. Shenoy, Byron N. Quinn, and Linda Yu

From the Department of Biochemistry & Molecular Biology, Oklahoma State University, Stillwater, Oklahoma 74078

Recombinant subunit IV mutants which identify the regions essential for restoration of bc₁ activity to the three-subunit core complex of Rhodobacter sphaeroides were generated and characterized. Four C-terminal truncated mutants: IV(1-109), IV(1-85), IV(1-76), and IV(1-40) had 100, 0, 0, and 0% of reconstitutive activity of the wild-type IV, indicating that residues 86-109 are essential. IV(1-109) is associated with the core complex in the same manner as the wild-type IV while mutants IV(1-85), IV(1-76), and IV(1-40) do not associate with the core complex, indicating that subunit IV requires its transmembrane helix region (residues 86-109) for assembly into the bc₁ complex. Since GST-IV(86-109) fusion protein has little reconstitutive activity, some region(s) in residues 1-85 are required for bc₁ activity restoration after subunit IV is incorporated into the complex through the transmembrane helix, presumably by interaction with cytochrome b in the core complex. The interacting regions are identified as residues 41-53 and 77-85, since mutants IV(21-109), IV(41-109), IV(54-109), and IV(77-109) had 95, 98, 53, and 53% of the reconstitutive activity of the wild-type IV. These two interacting regions are on the cytoplasmic side of the chromatophore membrane and closed to the DE loop and helix G of cytochrome b, respectively.

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