IDENTIFICATION OF THE UBIQUINONE-BINDING DOMAIN IN QPS1 OF SUCCINATE-UBIQUINONE REDUCTASE

An azidoubiquinone derivative, 3-azido-2-methyl-5-methoxy[H-3]-6-decyl-1,4-benzoquinone ([H-3]azido-Q), was used to study the ubiquinone-protein interaction and to identify ubiquinone-binding proteins in bovine heart mitochondrial succinate-ubiquinone reductase, When the reductase was incubated with [H-3]azido-Q and illuminated with long wavelength UV light, the decrease in the enzymatic activity correlated with the amount of azido-Q incorporated into the protein. When the illuminated, [H-3]azido-Q-treated reductase was extracted with organic solvent and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, radioactivity was found primarily in the QPs1 subunit. The [H-3]azido-Q-labeled QPs1 was purified from labeled reductase by a procedure involving ammonium sulfate fractionation, dialysis, organic solvent extraction, lyophilization, preparative sodium dodecyl sulfate polyacrylamide gel electrophoresis, and cold acetone precipitation, The purified, [H-3]azido-Q-labeled QPs1 protein was subjected to reductive carboxymethylation prior to digestion by trypsin, One azido-Q-linked peptide, with a retention time of 66.9 min, was obtained by high performance liquid chromatographic separation, The partial amino-terminal sequence of this peptide is GLTISQL-, indicating that this tryptic peptide comprises amino acid residues 113-140 of the revised amino acid sequence of QPs1. The Q-binding domain, using the proposed structure of QPs1, is probably located in the stretch connecting transmembrane helices 2 and 3 that extrude from the surface of the M side of the inner membrane.