Involvement of a carboxyl group in the interaction between succinate dehydrogenase and its membrane-anchoring protein (QPs) fraction.

The involvement of the carboxyl groups in the membrane-anchoring protein (QPs) in reconstitution of succinate dehydrogenase to form succinate-ubiquinone reductase is studied by using a carboxyl group modifying reagent, dicyclohexylcarbodiimide (DCCD). Inactivation of QPs by DCCD is found to be dependent on the temperature, pH, detergent, and DCCD concentration used. When QPs is treated with 300 molar excess DCCD at room temperature for 10 min, about 90% of the original reconstitutive activity is lost. When intact or reconstituted succinate-ubiquinone reductase formed from reconstitutively active succinate dehydrogenase and QPs is treated with DCCD under the same conditions, no loss of succinate-ubiquinone reductase activity is observed. However, when a mixture of reconstitutively inactive succinate dehydrogenase and QPs is treated with DCCD before being reconstituted with active succinate dehydrogenase, an inactivation behavior similar to that with QPs alone is observed. These results indicate that DCCD modifies the carboxyl groups of QPs which are essential for the interaction with succinate dehydrogenase to form succinate-ubiquinone reductase. Inactivation of QPs by DCCD parallels the incorporation of DCCD into QPs. About two carboxyl groups per molecule of QPs are essential for the interaction with succinate dehydrogenase. These essential carboxyl groups are located in the smaller subunit (Mr 13,000) of QPs. Modification of QPs by DCCD also alters the heme environment of cytochrome b560.