THE CARBOXYL-TERMINUS OF THE SACCHAROMYCES-CEREVISIAE SUCCINATE-DEHYDROGENASE MEMBRANE SUBUNIT, SDH4P, IS NECESSARY FOR UBIQUINONE REDUCTION AND ENZYME STABILITY

The succinate dehydrogenase (SDH) of Saccharomyces cerevisiae is compo sed of four nonidentical subunits encoded by the nuclear genes SDH1, S DH2, SDH3, and SDH4. The hydrophilic subunits, SDH1p and SDH2p, compri se the catalytic domain involved in succinate oxidation. They are anch ored to the inner mitochondrial membrane by two small, hydrophobic sub units, SDH3p and SDH4p, which are required for electron transfer and u biquinone reduction. Comparison of the deduced primary sequence of the yeast SDH4p subunit to SDH4p subunits from other species reveals the presence of an unusual 25-30 amino acid carboxyl-terminal extension fo llowing the last predicted transmembrane domain. The extension is pred icted to be on the cytoplasmic side of the inner mitochondrial membran e. To investigate the extension's function, three truncations were cre ated and characterized. The results reveal that the carboxyl-terminal extension is necessary for respiration and growth on nonfermentable ca rbon sources, for ubiquinone reduction, and for enzyme stability. Comb ined with inhibitor studies using a ubiquinone analog, our results sug gest that the extension and more specifically, residues 128-135 are in volved in the formation of a ubiquinone binding site. Our findings sup port a two ubiquinone binding site model for the S. cerevisiae SDH.