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Mutations in a helix-1 motif of the ATP synthase c- subunit of Bacillus pseudofirmus OF4 cause functional deficits and changes in c -ring stability and mobility on SDS-PAGE

, Abstract The ATP synthase of the alkaliphile Bacillus pseudofirmus OF4 has a tridecameric c- subunit rotor ring. Each c- subunit has an AxAxAxA motif near the center of the inner helix, where neutralophilic bacteria generally have GxGxGxG. Here, we studied the impact of four single and six multiple Ala-to-Gly chromosomal mutations in the A16xAxAxA22 motif on the capacity for non-fermentative growth and, for most of the mutants, on ATP synthesis by ADP + P i -loaded membrane vesicles at pH 7.5 and 10.5. SDS-PAGE analyses of the holo-ATP synthases were used to probe stability of the mutant c- rotors and mobility properties of the c- rotors as well as the monomeric c- subunits that are released from them by trichloroacetic acid treatment. Mutants containing an Ala16-to-Gly mutation exhibited the most severe functional defects. On SDS-PAGE, most of the mutant c- monomers exhibited increased mobility relative to the WT c -subunit, but among the intact c- rings, only Ala16-to-Gly containing mutants exhibited significantly increased mobility relative to the WT c- ring. The hypothesis that these c- rings have a decreased c-subunit stoichiometry is still untested but the functional impact of an Ala16-to-Gly mutation clearly depended upon additional Ala-to-Gly mutation(s) and