Membrane segment 5 (M5) is thought to play a direct role in cation transport by the sarcoplasmic reticulum Ca 2؉ -ATPase and the Na ؉ ,K ؉ -ATPase of animal cells. In this study, we have examined M5 of the yeast plasma membrane H ؉ -ATPase by alanine-scanning mutagenesis. Mutant enzymes were expressed behind an inducible heat-shock promoter in yeast secretory vesicles as described previously (Nakamoto, R. K., Rao, R., and Slayman, C. W. (1991) J. Biol. Chem. 266, 7940 -7949). Three substitutions (R695A, H701A, and L706A) led to misfolding of the H ؉ -ATPase as evidenced by extreme sensitivity to trypsin; the altered proteins were arrested in biogenesis, and the mutations behaved genetically as dominant lethals. The remaining mutants reached the secretory vesicles in sufficient amounts to be characterized in detail. One of them (Y691A) had no detectable ATPase activity and appeared, based on trypsinolysis in the presence and absence of ligands, to be blocked in the E 1 -to-E 2 step of the reaction cycle. Alanine substitution at an adjacent position (V692A) had substantial ATPase activity (54%), but was likewise affected in the E 1 -to-E 2 step, as evidenced by shifts in its apparent affinity for ATP, H ؉ , and orthovanadate. Among the mutants that were sufficiently active to be assayed for ATP-dependent H ؉ transport by acridine orange fluorescence quenching, none showed an appreciable defect in the coupling of transport to ATP hydrolysis. The only residue for which the data pointed to a possible role in cation liganding was Ser-699, where removal of the hydroxyl group (S699A and S699C) led to a modest acid shift in the pH dependence of the ATPase. This change was substantially smaller than the 13-30-fold decrease in K ؉ affinity seen in corresponding mutants of the Na ؉ ,K ؉ -ATPase (Arguello, J. M., and Lingrel, J. B (1995) J. Biol. Chem. 270, 22764 -22771). Taken together, the results do not give firm evidence for a transport site in M5 of the yeast H ؉ -ATPase, but indicate a critical role for this membrane segment in protein folding and in the conformational changes that accompany the reaction cycle. It is therefore worth noting that the mutationally sensitive residues lie along one face of a putative ␣-helix.
