Stalk segment 4 of the yeast plasma membrane H+-ATPase -: Mutational evidence for a role in the E1-E2 conformational change

In the P-2-type ATPases, there is growing evidence that four alpha-helical stalk segments connect the cytoplasmic part of the molecule, responsible for ATP binding and hydrolysis, to the membrane-embedded part that mediates cation transport. The present study has focused on stalk segment 4, which displays a significant degree of sequence conservation among P-2-ATPases. When site-directed mutants in this region of the yeast plasma membrane H+-ATPase were constructed and expressed in secretory vesicles, more than half of the amino acid substitutions led to a severalfold decrease in the rate of ATP hydrolysis, although they had little or no effect on the coupling between hydrolysis and transport. Strikingly, mutant ATPases bearing single substitutions of 13 consecutive residues from Ile-359 through Gly-371 were highly resistant to inorganic orthovanadate, with IC50 values at least 10-fold above those seen in the wild-type enzyme. Most of the same mutants also displayed a significant reduction in the K-m for MgATP and an increase in the pH optimum for ATP hydrolysis. Taken together, these changes in kinetic behavior point to a shift in equilibrium from the E-2 conformation of the ATPase toward the E-1 conformation. The residues from Ile-359 through Gly-371 would occupy three full turns of an alpha-helix, suggesting that this portion of stalk segment 4 may provide a conformationally active link between catalytic sites in the cytoplasm and cation-binding sites in the membrane.