Several stresses cause additional activation to the glucose-stimulated plasma membrane H+-ATPase activity of yeast, but it is not clear how this is achieved. We recently reported that cells lacking the integral plasma membrane heat shock protein Hsp30 display enhanced increases in plasma membrane H+-ATPase activity with either heat shock or weak organic acid stress (Piper, P.W., Ortiz-Calderon, C., Holyoak, C., Coote, P. and Cole, M. (1997) Cell Stress and Chaperones 2, 12-24), indicating that the stress induction of Hsp30 acts to reduce stress stimulation of the H+-ATPase. In this study it is shown that Hsp30 acts to reduce the Vmax of H+-ATPase in heat shocked cells. Its action is lost with deletion of the C-terminal 11 amino acids of the H+-ATPase, a deletion that does not abolish the stress stimulation of this enzyme. Mutation of the Thr-912 residue within the C-terminal domain of H+-ATPase, a potential site of phosphorylation by the Ca2+-calmodulin-dependent protein kinase, also abolishes any effect of Hsp30. Hsp30 may therefore be acting on a Thr-912 phosphorylated form of the H+-ATPase.