Proton pump interactor, isoform 1 (PPI1) is a novel interactor of the C-terminus of Arabidopsis thaliana plasma membrane H+-ATPase (EC 3.6.3.6) (Morandini P, Valera M, Albumi C, Bonza MC, Giacometti S, Ravera G, Murgia I, Soave C & De Michelis MI (2002) Plant J 31, 487–497). We produced two fusion proteins consisting of, respectively, the first 88 amino acids or the entire protein deleted of the last 24 hydrophobic amino acids, and we show that the latter protein has a threefold higher affinity for the H+-ATPase. PPI1-induced stimulation of H+-ATPase activity dramatically decreased with the increase of pH above pH 6.8, but became largely pH-independent when the enzyme C-terminus was displaced by fusicoccin-induced binding of 14-3-3 proteins. The latter treatment did not affect PPI1 affinity for the H+-ATPase. These results indicate that PPI1 can bind the H+-ATPase independently of the C-terminus conformation, but is not able to suppress the C-terminus auto-inhibitory action. [ABSTRACT FROM AUTHOR]