ABP(57) is an auxin-binding protein that possesses receptor function. In this study, a protocol for ABP(57) purification was developed on the basis of cross-reactivity shown between ABP(57) and antisera raised against bovine serum albumin, which enabled us to purify ABP(57) with a high yield and to further characterize it. ABP(57) activates plant plasma membrane H(+)-ATPase (PM H(+)-ATPase) via direct interaction. The binding of indole-3-acetic acid (IAA) to the primary binding site on ABP(57) caused a marked increase in the affinity of ABP(57) for PM H(+)-ATPase, which was accompanied by a change in ABP(57) conformation. Meanwhile, additional IAA binding to the secondary site on ABP(57) nullified the initial effect without inducing further conformational change. When ABP(57) with IAA occupying only the primary site interacted with PM H(+)-ATPase, no IAA could access the secondary site. These results suggest that IAA-induced biphasic alteration in the affinity of ABP(57) for PM H(+)-ATPase correlates with a bell-shaped dose response of the enzyme to IAA. There is also a possibility that, whereas the stimulation phase of the response is associated with a conformational change of ABP(57), the destimulation phase probably results from hindrance arising directly from the presence of IAA at the secondary site.