Two isoforms of soluble auxin receptor in rice (Oryza sativa L.) plants:: Binding property for auxin and interaction with plasma membrane H+-ATPase

An auxin receptor protein, isolated from the soluble fractions of rice shoots and roots, was characterised in terms of the affinity and specificity for IAA and the modulating effect on H+-ATPase of plant plasma membrane. The receptor protein gives a biphasic binding isotherm for IAA, indicating the existence of the primary and secondary binding sites. The predominant isoform of the receptor in roots shows much higher affinity to IAA compared with that in shoots. Being monomeric protein with about the same molecular mass (57-58 kDa) and showing a similar chromatographic behaviour, both isoforms mediate IAA-induced modulation of the plasma membrane H+-ATPase in the respective IAA concentration ranges separated by ca. 3 orders of magnitudes (10(-)10-10(-)7 M vs. 10(-7)-10(-4) M). Analysis of kinetic data of the H+-ATPase activity revealed that the receptor per se functions as an effector of the enzyme, causing a decrease in K-m and an increase in V(m)ax through protein-protein interaction at a 1:1 ratio. Further, it appeared that, while IAA does not affect by itself the kinetic parameters of the H+-ATPase, the auxin exerts its effect via the receptor, biphasically regulating the efficiency of the effector molecule probably by inducing two-phase conformational changes that involve IAA binding to two separate binding sites. It was also found that other active auxins examined, such as indole-3-propionic acid, 1-naphthalene acetic acid and 2,4-dichlorophenoxyacetic acid, do not work together with the receptor to elicit the same response of the H+-ATPase as seen with IAA.