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.
Auxin-binding proteins (ABPs)1 are a class of low abundance proteins in plants that bind active auxins with high affinity and specificity. As a result of ABP-auxin binding, ABP might initiate the auxin signal pathways leading to various cellular responses and thus have a plant hormone receptor function. Extensive studies have led to the identification of a number of ABPs in both membrane and soluble fractions, of which the best characterized in terms of cellular location, biochemical nature, and putative receptor function is ABP1 (for reviews, see Refs. 1-5). The main natural auxin in most plants is indole-3-acetic acid (IAA), which appears to exert many effects in plants, including, most importantly, cell elongation. In fact, the term "auxin" is used to describe chemical substances that stimulate elongation growth in coleoptiles and many stems.There is no clear explanation of the mechanism by which auxin regulates cell growth. The immediate effect of exposure of plant tissues to auxin is proton excretion, occurring within minutes. The resulting apoplastic acidification provides a favorable condition for cell wall loosening, which could be an early part of auxin-induced cell expansion (1, 3, 6). H ϩ -ATPase activity of the plasma membrane (PM) in plants is responsible for proton extrusion from the cell. Therefore, the acidification of the cell wall space is thought to result from activation of the electrogenic H ϩ -ATPase in some manner. There is evidence that auxin stimulates plant PM H ϩ -ATPase, which is correlated with an increased affinity of the enzyme for ATP (7) but does not involve a direct effect of auxin on the enzyme itself (8, 9).We have previously isolated two isoforms of a soluble ABP in rice plants, one fr...