A Soluble Auxin-binding Protein, ABP57

Abstract

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...

Overexpression of the Auxin Binding PROTEIN1 Modulates PIN-Dependent Auxin Transport in Tobacco Cells

“…It has been shown that the auxin-regulated activation of H + -ATPase through phosphorylation of the penultimate threonine is indeed independent on TIR1/AFB-dependent auxin signalling, thus supporting the involvement of ABP1 [19]. Furthermore in rice direct interaction between ABP 57 and H + -ATPase has been proven in vitro [20]. It has been suggested that the C-terminus of ABP1 is likely to convey the signal for auxin-induced H + extrusion by H + -ATPase into the cell wall, and that the concurrent K + influx is followed by water uptake and turgor-driven cell expansion [21].…”

Auxin polar transport is essential for the development of zygote and embryo in Nicotiana tabacum L. and correlated with ABP1 and PM H+-ATPase activities

“…By using ATP as the energy source, PM H + -ATPase pumps H + from cytoplasm to the cell exterior, thus forming an electrochemical gradient across the PM and constituting the driving force for nutrient and K + uptake into the cells (Bouche-Pillon et al , 1994; Sondergaard et al , 2004). PM H + -ATPase is regarded as a downstream molecule of the auxin-dependent signal transduction pathway (Kim et al , 2001; Rober-Kleber et al , 2003; Sondergaard et al , 2004; Christian et al , 2006). Evidence has shown that the final target protein in auxin-induced cell acidification and elongation is the PM H + -ATPase, for auxin can improve the transcription and translation of PM H + -ATPase as well as the activity of this protein (Harper et al , 1994; Frías et al , 1996; Rober-Kleber et al , 2003; Sondergaard et al , 2004).…”

Interaction of zinc and IAA alleviate aluminum-induced damage on photosystems via promoting proton motive force and reducing proton gradient in alfalfa

“…In the photosynthetic process, H + -ATPase extrudes proton to create an electrochemical gradient (proton gradient, ΔpH) between the lumen and stroma by consuming ATP, which regulates electron transfer in photosystems and finally affects CO 2 assimilation [16,26,27]. Auxin functions as a systemic signaling compound affecting H + transfer in cells and H + secretion from roots by activating plasma membrane (PM) H + -ATPase [28,29]. Thus, IAA and H + -ATPase play an important role in regulating membrane potential and proton gradient (ΔpH).…”