Auxins receptors/binding proteins

The existence of auxin receptors/binding proteins, the likely target sites of auxin herbicides, has been the subject of considerable research in recent years (for reviews, see Refs. 4, 173, and 179-181). There seems little... 

Napier, R. M., and M. A. Venis, From auxin-binding protein to plant hormone receptor Trends Biochem. Sci. 16 72— 75, 1991. 

Auxin Binding Protein Receptor 1LR5 His 1 HiSj 3 Glu 42 His H20... 

Auxins are synthesized in apical buds (at the tip) of growing shoots. They stimulate growth of the main shoot and inhibit lateral shoot development. A class of auxin-binding membrane proteins may represent auxin receptors. Auxin action involves pumping protons out of the cell, possibly in conjunction with a membrane ATPase. 

There are three major classes of synthetic auxins the aryl acetic acids, which include indoleacetic acid (lAA) itself (Fig. 1 structure 2-1) and 1-naphthyl acetic acid (2-4), phenoxy acetic acids, represented by 2,4-dichlorophenoxy acetic acid (2,4-D) (2-7) and the benzoic acids, e.g. 2,3,6-trichloro benzoic acid (2-9). It remains the case however, that there is no stmcture-activity proposal which satisfactorily covers all molecules which are known to have auxin activity. As is well known, auxins have multiple effects, including promotion of cell elongation, cell division and gene expression. There may well be different receptors for each effect, and differences between receptors. An auxin binding protein (ABP), has now been characterised, and is possibly a receptor controlling cell elongation [3]. 

Further differences lie in the reported affinities for the auxin-binding proteins K(j for tobacco is 6 X 10 M [ 15] K s for pea are 10 M and 6 X 10 M [3] while K s for mung bean are 3X 10 M [12] and 10" M [11]. The latter values represent affinities that are 50- to 500-fold lower than for pea and tobacco material. We therefore think it is premature to consider the auxin-binding proteins from mung bean as true receptors. 

The next step in the perception-transduction mechanism of auxin should be more specific. We plan to use the reinitiating nuclei from tobacco in reconstitution experiments with partially purified auxin-binding protein. We have shown in vivo that several genes are specifically induced by auxin. We hope to be able to show the same induction in vitro by addition of auxin-receptor complex to nuclei isolated from auxin-starved cells, where these genes are normally not expressed. 

Without doubt, auxin binding sites in maize membranes have been studied the most extensively in a number of laboratories. Several lines of correlative evidence suggest that the sites have a receptor function [21 ] as do the more direct observations of inhibition of auxin-induced growth by an affinity-purified antibody to the auxin binding protein [12] and functional reconstitution of binding protein and ATPase in a bilayer lipid membrane [18]. The binding sites can be readily solubilized from the membranes using an acetone procedure and a native Mr of 40-45 kDa is obtained on gel filtration [19]. 

Matthyssee and Phillips (20) isolated two nuclear proteins, from tobacco cells, that bound specifically to 2,4-D. Receptor proteins for auxins, kinetins, and GA have been found (21). Sub-cellular fractions from bean leaves were recently shown to bind abscisic acid (22). Preliminary experiments (22) indicated that maximum ABA binding activity coincides with the activities of membrane-bound Mg -dependent, K+-stimulated ATPase and glucan synthetase. Table I of Biswas and Roy (21) lists hormone receptor proteins reported in plant tissue. For a protein to qualify as a receptor molecule, it should have a high stereo-specific binding capacity (Kd 10 6 to 10 SM) for its particular hormone. In com coleoptiles, both IAA and NAA are equally effective in inducing cell elongations fractions of plasma membrane and endoplasmic reticular membrane contain receptor proteins with Kd values of 10 M to 10 M for auxins (5, 18). When one considers procedural... 

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