Auxin binding protein

ABP1 was first detected in 1972 in crude membrane preparations of etiolated maize cole-optiles and purified in 1985. It has since been found in all green plants, including the bryophytes and pteridophytes, and in many tissues. The maize ABP1 cDNA encodes a 163-amino-acid protein and the mature protein has a molecular weight of 22 kDa containing a high-mannose oligosaccharide. Interestingly, it is localised at the endoplasmic reticulum (ER) and, like all proteins destined for delivery to the ER, carries a signal sequence 38 residues long at the C-terminus. To function as a receptor it is thought to associate with a membrane-bound 'docking' protein.

A comparison of ABP1 sequence data from a range of species indicated three highly conserved sequences, termed boxes A, B and C. Antibodies raised to box A were shown to have auxin-tike activity, indicating an important role at the binding site of these 15 amino acids. The exact roles of boxes B and C remain uncertain.

Edgerton and colleagues (1994) studied auxin binding to ABP1 in isolated maize micro-somes and found the characteristics very similar to those predicted solely on biological data by Katekar (1979) (Figure 7.4).

More recently (2001), the protein has been crystallised and a role for a metal ion has been suggested as an ideal carboxylic acid coordination group. The metal ion, likely to be Zn2+, is complexed to three histidine and one glutamic acid residue in box A.

All available data, reviewed by Napier et al. (2002), have shown ABP1 to be active to auxins at the surface of the plasma membrane, despite carrying the ER sequence. These include the early responses to auxin action, such as promoting ion fluxes.

The suggestion that herbicides might bind to this site and alter plasma membrane function has been proposed by Hull and Cobb (1998) . In their study, highly purified plasma

Figure 7.4 A topographic model of the auxin receptor viewed from the side. This model proposes that the auxin receptor possesses regions to accept a carboxyl group, the methylene carbon of IAA (a), the indole ring (Ar1, Ar2) and adjacent areas to the indole ring (d/e) (after Katekar, 1979).

CH3 i 3 O-C-COOCH

Cl methyl ester of dichlorprop

O-C-COOCH3

Cl diclofop-methyl

Figure 7.5 Structures of the methyl ester of dichlorprop (which controls dicotyledonous weeds in monocotyledonous crops) and diclofop-methyl (which controls monocotyledonous weeds in dicotyledonous crops).

membrane vesicles were isolated from the monocotyledonous weed black-grass (Alopecurus myosuroides Huds.) and the dicotyledonous crop sugar beet (Beta vulgaris) and H+-efflux measured in the presence and absence of herbicides. They found that while auxin-type herbicides in general did not affect H+-efflux, the aryloxyphenoxypropionate diclofop-methyl was highly inhibitory and 2,4-D gave a slight increase in activity. Auxins are known to antagonise the action of these 'fop' herbicides in the field when present in mixtures. Since the fops are able to depolarise the plasma membrane potential by inhibiting ATP-ase, perhaps the auxin repolarises the potential by stimulating ATP-ase activity and so restores cytoplasmic homeostasis. It may also be speculated that an interaction of these herbicidal molecules could occur at ABP1. To speculate further, perhaps this interaction could also account for the differences in selectivity observed in the field between two similar herbicides (Figure 7.5).

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