Wheat-germ lectin structure

The 93- to 95-residue neurophysins act as carriers for vasopressin and ocytocin, forming specific complexes with them. Neurophysins contain 14 cysteine residues, which form seven disulfide bonds. There is a striking similarity in sequence between the neurophysins, snake venom toxins, a wheat germ lectin (agglutinin), a ragweed pollen allergen, and a small plant protein called hevein. On the basis of the alignment of cysteine residues, Drenth proposed that all of these proteins have a disulfide-linked core whose structure is shown in Fig. 30-16. 

C.S. Wright. 1992. Crystal structure of a wheat germ agglutinin/ glycophorin-sialoglycopeptide receptor complex Structural basis for cooperative lectin-cell binding J Biol. Chem. 267 14345-14352. (PubMed)... 

Truncated structures of sialic acids show the moieties required for binding to various sialic acid-specific lectins. LFA is Limaxfiavus agglutinin [231] and WGA is wheat germ agglutinin [5]. The binding importance of sialic acid C-4 and C-7 hydroxyls for MAG has not yet been determined. R indicates that LFA tolerates a variety of equatorial bulky substituents on the C-5 W-acyl group of sialic acids... 

Wheat germ agglutinin is the most studied of these lectins and also one of the most useful in its biomedical application. The lectin is a homodimer composed of subunits of Mr = 23 600 Da which dissociates into monomers at acid pH. It is a pure, metal-free protein devoid of carbohydrate residues, which is isolated as a mixture of four isolectins differing in electrophoretic mobility [116,117]. The lectin has been sequenced (171 amino acids per polypeptide chain) and its X-ray crystallographic structure determined at a 2.0 A resolution [118,119]. 

The crystal structure of the non-covalent complex formed between wheat-germ agglutinin and A-acetylneuraminic acid has been determined from an electron density difference map at 2.8 A resolution. The map exhibits two strong binding sites on the lectin dimer molecule, which are located in corresponding faces of the protomer-protomer interface. The iV-acetylneuraminic acid molecule is oriented... 

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