A lectin is any protein incorporating one or more (frequently two) sites highly specific for carbohydrate binding, occurring in the tissues of most living organisms. Lectins are of interest because of their wide variety of properties and potential applications for example in pharmacology, immunology, cancer therapy and agriculture, and occur also as a component of toxins such as type II RIP's. Protein-carbohydrate interactions are as yet not well understood (Toone, 1994). One object of this research is to characterise protein topographies responsible for driving molecular mechanisms involved in sugar complexation.
Nettle lectin (from Urtica dioica), one of the smallest known lectin proteins, has been crystallized both with and without bound NN'N''- chitotriose, and its structure is currently being determined. The amino acid sequence indicates the presence of hevein domains similar to the sub-structures observed in wheatgerm agglutinin, and we have modelled the domains of nettle lectin on the basis of WGA coordinates. The alignment also serves to indicate the residues most likely to be involved in forming the sugar binding sites.
The type II RIP mistletoe lectin ML1 (from viscum album) has been crystallized and its structure determined at low resolution by molecular replacement using ricin (30-40% homologous) coordinates. It is of importance through its use in homeopathy and potentially in medical applications, being more toxic than ricin. The research program involves studies of other ML isolectins, the separate A and B chains, the reconstituted A and B heterodimer and development of an expression system. We are also looking at the related structures of ricin agglutinin and mormordin, a type I RIP.
Crystals have recently been obtained of the lectin from edible mushroom (ostreatus pleuratus). This is of interest as a phosphatase activity enhancement enzyme with potential applications in agricultural fertilization. Other lectins being purified or crystallized include daffodil (narcissus sp carlton) known to bind to HIV-1 GP120 coat glycoprotein, laba (a bark lectin from laburnum with high toxicity) and vicia villosa agglutinin.