About the Lectin

Purified wheat germ agglutinin is perhaps the most widely used lectin. It has been used in diagnostic microbiology for the rapid identification of Neisseria gonorrhoeae ^1,2,3 . WGA agglutinates a high percentage of Giardia lamblia trophozoites, and the FITC labeled lectin stains the majority of trophozoites as well ⁴ . Fungal cell walls are composed primarily of chitin, and WGA has been used to detect the distribution of chitin in these organisms. WGA has also been shown to inhibit fungal growth in Trichoderma viride, indicating the importance of the cell wall component in this process ⁵ . Conjugated forms of the lectin have been used as a retrograde neuronal tracer ^6,7,8 although it has been reported that the lectin will transport in the anterograde direction as well ⁹ . Recently, EY Laboratories has produced two conjugates of WGA; one form is transported but stops at the synapse, while another form transports and crosses the synaptic junction ¹⁰ . Purified WGA binds to a number of glycoconjugates including bovine fetuin ¹¹ , Staph. aureus acid and a peptidoglycan produced by Micrococcus luteus ¹² , the capsular polysaccharide of pneumococcus S14 ¹³ , and human glycosphingolipids and carcinoembryonic antigens ¹⁴ . WGA will also precipitate ovine submaxillary mucin (OSM) and desialylated OSM ¹¹ . A column of immobilized WGA was shown to bind a much larger fraction of prostatic acid phosphase from prostate carcinoma than from benign prostate hyperplasia tissue, although enzyme preparations from the two sources were catalytically indistinguishable ¹⁵ . WGA reacts strongly with the chitobiose core of asparagine linked oligosaccharides, specifically with the Man β(1,4)GlcNAc β(1,4)GlcNAc trisaccharide ¹⁶ . It is important to note that other lectins sharing similar carbohydrate binding properties with WGA are not necessarily useful in all of the applications described above. This fact shows that lectins, WGA in particular, can have basic similarities but still have a number of unique properties.

It has been suggested that WGA also has an affinity for N-acetylneuraminic acid (Neu5Ac , sialic acid), since the binding of WGA to animal cells can be decreased or eliminated by treatment of the cells with neuraminidase. Oddly, neuraminidase treated human erythrocytes react slightly stronger with the lectin. The precipitation of OSM by the lectin is indicative of the sialic acid specificity since OSM is devoid of GlcNAc residues. Desialylated OSM contains terminal O-linked α–GalNAc residues. WGA also recognizes this carbohydrate, but to a lesser degree than either GlcNAc or sialic acid. Several analogs of Neu5Ac are also inhibitors of WGA but N-acetylglycolylneuraminic acid (Neu5Gc) is not inhibitory. Both GlcNAc and Neu5Ac are commonly found in cellular glycoproteins and in various tissue types. For this reason conjugates of WGA have been widely applied to histochemical staining applications ¹⁷ and for the purification of glycoproteins and glycopeptides ^18,19,20 . WGA has also been applied to studies of the neuronal cell adhesion molecule ²¹ .

WGA is the first lectin to have its complete amino acid sequence determined ²² The purified lectin is actually a mixture of several isolectins ²³ Recent experiments using high performance hydrophobic interaction chromatography indicate that there are as many as six isolectins ²⁴ . Native WGA can be modified by succinylation to yield a lectin which no longer reacts with sialic acid but which still retains its other carbohydrate binding properties ²⁵ .

The X-ray crystal structure of WGA complexed with sialic acid-containing oligosaccharides has been solved, showing that two molecules of sialyllactose binds to a molecule of WGA ^26-28 .

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