IDGFs as lectins

An intriguing possibility is that the IDGFs could interact as lectins with the insulin receptor itself, which is already known to bind other lectins. Wheat germ agglutinin (WGA), a mitogenic lectin for which the receptor sugar is N-acetyl-glucosamine, binds to the mammalian insulin receptor (see http:\\ www.vectorlabs.com\frames\F~KTectins.htm), suggesting that its mitogenic activity could be mediated in part through the insulin receptor pathway. Further evidence supporting interaction between mitogenic lectins and the insulin receptor comes from demonstrations that some lectins compete with insulin for binding to cells expressing the insulin receptor (Rouiller et al 1986) and glycosylation mutants show altered insulin binding (Podskalny et al 1984). Furthermore, when one of the acceptor asparagines for glycosylation in the mammalian insulin receptor is altered by site-directed mutagenesis, the mutant receptor shows a defect in ligand-induced internalization, major alterations in tyrosine kinase activity and autophosphorylation, and is unable to transduce the signal for either glycogen or DNA synthesis (Leconte et al 1994). All N-linked oligosaccharides have the same pentasaccharide core Mannose a-1,3 (Mannose a-1,6) Mannose ^-1,4 GlcNAc ^-1,4 GlcNAc, which is recognized by numerous lectins (Di Virgilio 1997). Since chitin is a ^-1,4-linked GlcNAc linear polymer, it seems likely that chitin-specific lectins might also interact with N-linked oligosaccharides.

A role for lectins in regulation of the Drosophila insulin receptor is suggested by the fact that the receptor showed increased basal autophosphorylation when purified using a lentil (mannose-specific) lectin column (Ruan et al 1995). However, the Drosophila insulin receptor has a different lectin-binding specificity than the mammalian homologue, showing a higher affinity for mannose-specific lectins than for WGA (Marin-Hincapie & Garofalo 1995).

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