The effect of lectins on desensitisation of locust muscle glutamate receptors.

At the excitatory neuromuscular junction of the locust, Schistocerca gregaria, desensitisation to L-glutamate is blocked by the lectin concanavalin A (Con A). In this study a range of lectins has been used to assess the influence of simple sugar binding specificity on desensitisation block. Pea and lentil lectins have similar simple sugar specificities (mannose/glucose) to Con A and block desensitisation in a similar manner. Soybean and wheatgerm lectins have other simple sugar specificities and do not block desensitisation of the locust muscle glutamate receptor. Native Con A is a tetramer at pH 7 but at lower pH or following succinylation (S-Con A) it becomes a dimer, with reduced biological activity. S-Con A does not block desensitisation but it does bind to locust muscle and protects the glutamate receptor from the desensitisation block caused by Con A. When Con A is applied to a desensitised neuromuscular junction the ongoing desensitisation is not blocked. It appears that desensitisation block by Con A, pea and lentil lectins is dependent on lectin binding to mannose or glucose moieties on or in the region of the glutamate receptor, and that these moieties are masked from lectin when the receptor is in its desensitised state.