The binding of [125I]wheat germ agglutinin ([125I]WGA) of high specific activity to Chinese hamster ovary (CHO) cells has been examined over a millionfold range of WGA concentrations and correlated with the phenomena of agglutination and capping by WGA. Analysis of the binding data by the method of Scatchard gives a complex curve indicative of positive cooperativity amongst high-affinity binding sites. Binding assays performed under conditions which inhibit capping and/or agglutination, such as low temperature or glutaraldehyde fixation, give similarly complex binding curves. Thus, the gross mobility of WGA receptors in the membrane does not appear to be responsible for the cooperative binding of WGA to CHO cells.

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