Cooperative Binding of Concanavalin A to Thymocytes at 4oC and Micro-redistribution of Concanavalin A Receptors

Abstract

The mode of binding of 125I-labeled concanavalin A [con A] and succinyl-concanavalin A [S-con A] to rat thymocytes at 4.degree. C was investigated. Simultaneously, the free binding sites of the cell-bound lectin molecules were quantified by horseradish peroxidase binding. Con A showed cooperative binding, while S-con A did not. The number of molecules of con A bound to the cell surface when it was saturated was twice the number of molecules of S-con A. The binding of native con A to thymocytes at 4.degree. C apparently brings about a cooperative modification of the membrane which leads to appearance of new receptors. Divalent S-con A has no such effect. Horseradish peroxidase binding to cell-bound lectin was related to the immobilization of membrane receptors; the more they are immobilized, the more receptor-associated lectin can bind horseradish peroxidase. Post-binding events, termed micro-redistribution, occurred at 4.degree. C when con A or S-con A binds to cells. A cooperative restriction of the micromobility of cell receptors is produced by increasing concentrations of con A. S-con A does not restrict cell receptor mobility at any concentration tested. The results are discussed in terms of cell stimulation and cell agglutination.