Interaction of concanavalin A with rabbit thymocyte plasma membranes. Distinction between low affinity assoication and positively cooperative binding mediated by a specific glycoprotein.

1. We have analyzed the interaction of the mitogenic lectin, concanavalin A, with purified plasma membranes isolated from rabbit thymocytes. 2. Scatchard analyses show that in native membranes binding is positively cooperative at low concanavalin A concentrations and non-interacting at high lectin levels. 3. In contrast, membranes treated with 0.0064 M glutaraldehyde exhibit diphasic Scatchard plots, indicating the presence of high- and low-affinity binding sites. The high-affinity zone corresponds to the region of positive cooperativity in native membranes. 4. The number of high-affinity binding sites per cell-equivalent corresponds approximately to the number of glycoprotein (mol. wt. 55000) molecules (1-10(6)/cell), but account for less than 25% of the total lectin binding. 5. Treatment of membranes with 0.0064 M glutaraldehyde selectively crosslinks the glycoprotein (mol. wt. 55000) and its multimers, correlating directly with the modifications of concanavalin A-binding. 6. We conclude that high-affinity binding of concanavalin A to thymocyte membranes is a cooperative process mediated by the glycoprotein (mol. wt. 55000). We further conclude that the bulk of concanavalin A binding is through low-affinity associations, not involving specific membrane macromolecules.