Does lectin–receptor complex formation produce zones of restricted mobility within the membrane?

Abstract

IT is well established that component lipids and proteins are mobile in the plane of the membrane^1,2. This lateral mobility could have an important role in the transmission of signals through the membrane. For example, there is much evidence that aggregation of receptors induced by divalent antibodies or by chemical cross-linking can reproduce the biological action of the corresponding ligands^3–5. It was shown recently that receptors for monovalent ligands like insulin and epidermal growth factor aggregate into patches on the cell surface after interaction with the corresponding hormone. Clustered receptors accumulate at coated regions of the membrane and become internalised in endocytic vesicles^6,7. Aggregation is dependent on receptor occupancy by the active ligand⁷. Here we show that lateral mobility is also dependent on receptor occupancy. Our approach has been to compare the lateral mobilities of free and ligand-bound receptors using cultured cells placed in a steady electric field. This permits the measurement of lateral long-range (over the whole cell) mobility of the receptors with and without the corresponding ligands⁸. We have compared this method with the dynamic method of fluorescence photobleaching recovery (FPR) which only permits measurement of the mobility of fluorescent ligand-receptor complexes in the range of the size of the laser beam diameter (∼ 3 μm)^9–12. We have studied the lateral mobility of concanavalin A (Con A) and ricin receptors from non-malignant fibroblasts 3T3 and malignant neuroblastoma NS-20 and C₆ glial cells with both techniques. Our results suggest that lectin-receptor formation results in the production of zones of restricted mobility in the membrane.