Transmembrane interactions and the mechanism of capping of surface receptors by their specific ligands

The mechanism of capping of cell surface receptors has been examined by a double fluorescence staining procedure that permitted simultaneous observations of the distribution of a surface-bound ligand together with intracellular actin or myosin. At an early stage in the capping of the T-25 antigen or the H2 histocompatibility antigens on mouse splenic T lymphocytes, or of concanavalin A receptors on HeLa cells, when the specific receptors in question were collected into patches that were distributed over the entire cell surface, the intracellular membrane-associated actin or myosin was also accumulated into patches that were located directly under the receptor patches. These and other results have led us to propose a general molecular mechanism for the process of capping, in which actin and myosin are directly involved. It is suggested that membrane-associated actin is directly or indirectly bound to an integral protein or class of proteins, X, in the plasma membranes of eukaryotic cells. When any receptor in the membrane is aggregated by an external multivalent ligand, the aggregate binds effectively to X, whereas unaggregated receptors do not bind to X. The receptor aggregates, linked to actin (and myosin) through X, are then actively collected into a cap by an analogue of the actin--myosin sliding filament mechanism of muscle contraction.