A detailed kinetic analysis of the distribution of cytoplasmic myosin during the capping of various lymphocytic surface molecules revealed two distinct capping mechanisms. (a) Some cell surface molecules, including immunoglobulin, Fc receptor, and thymus leukemia antigen, all cap spontaneously in a small fraction of lymphocytes during locomotion. Cytoplasmic myosin becomes concentrated in the cytoplasm underlying these spontaneous caps. Exposure to specific antibodies causes all three of these surface molecules to cap rapidly with a concomitant redistribution of cytoplasmic myosin to the area of the cap. These antibodies also stimulate cell locomotion. (b) Other lymphocyte surface molecules, including H2 and Thy.1, do not cap spontaneously. Moreover, exposure to antibodies to these molecules causes them to cap slowly without a redistribution of cytoplasmic myosin or stimulation of cell locomotion. Exposure to concanavalin A gives a response intermediate between these two extremes. We believe that the first type of capping is active and may involve a direct link between the surface molecules and the cytoplasmic contractile apparatus. The second type of capping appears to result simply from aggregation of cross-linked molecules in the plane of the membrane.

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