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The ultrastructural distribution of membrane-bound immunoglobulin (Ig) on mouse spleen lymphocytes has been studied by labeling the surface Ig with anti-mouse Ig antibody conjugated t o ferritin (FT). The distribution is temperature-dependent. At 0 OC -4 OC FTzppears p t r i b u t e d in small clusters over the entire surface, whereas at 2 0 C -22 C, in the great majority of the labeled cells, it is concentrated over one pole. This pole corresponds t o that part of the cell which contains the G o l d complex and most of the cellular organelles, and which forms the uropod in moving lymphocytes. This polar redistribution is accompanied by endocytosis of part of the labeled membrane.The redistribution of label occurring at room temperature indicates that labeled and unlabeled components of the plasma membrane can segregate from each other and suggests that plasma membrane is fluid at room temperature. The diffuse distribution found at 0 OC -4 OC is probably close (although not identical) t o the distribution in vivo before labeling, while the polar distribution at higher temperature is probably induced by the cross-linking of the surface Ig molecules by the antibody-FT conjugate and by a metabolically-dependent flow of the aggregates towards the "tail" of the lymphocyte. The mechanism involved is considered to be the same mechanism responsible for membrane flow during cell movement. A detailed model of the structure and rheological properties of the lymphocyte membrane is proposed, which can explain the present results and those of others concerning the behavior of surface components of lymphocytes and other cells.

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The effect of concanavalin A (Con A) on the capping of mouse lymphocyte surface immunoglobulin (surface lg), cross-linked by rabbit anti-mouse Ig antibody, and on the capping of mouse thymocyte 0 antigen, cross-linked by anti-0 alloantibody and rabbit anti-mouse lg antibody, has been studied by immunofluorescence, using fluorescein conjugated Con A and rhodamine-conjugated anti-mouse lg antibody, and by electron microscopy, using native or fluorescein-conjugated Con A and ferritin-conjugated anti-mouse lg antibody.Prior incubation of the cells with Con A inhibited only partially capping of surface Ig, whereas it blocked almost completely capping of 0 antigens. Both on cells with rings and on cells with caps the staining for surface Ig or 0 antigen was superimposed to the staining for Con A. When Con A receptors on spleen cells were capped by Con A at concentrations of 10 ug/ml or higher, and the distribution of surface lg was examined under noncapping conditions, all detectable surface lg were found in the caps. As shown by electron microscopy, surface lg remained dispersed in a layer of Con A. The ability of Con A to cap surface ig was not alterecl by the presence of colchicine or vinblastine. These results suggest that surface lg are cross-linked by Con A to other Con A receptors. In these conditions surface lg behave essentially as Con A receptors, as for example, in their sensitivity to cytochalasin B during inhibition or reversal of capping induced by this drug. The behavior of surface lg parallels that of Con A receptors also in the presence of vinblastine. It is concluded that in the presence of Con A, antimitotic drugs do not modify directly the interaction between Con A receptors and surface lg, but probably influence the capping ability of the Con A receptors or, more in general, affect the ability to elicit movements over the cell surface. The role in capping of cytochalasin-sensitive and vinblastine-sensitive structures is discussed. Both types of structures appear to play an active role in the formation of a cap, although the former probably corresponds to the main mechanical system responsible for the active displacement of cytoplasmic and surface material.

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