Ultrastructural analyses have revealed striking similarities between Concanavalin A capping and phagocytosis in leukocytes. Both processes involve extensive membrane movement to form a protuberance or pseudopods; a dense network of microfilaments is recruited into both the protuberance and the pseudopods; microtubules are disassembled either generally (capping) or in the local region of the pseudopods (phagocytosis); and cells generally depleted of microtubules by colchicine show polarized phagocytosis via the microfilament-rich protuberance rather than uniform peripheral ingestion of particles via individual pseudopods. Cap formation can thus be viewed as occurring as an exaggeration of the same ultrastructural events that mediate phagocytosis. Similar changes in cell surface topography also accompany capping and phagocytosis. Thus, in nonfixed cells, Concanavalin A-receptor complexes aggregate into the region of the protuberance in colchicine-treated leukocytes (conventional capping) or into the region of pseudopod formation in phagocytizing leukocytes. In the latter case, the movement of lectin-receptor complexes occurs from membrane overlying peripheral microtubules into filament-rich pseudopods that exclude microtubules. These data provide evidence against a role for microtubules as "anchors" for lectin receptors. Rather, they indicate a preferential movement of cell surface Concanavalin A-receptor complexes towards areas of extensive (the protuberance) or localized (pseudopods) microfilament concentration. In conventional capping, Concanavalin A must be added to the colchicine-treated cells before fixation in order to demonstrate movement of receptors from a diffuse distribution into the protuberance. However, Convanavalin A receptors are enriched in the membrane associated with phagocytic particles as compared to the remaining membrane. This particle-induced redistribution of receptors is particularly prominent in colchicine-treated cells that phagocytize and are then fixed and Concanavalin A labeled; both lectin receptors and beads are concentrated over the protuberance. Thus, the final analogy between conventionally capped and phagocytic cells is that in both cases the properties of the plasma membrane in regions of microfilament concentration are modified by Concanavalin A itself (capping) or by the phagocytized particle, to limit locally the diffusion of Concanavalin A receptors.

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