Direct evidence for microfilament-mediated capping of surface receptors on crawling fibroblasts.

On moving fibroblasts, cell-surface receptors cross-linked by antibodies or lectins are cleared centripetally from regions of lamellar cytoplasm and collect as a cap over the perinuclear region. Current theories of the mechanism of receptor redistribution on cultured cells variously implicate membrane flow, lipid flow, surface waves and linkage to the cytoskeleton. The last, the anchorage model, is based on observations that ligand-induced clusters of receptors on a variety of cell types either attach to actin or align over structures containing actin, myosin and alpha-actinin. I show here that the capping of antibody receptors on crawling chick embryo fibroblasts is highly coordinated with the apparent centripetal movements of arcs, which are part of a dorsal cortical actin-microfilament sheath (DCMS). This phenomenon can be directly observed in living cells. The data support the anchorage model of membrane receptor mobility and suggest that there is a continuous flow of actin associated with fibroblast locomotion.