Focal exocytosis of VAMP3-containing vesicles at sites of phagosome formation

Phagocytosis involves the receptor-mediated extension of plasmalemmal protr usions, called pseudopods, which fuse at their tip to engulf a particle. Ac tin polymerizes under the nascent phagosome and may propel the protrusion o f pseudopods. Alternatively, membrane extension could result from the local ized insertion of intracellular membranes into the plasmalemma next to the particle. Here we show focal accumulation of VAMP3-containing vesicles, lik ely derived from recycling endosomes, in the vicinity of the nascent phagos ome. Using green fluorescent protein (GFP) as both a fluorescent indicator and an exofacial epitope tag, we show that polarized fusion of VAMP3 vesicl es precedes phagosome sealing, It is therefore likely that targeted deliver y of endomembranes contributes to the elongation of pseudopods, In addition to mediating pseudopod formation, receptor-triggered focal secretion of en dosomes may contribute to polarized membrane extension in processes such as lamellipodial elongation or chemotaxis.