SYK KINASE, TYROSINE-PHOSPHORYLATED PROTEINS AND ACTIN-FILAMENTS ACCUMULATE AT FORMING PHAGOSOMES DURING FC-GAMMA RECEPTOR-MEDIATED PHAGOCYTOSIS

Phagocytosis mediated by Fc gamma receptors (Fc gamma Rs) is thought t o be regulated by a cascade of tyrosine phosphorylation events that fi nally leads to the rearrangement of submembranous actin-based cytoskel eton and internalization of particles. Suggestions concerning the func tional relationship between protein tyrosine kinases, their substrates , and actin filament reorganization prompted us to determine cellular distribution of these elements during uptake of IgG-coated particles i n murine thio-macrophages. We found that the onset of uptake of the pa rticles was accompanied by tyrosine phosphorylation of several protein s, among which 90, 50, 40, 30, and 25 kDa polypeptides were distinguis hed. In most of the proteins the tyrosine hyperphosphorylation persist ed up to 3 min of the uptake; however, kinetics of the phosphorylation of individual proteins varied. Immunofluorescence data showed that th e phosphotyrosine-bearing proteins were localized in regions of the pa rticle uptake, being concentrated at phagocytic cups and nascent phago somes. The local enrichment in tyrosine phosphorylated proteins was co rrelated with accumulation of actin filaments at these early stages of phagosome formation. During phagosome maturation, both tyrosine phosp horylated proteins and microfilaments disappeared from the periphagoso mal regions. Syk, one of the tyrosine kinases, was translocated to the regions where Fc gamma R-mediated phagocytosis had started. On the co ntrary, no enrichment in phosphatidylinositol 3-kinase was detected in these places. (C) 1997 Wiley-Liss, Inc.