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R Is an Active Stage of Phagocytosis That Involves Receptor Clustering and Phosphorylation1
adkowska*
ska*
* Department of Cell Biology, Nencki Institute of Experimental Biology, Warsaw, Poland; and
Department of Molecular Genetics, Medical Research Institute, Tokyo Medical and Dental University, Tokyo, Japan
Fc
R mediate the phagocytosis of IgG-coated particles and the clearance of IgG immune complexes. By dissecting binding from internalization of the particles, we found that the binding stage, rather than particle internalization, triggered tyrosine phosphorylation of FcR and accompanying proteins. High amounts of Lyn kinase were found to associate with particles isolated at the binding stage from J774 cells. PP2 (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine), an Src kinase inhibitor, but not piceatannol, an inhibitor of Syk kinase, reduced the amount of Lyn associated with the bound particles and simultaneously diminished the binding of IgG-coated particles. Studies of baby hamster kidney cells transfected with wild-type and mutant FcRIIA revealed that the ability of the receptor to bind particles was significantly reduced when phosphorylation of the receptor was abrogated by Y298F substitution in the receptor signaling motif. Under these conditions, binding of immune complexes of aggregated IgG was depressed to a lesser extent. A similar effect was exerted on the binding ability of wild-type FcRIIA by PP2. Moreover, expression of mutant kinase-inactive Lyn K275R inhibited both FcRIIA phosphorylation and IgG-opsonized particle binding. To gain insight into the mechanism by which protein tyrosine phosphorylation can control FcR-mediated binding, we investigated the efficiency of clustering of wild-type and Y298F-substituted FcRIIA upon binding of immune complexes. We found that a lack of FcRIIA phosphorylation led to an impairment of receptor clustering. The results indicate that phosphorylation of FcR and accompanying proteins, dependent on Src kinase activity, facilitates the clustering of activated receptors that is required for efficient particle binding.
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