SUBSTITUTIONS AND DELETIONS IN THE CYTOPLASMIC DOMAIN OF THE PHAGOCYTIC RECEPTOR FC-GAMMA-RIIA - EFFECT ON RECEPTOR TYROSINE PHOSPHORYLATION AND PHAGOCYTOSIS

Fc gamma RIIA in the absence of other Pc receptors or receptor subunit s induces the ingestion of IgG-coated cells. The cytoplasmic domain of Fc gamma RIIA contains two Y-x-x-L sequences similar to those in othe r Ig gene family receptors plus an additional tyrosine residue not in a Y-x-x-L motif. Upon cross-linking, Fc gamma RIIA is phosphorylated o n tyrosine and the cytoplasmic tyrosines, Y275 (Y1), Y282 (Y2), and Y2 98 (Y3), may be important for its phagocytic activity. Because COS-1 c ells can serve as a model for examining molecular structures involved in phagocytosis, substitutions and deletions were introduced into the cytoplasmic domain of Fc gamma RIIA and examined in COS-1 cell transfe ctants for their effects on phagocytosis and tyrosine phosphorylation. Disruption of a single cytoplasmic Y-x-x-L motif by substitution of t yrosine Y2 or Y3 by phenylalanine or by removing the threonine and leu cine residues within the motif inhibited phagocytosis 50% to 65%. Tyro sine phosphorylation of Fc gamma RIIA also was inhibited, although to a greater extent by the substitution of Y3 than of Y2. Replacement of the N-terminal first cytoplasmic domain tyrosine, Y1, which is not wit hin a typical Y-x-x-L, by itself did not inhibit phagocytosis, but rep lacement of Y1 in mutants lacking Y2 or Y3 virtually eliminated phagoc ytic activity and receptor tyrosine phosphorylation. Thus, at least tw o cytoplasmic tyrosines, including at least one typical single Y-x-x-L motif, are required for phagocytosis by Fc gamma RIIA. The data sugge st that there is a close but not a simple relationship between phospho rylation of the Fc gamma RIIA cytoplasmic tyrosines and Fc gamma RIIA- mediated phagocytosis. Y3 appears to be particularly important because its removal by truncation or replacement with phenylalanine inhibits both tyrosine phosphorylation and phagocytosis in parallel. Alteration s in the 12 residue proline-containing sequence between the two Y-x-x- L motifs also reduced phagocytic activity and tyrosine phosphorylation . Thus, the specific structure of the Fc gamma RIIA cytoplasmic domain accounts for its ability to stimulate phagocytosis in the absence of other subunits. (C) 1994 by The American Society of Hematology.