Biochemical signal transmitted by Fc gamma receptors: phospholipase A2 activity of Fc gamma 2b receptor of murine macrophage cell line P388D1.

The detergent lysate of the P388D1 macrophage cell line was subjected to affinity chromatography on two different media, Sepharose coupled to heat-aggregated human IgG (IgG-Sepharose) and Sepharose coupled to the phosphatidylcholine analog rac-1-(9-carboxyl)nonyl-2-hexadecylglycero-3-phosphocholine (PC-Sepharose). Both IgG- and phosphatidylcholine-binding proteins were further purified by Sephadex G-100 gel filtration and isoelectric focusing in the presence of 6 M urea. The isolated IgG-binding proteins specifically bound to IgG2a, but not to IgG2b, whereas the isolated phosphatidylcholine-binding proteins specifically bound to IgG2b but not to IgG2a. Phosphatidylcholine-binding proteins possessed a typical phospholipase A2 activity (phosphatide 2-acylhydrolase, EC 3.1.1.4), which was maximal (10 mumol/min per mg of protein) at pH 9.5, depended on Ca2+, and was specific for cleavage of fatty acid from the C-2 position of the glycerol backbone of phosphatidylcholine. The noted enzymatic activity was augmented 4-fold by preincubating phosphatidylcholine-binding proteins with heat-aggregated murine IgG2b but not with IgG2a. IgG-binding proteins, on the other hand, are devoid of any detectable phospholipase A2 activity. Thus, the functional significance of Fc gamma 2b receptor of P388D1 macrophage cell line would be the generation of phospholipase A2 activity at the cell surface upon specific binding to Fc gamma 2b fragment.