DIFFERENTIAL-EFFECTS OF B-CELL RECEPTOR AND B-CELL RECEPTOR-FC-GAMMA-RIIB1 ENGAGEMENT ON DOCKING OF CSK TO GTPASE-ACTIVATING PROTEIN (GAP)-ASSOCIATED P62

The stimulatory and inhibitory pathways initiated by engagement of sti mulatory receptors such as the B cell receptor for antigen (BCR) and i nhibitory receptors such as Fc gamma receptors of the IIB1 type (Fc ga mma RIIB1) intersect in ways that art: poorly understood at the molecu lar level. Because the tyrosine kinase Csk is a potential negative reg ulator of lymphocyte activation, we examined the effects of BCR and Fc gamma RIIB1 engagement on the binding of Csk to phosphotyrosine-conta ining proteins. Stimulation of a B lymphoma cell Line, A20, with intac t anti-IgG antibody induced a direct, SH2-mediated association between Csk and a 62-kD phosphotyrosine-containing protein that was identifie d as RasGTPase-activating protein-associated p62 (GAP-A.p62). In contr ast, stimulation of A20 cells with anti-IgG F(ab')(2) resulted in litt le increase in the association of Csk with GAP-A.p62. The effect of Fc gamma RIIB1 engagement on this association was abolished by blockade of Fc gamma RIIB1 with the monoclonal antibody 2.4G2. Furthermore, the increased association between Csk and GAP-A.p62 seen upon stimulation with intact anti-Ig was abrogated in the Fc gamma RIIB1-defcient cell line IIA1.6 and recovered when Fc gamma RIIB1 expression was restored by transfection. The differential effects of BCR and BCR-Fc gamma RII B1-mediated signaling on the phosphorylation of GAP-A.p62 and its asso ciation with Csk suggest that docking of Csk to GAP-A.p62 may function in the negative regulation of antigen receptor-mediated signals in B cells.