Rac-mediated Stimulation of Phospholipase Cγ2 Amplifies B Cell Receptor-induced Calcium Signaling

The Rho GTPase Rac is crucially involved in controlling multiple B cell functions, including those regulated by the B cell receptor ( BCR) through increased cytosolic Ca2+. The underlying molecular mechanisms and their relevance to the functions of intact B cells have thus far remained unknown. We have previously shown that the activity of phospholipase C gamma(2) ( PLC gamma(2)), a key constituent of the BCR signalosome, is stimulated by activated Rac through direct protein-protein interaction. Here, we use a Rac-resistant mutant of PLC gamma(2) to functionally reconstitute cultured PLC gamma(2)-deficient DT40 B cells and to examine the effects of the Rac-PLC gamma(2) interaction on BCR-mediated changes of intracellular Ca2+ and regulation of Ca2+-regulated and nuclear-factor-of-activated-T-cell-regulated gene transcription at the level of single, intact B cells. The results show that the functional Rac-PLC gamma(2) interaction causes marked increases in the following: ( i) sensitivity of B cells to BCR ligation; ( ii) BCR-mediated Ca2+ release from intracellular stores; ( iii) Ca2+ entry from the extracellular compartment; and ( iv) nuclear translocation of the Ca2+ regulated nuclear factor of activated T cells. Hence, Rac-mediated stimulation of PLC gamma(2) activity serves to amplify B cell receptor-induced Ca2+ signaling.