Rac is involved in early TCR signaling

The GTPase Pac controls signaling pathways often related to actin polymerization in various cell types. In T lymphocytes, Pac is activated by Vav, a major component of the multiprotein transduction complex associated to the TCR. Although profound signaling defects have been observed in Vav-deficient mice, a role of Pac in the corresponding early TCR signaling has not been tested directly. This question was investigated in Jurkat T cells transfected with either a dominant-negative (RacN17) or a constitutively active (RacV12) form of Rac, In T cells expressing either RacN17 or RacV12, the anti-CD3-induced Ca2+ response and production of inositol-1,4,5-trisphosphate were inhibited, The basal level of phosphatidylinositol-4,5-bisphosphate was not significantly diminished by Pac mutants. The major inhibitory effect of Pac mutants on Ca2+ signaling is exerted on the activity of phospholipase C-gamma and, before that, on the phosphorylation of ZAP-70 and of the linker molecule for activation of T cells, LAT, An anti-CD3-induced increase in actin polymerization was observed in control cells but not in cells transfected with a Pac mutant. In addition, latrunculin, which binds to monomeric actin, simultaneously inhibited basal and CD3-induced actin polymerization and Ca2+ signaling, These findings suggest a link between the effects exerted by Pac mutants on cortical actin polymerization and on TCR signaling. Pac cycling between its GTP- and GDP-bound states is necessary for this signaling. Alterations observed in early TCR-dependent signals suggest that Pac contributes to the assembly of the TCR-associated multiprotein transduction complex.