Activation of Heterotrimeric GTP-Binding Proteins Upon TCR/CD3 Engagement

Thymically derived lymphocytes perceive antigenic signals via the T-cell antigen receptor-CD3 molecular complex (1 ,2 ). The TCR/CD3 is a multimeric complex of proteins composed of an α/β heterodimer that is responsible for the recognition of the antigenic signal (in the case of T cells a peptide in the context of self-MHC class I or II), and the associated CD3 (γ,δ,ε) and ζ proteins that are responsible for transducing the signal into the intracellular environment (3 ). It has been demonstrated that the signaling pathways involve protein tyrosine kinases and phosphatases (4 ), the Rho family of small guanosine triphosphate (GTP)-binding proteins (5 ), and the heterotrimeric GTP-binding proteins (G proteins) of the Gq family (6 ,7 ). The involvement of G proteins in TCR/CD3-mediated signaling is of particular interest in view of recent reports suggesting a crosstalk between G protein coupled-receptors and tyrosine kinase receptors (8 ,9 ), and the demonstration that Gαq and Gβ/γ subunits can activate lymphocyte-specific tyrosine kinases (10 ,11 ). Thus, an assay that would measure the activation of heterotrimeric G proteins upon TCR/CD3 engagement would be a useful technique.