Towards unraveling the complexity of T cell signal transduction

Activation of resting T lymphocytes through the T cell antigen receptor complex is initiated by critical phosphorylation and dephosphorylation events that regulate the function and interaction of a number of signaling molecules. Key elements in these reactions are members of the Src, Syk and Csk families of protein tyrosine kinases (PTKs) and the phosphotyrosine phosphatases (PTPases) that regulate and/or counteract them, such as CD45. The PTKs can autophosphorylate and phosphorylate each other at multiple sites and, as the result of these interactions, they are induced to phosphorylate other cellular proteins. These phosphorylation events lead to modulation of enzymatic activities and/or serve as binding sites for other signaling molecules having phosphotyrosine-binding Src homology 2 (SH2) domains. As a result, these proteins translocate to the receptor complexes and are juxtaposed to the kinases that phosphorylate them. Some of the SH2-domain-containing polypeptides lack enzymatic activities and, instead, serve as adapter molecules that couple the signal to downstream effectors, such as regulators of the Ras proteins, and further into serine/threonine-specific protein kinase cascades. Through largely unknown steps these reactions lead to the transcription of previously silent genes, activation of lymphocyte effector functions, progression through the cell cycle and cell proliferation.