| Protein tyrosine kinases have been implicated in signal transduction in mature and developing T lymphocytes after the stimulation of many cell surface molecules including the T cell antigen receptor, coreceptors CD4 and CD8, CD5, Thy-1, Ly6, CD2, and CD28. The identities of many of these tyrosine kinases, however, remain unknown. We have isolated a novel murine tyrosine kinase, called TSK for T cell-specific kinase, which is selectively expressed in T lymphocytes. The Tsk cDNA clone encodes a polypeptide of predicted molecular weight 72kd, with sequence similarities to both the src and abl families of tyrosine kinases. Sequence comparisons indicate that TSK contains one SH2 domain and one SH3 domain, but lacks the negative regulatory tyrosine (src Y527) common to src-family kinases. The amino terminal portion of the deduced TSK protein also contains one pleckstrin homology domain and a putative SH3-binding site sequence. TSK, along with the recently described Bruton's tyrosine kinase (BTK), murine TECA and drosophila src28C, constitute a new family of tyrosine kinases. Interestingly, BTK, as indicated by its mutation in X-linked agammaglobulinemia (XLA), is involved in the regulation of B cell development. Tsk expression is developmentally regulated. Steady-state Tsk mRNA levels are five- to ten-fold higher in thymocytes than in peripheral T cells. Tsk, while not detected in hematopoietic precursor cells, is expressed at low levels in the most immature thymocytes. Tsk expression then increases as thymocytes enter the developmental stage at which selection occurs. Thymic T cell development, repertoire selection, and mature T cell activation require TCR-dependent signaling events which lead to the incongruous outcomes of positive selection, negative selection, and proliferation with cytokine production, respectively. This paradox, and the current knowledge of TCR-associated tyrosine kinases during thymic maturation, suggest a changing signaling mechanism through the TCR and associated surface molecules in maturing thymocytes and peripheral T cells. The abundant expression of Tsk at a crucial step in thymic development is highly suggestive of a role for TSK in early T lymphocyte differentiation and selection. As a mechanism to study the role of TSK in thymic T cell development we have produced mice expressing a kinase-inactive mutant of Tsk, the expression of which is directed to immature thymocytes via the proximal lck promoter. These mice will provide useful tools for the elucidation of the role of Tsk in thymic ontogeny. |
