Regulation of protein kinase C theta function during T cell activation by Lck-mediated tyrosine phosphorylation

Protein kinase C theta (PKC theta) is a novel Ca2+-independent PKC isoform, which is selectively expressed in skeletal muscle and hematopoietic cells, especially T cells. In T cells, it colocalizes with the T cell antigen rec eptor (TCR).CD3 complex in antigen-stimulated T cells and is involved in th e transcriptional activation of the interleukin-2 gene. In the present stud y, we report that PKC theta is tyrosine phosphorylated in Jurkat T cells up on TCR.CD3 activation. The Src family protein-tyrosine kinase, Lck, was cri tical in TCR-induced tyrosine phosphorylation of PKC theta. Lck phosphoryla ted and was associated with the regulatory domain of PKC theta both in vitr o and in intact cells. This association was constitutive, but it was enhanc ed by T cell activation, with both Src-homology 2 and Src-homology 3 domain s of Lck contributing to it. Tyrosine 90 (Tyr-90) in the regulatory domain of PKC theta was identified as the major phosphorylation site by Lck. A con stitutively active mutant of PKC theta (A148E) could enhance proliferation of Jurkat T cells and synergized with ionomycin to induce nuclear factor of T cells activity. However, mutation of Tyr-90 into phenylalanine markedly reduced (or abolished) these activities. These results suggest that Lck pla ys an important role in tyrosine phosphorylation of PKC theta, which may in turn modulate the physiological functions of PKC theta during TCR-induced T cell activation.