Role of protein kinase C in T-cell antigen receptor regulation of p21^ras: Evidence that two p21^ras regulatory pathways coexist in T cells

T-lymphocyte activation via the antigen receptor complex (TCR) results in accumulation of p21^ras in the active GTP-bound state. Stimulation of protein kinase C (PKC) can also activate p21^ras, and it has been proposed that the TCR effect on p21^ras occurs as a consequence of TCR regulation of PKC. To test the role of PKC in TCR regulation of p21^ras, a permeabilized cell system was used to examine TCR regulation of p21^ras under conditions in which TCR activation of PKC was blocked, first by using a PKC pseudosubstrate peptide inhibitor and second by using ionic conditions that prevent phosphatidyl inositol hydrolysis and hence diacylglycerol production and PKC stimulation. The data show that TCR-induced p21^ras activation is not mediated exclusively by PKC. Thus, in the absence of PKC stimulation, the TCR was still able to induce accumulation of p21^ras-GTP complexes, and this stimulation correlated with an inactivation of p21^ras GTPase-activating proteins. The protein tyrosine kinase inhibitor herbimycin could prevent the non-PKC-mediated, TCR-induced stimulation of p21^ras. These data indicate that two mechanisms for p21^ras regulation coexist in T cells: one PKC mediated and one not. The TCR can apparently couple to p21^ras via a non-PKC-controlled route that may involve tyrosine kinases.