A PAK1-PIX-PKL complex is activated by the T-cell receptor independent of Nck, Slp-76 and LAT

Given the importance of the Rho GTPase family member Rad and the Rac1/Cdc42 effector PAK1 in T-cell activation, we investigated the requirements for t heir activation by the T-cell receptor (TCR), Rad and PAK1 activation requi red the tyrosine kinases ZAP-70 and Syk, but not the cytoplasmic adaptor Sl p-76, Surprisingly, PAK1 was activated in the absence of the transmembrane adaptor LAT while Rad was not. However, efficient PAK1 activation required its binding sites for Rho GTPases and for PIX, a guanine nucleotide exchang e factor for Rho GTPases, The overexpression of beta PIX that either cannot bind PAK1 or lacks GEF function blocked PAK1 activation. These data sugges t that PAK1-PIX complex is recruited to appropriate sites for activation an d that PIX is required for Rho family GTPase activation upstream of PAK1, F urthermore, we detected a stable trimolecular complex of PAK1, PIX and the paxillin kinase linker p95PKL, Taken together, these data show that PAK1 co ntained in this trimolecular complex is activated by a novel LAT-and Slp-76 -independent pathway following TCR stimulation.