A NCK-PAK1 SIGNALING MODULE IS REQUIRED FOR T-CELL RECEPTOR-MEDIATED ACTIVATION OF NFAT, BUT NOT OF JNK

The T-cell antigen receptor (TCR) triggers a signaling cascade initiat ed by the tyrosine kinase Lck and requiring the proto-oncogene p95(vav ). Vav is activated by Lck and can function as a guanine nucleotide ex change factor for the Rho-family GTPases, Rad and Cdc42, To investigat e the involvement of these GTPases in TCR signaling, we focused on the ir well characterized effector, Pak1. This serine/threonine kinase is activated by GTP-bound Rad or Cdc42. However, its role in mediating do wnstream signaling events is controversial. We observed rapid, TCR-dep endent activation of Pak1 and TCR-inducible association of Pak1 with N ck, which was tyrosine phosphorylated following stimulation. Pak1 acti vation occurred independently of Ras activation or calcium flux, but w as dependent on the Lck tyrosine kinase, and was downstream of Vav and Cdc42, Dominant negative Pak1 or Nck specifically inhibited TCR-media ted activation of the nuclear factor of activated T cells (NFAT) trans cription factor. TCR-mediated activation of Erk2 was also inhibited by dominant negative Pak, However, Pak1 activation was neither necessary nor sufficient for TCR-dependent c-Jun N-terminal kinase (JNK) activa tion. Therefore, Pak1 acts downstream of Vav and is required for activ ation of Erk2 and NFAT by a JNK-independent pathway. This is the first demonstration of a requirement for Pak to mediate the regulation of g ene expression by an extracellular ligand.