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 initiated by the tyrosine kinase Lck and requiring the proto-oncogene p95(vav). Vav is activated by Lck and can function as a guanine nucleotide exchange factor for the Rho-family GTPases, Rac1 and Cdc42. To investigate the involvement of these GTPases in TCR signaling, we focused on their well characterized effector, Pak1. This serine/threonine kinase is activated by GTP-bound Rac1 or Cdc42. However, its role in mediating downstream signaling events is controversial. We observed rapid, TCR-dependent activation of Pak1 and TCR-inducible association of Pak1 with Nck, which was tyrosine phosphorylated following stimulation. Pak1 activation occurred independently of Ras activation or calcium flux, but was dependent on the Lck tyrosine kinase, and was downstream of Vav and Cdc42. Dominant negative Pak1 or Nck specifically inhibited TCR-mediated activation of the nuclear factor of activated T cells (NFAT) transcription 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) activation. Therefore, Pak1 acts downstream of Vav and is required for activation of Erk2 and NFAT by a JNK-independent pathway. This is the first demonstration of a requirement for Pak to mediate the regulation of gene expression by an extracellular ligand.