Fig. 7. Proposed model for PAK1 and Rac1 activation by the TCR. PAK1 activation requires Lck and ZAP-70, but occurs independently of LAT and Slp-76. Rac1 activation requires ZAP-70 and LAT, but is independent of Slp-76. These contrast the requirement of ZAP-70, LAT and Slp-76 for Ras and phosphatidylinositol pathway activation. Each of these divergent pathways is required to activate the transcriptional responses of T-cell activation. PAK1 activity most likely contributes to cytoskeletal reorganization as well. Undoubtedly, cytoskeletal rearrangements require contributions from many other components besides PAK1, but these have been omitted for clarity.