Tyrosine phosphorylateion is believed to be a general activation mechansim for the Vav family. VAV1 Tyr-174 binds to the Dbl homology region, inhibiting GEF activity. Phosphorylation of this residue by Syk relieves inhibition, activating Vav1. In Jurkat cells T-cell receptor activation leads to increased Vav2 tyrosine phosphorylation; the expression of Lck, Fyn, Zap70, or Syk stimulated this phosphorylation. Vav is regulated downstream of the thrombin and thrombopoietin receptors (Miyakawa et al. 1997) and integrins, including the major platelet integrin alphaIIbbeta3. Vav family proteins are involved in filopodia and lamellipodia formation; mouse platelets deficient in Vav1 and Vav3 exhibit reduced filopodia and lamellipodia formation during spreading on fibrinogen. This is accompanied by reduced alphaIIbbeta3-mediated PLCgamma2 tyrosine phosphorylation and reduced Ca(2+) mobilization (Pearce et al. 2007).