TI - Basic fibroblast growth factor stimulates activation of Rac1 through a p85 betaPIX phosphorylation -dependent pathway . AB - In a previous study we reported that phosphorylation of p85 betaPIX , a guanine nucleotide exchange factor ( GEF ) for Rac1/Cdc42 , is a signal for translocation of the PIX complex to neuronal growth cones and is associated with basic fibroblast growth factor ( bFGF ) -induced neurite outgrowth . However , the issue of whether p85 betaPIX phosphorylation affects GEF activity on Rac1/Cdc42 is yet to be explored . Here we show that Rac1 activation occurs in a p85 betaPIX phosphorylation -dependent manner . A GST-PBD binding assay reveals that Rac1 is activated in a dose - and time -dependent manner in PC12 cells in response to bFGF . Inhibition of ERK or PAK2 , the kinases upstream of p85 betaPIX in the bFGF signaling , prevents Rac1 activation , suggesting that phosphorylation of p85 betaPIX functions upstream of Rac1 activation . To directly address this issue , transfection studies with wild-type and mutant p85 betaPIX ( S525A/T526A , a non-phosphorylatable form ) were performed . Expression of mutant PIX markedly inhibits both bFGF - and nerve growth factor ( NGF ) -induced activation of Rac1 , indicating that phosphorylation of p85 betaPIX is responsible for activation of this G protein . Both wild-type and mutant p85 betaPIX displaying negative GEF activity (L238R/L239S) are similarly recruited to growth cones , suggesting that Rac1 activation is not essential for translocation of the PIX complex (PAK2-p85 betaPIX-Rac1) . However , expression of mutant p85 betaPIX (L238R/L239S) results in retraction of the pre-existing neurites . Our results provide evidence that bFGF - and NGF -induced phosphorylation of p85 betaPIX mediates Rac1 activation , which in turn regulates cytoskeletal reorganization at growth cones , but not translocation of the PIX complex .