Protein kinase A-dependent phosphorylation modulates β1Pix guanine nucleotide exchange factor activity through 14-3-3β binding

beta(1)Pix is a guanine nucleotide exchange factor (GEF) for the small GTPases Rac and Cdc42 which has been shown to mediate signaling pathways leading to cytoskeletal reorganization. In the present study, we show that the basal association between endogenous beta Pix and endogenous 14-3-3 beta was increased after forskolin stimulation and significantly inhibited by protein kinase A inhibitor. However, forskolin stimulation failed to increase the interaction between 14-3-3 beta and a beta(1)Pix mutant that is insensitive to protein kinase A phosphorylation, beta(1)Pix(S516A, T526A). We present evidence indicating that forskolin-induced binding of 14-3-3 beta to beta(1)Pix results in inhibition of Rac1 GTP loading in 293 cells and in vitro. Furthermore, we show that deletion of 10 amino acid residues within the leucine zipper domain is sufficient to block beta(1)Pix homodimerization and 14-3-3 beta binding and modulates beta(1)Pix-GEF activity. These residues also play a crucial role in beta(1)Pix intracellular localization. These results indicate that 14-3-3 beta negatively affects the GEF activity of dimeric beta(1)Pix only. Altogether, these results provide a mechanistic insight into the role of 14-3-3 beta in modulating beta(1)Pix-GEF activity.