Activates CDC42, a member of the Ras-like family of Rho- and Rac proteins, by exchanging bound GDP for free GTP. Plays a role in regulating the actin cytoskeleton and cell shape. Activates MAPK8. Homooligomer. Concentrated in filopodia and poorly detected at lamellipodia. Binds along the sides of actin fibers. Detected in brain, lung, liver, skeletal muscle, kidney, testis and cultured hippocampal neurons. The part of the protein spanning the actin filament-binding domain together with the DH domain and the first PH domain is necessary and sufficient for microspike formation. Activation of MAPK8 requires the presence of all domains with the exception of the actin filament-binding domain.