Activates CDC42, a member of the Ras-like family of Rho- and Rac proteins, by exchanging bound GDP for free GTP. Activates MAPK8 (By similarity). Plays a role in regulating the actin cytoskeleton and cell shape. Promotes the formation of lamellipodia. Homooligomer. Concentrated in filopodia and poorly detected at lamellipodia. Binds along the sides of actin fibers (By similarity). Detected in thymus, lung, heart, skeletal muscle, small intestine, liver, kidney, spleen and testis. Expressed in all parts of the brain and in the spinal cord at embryonic, postnatal, and adult stages. Levels of expression are lower in postnatal and adult tissues than in embryonic tissues. 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 (By similarity).