Activates CDC42, a member of the Ras-like family of Rho- and Rac proteins, by exchanging bound GDP for free GTP. Mediates VEGF-induced CDC42 activation. May regulate proangiogenic action of VEGF in vascular endothelial cells, including network formation, directional movement and proliferation. May play a role in regulating the actin cytoskeleton and cell shape (By similarity). In peripheral membrane ruffles, colocolizes with F-actin. Expressed in highly vascularized tissues, such as lung, kidney and ovary. Detected at 10 dpc in the intersomitic vessels, dorsal aorta and brain vasculature. In retina, strongly expressed in the developing retinal vasculature at postnatal day 4 and down-regulated in preformed vessels at postnatal day 8. Predominantly detected in capillaries and veins, and particularly at the advancing vascular fronts, as compared to arteries.