GTP-binding protein that possesses intrinsic GTPase activity. Contributes to the polarizing activity of KRIT1 and CDH5 in the establishment and maintenance of correct endothelial cell polarity and vascular lumen. Required for the localization of phosphorylated PRKCZ, PARD3 and TIAM1 to the cell junction. Plays a role in the establishment of basal endothelial barrier function (By similarity). GTP + H2O = GDP + H(+) + phosphate Activated by guanine nucleotide-exchange factor (GEF) EPAC2 in a cAMP-dependent manner. Heterodimer with RAP1GAP (By similarity). Interacts with EPAC2 (By similarity). Interacts with SGSM1 (By similarity). Interacts with SGSM2 (By similarity). Interacts with SGSM3 (By similarity). Interacts with KRIT1 (By similarity). Interacts with RAP1GDS1 (By similarity). May shuttle between plasma membrane and cytosol (By similarity). Presence of KRIT1 and CDH5 is required for its localization to the cell junction (By similarity).