GTPase activator for the nuclear Ras-related regulatory proteins Rap1 and Rap2 in vitro, converting them to the putatively inactive GDP-bound state (PubMed:9346962). Affects cell cycle progression (By similarity). Interacts with RRP1B; the interaction leads to inhibition of SIPA1 GTPase activity. Mostly localized in the perinuclear membraneous region. Expressed in fetal as well as in adult tissues. Expressed abundantly in the lymphoid tissues such as thymus, spleen and peripheral blood lymphocytes and also shows a significant expression in the spinal cord. Repressed by 12-O-tetradecanoylphorbol-13-acetate (TPA) in promyelocytic HL-60 cells.