GTPase activator for the nuclear Ras-related regulatory proteins Rap1, Rsr1 and Ran in vitro, converting them to the putatively inactive GDP-bound state. Affects cell cycle progression. Interacts with RRP1B; the interaction leads to inhibition of SIPA1 GTPase activity. Mostly localized in the perinuclear membraneous region. Preferentially expressed in both fetal and adult lymphohematopoietic tissues. By mitogens.