RASA3

RASA3 (RAS p21 protein activator 3, also known as GAP1(IP4BP)) is a bifunctional GTPase-activating protein that catalyzes GTP hydrolysis on both RAS family GTPases (H-Ras, N-Ras, K-Ras, R-Ras) and RAP1, converting them from active GTP-bound to inactive GDP-bound states. The protein has a characteristic domain architecture: two N-terminal C2 domains for calcium-dependent phospholipid binding, a central RasGAP catalytic domain containing the arginine finger mechanism, a pleckstrin homology (PH) domain that binds phosphoinositides (PIP3, PI(4,5)P2) and inositol 1,3,4,5-tetrakisphosphate (IP4), and a C-terminal Btk-type zinc finger. RASA3 was originally identified as the first known high-affinity receptor for IP4. Critically, PIP3 binding to the PH domain inhibits RASA3's GAP activity, providing a negative feedback link between PI3K signaling and Ras/Rap1 activity - when PI3K generates PIP3, RASA3 is inactivated, relieving its restraint on Ras/Rap1 signaling. RASA3 is predominantly cytosolic with regulated membrane association via its C2 and PH domains; proper membrane targeting is essential for function. In platelets, RASA3 is the major Rap1-GAP that counterbalances CalDAG-GEFI (Rap-GEF), setting the activation threshold to prevent inappropriate platelet activation and integrin alphaIIbbeta3 engagement. Mouse scat mutant (G125V, disrupts membrane targeting) exhibits severe thrombocytopenia and anemia with ~94% lethality by 30 days due to unchecked Ras activity in hematopoietic cells. In T lymphocytes, RASA3 acts as a gatekeeper restraining Rap1-driven LFA-1 integrin activation until proper TCR/costimulation occurs; RASA3-deficient T cells show constitutive high-affinity integrin states, impaired lymph node egress, and dysregulated immune responses. Human biallelic RASA3 loss-of-function mutations cause Rasopathy-like syndrome with developmental abnormalities and bone marrow failure. Expression is enriched in hematopoietic cells, platelets, megakaryocytes, and T cells.