GAP1 (also called IP4BP, RASA3/Ras GTPase-activating protein 3, and RAS p21 protein activator (GTPase activating protein) 3/GAPIII/MGC46517/MGC47588)) is a member of the GAP1 family of GTPase-activating proteins, along with RASAL1, GAP1(m), and CAPRI. With the notable exception of GAP1(m), they all possess an arginine finger-dependent GAP activity on the Ras-related protein Rap1. GAP1(IP4BP) contains two C2 domains, a PH domain, a RasGAP domain, and a BTK domain. Its C2 domains, like those of GAP1M, do not contain the C2 motif that is known to be required for calcium-dependent phospholipid binding. GAP1(IP4BP) is regulated by the binding of its PH domains to phophoinositides, PIP3 (phosphatidylinositol 3,4,5-trisphosphate) and PIP2 (phosphatidylinositol 4,5-bisphosphate). It suppresses RAS, enhancing the weak intrinsic GTPase activity of RAS proteins resulting in the inactive GDP-bound form of RAS, allowing control of cellular proliferation and differentiation. GAP1(IP4BP) binds tyrosine-protein kinase, HCK. Members here include humans, chickens, frogs, and fish. PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N-terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes.
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