GAP1IP4BP Contains a Novel Group I Pleckstrin Homology Domain That Directs Constitutive Plasma Membrane Association

The group I family of pleckstrin homology (PH) domains are characterized by their inherent ability to specifically bind phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P 3 ) and its corresponding inositol head-group inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P 4 ). In vivo this interaction results in the regulated plasma membrane recruitment of cytosolic group I PH domain-containing proteins following agonist-stimulated PtdIns(3,4,5)P 3 production. Among group I PH domain-containing proteins, the Ras GTPase-activating protein GAP1 IP4BP is unique in being constitutively associated with the plasma membrane. Here we show that, although the GAP1 IP4BP PH domain interacts with PtdIns(3,4,5)P 3 , it also binds, with a comparable affinity, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P 2 ) ( K d values of 0.5 ± 0.2 and 0.8 ± 0.5 μ m , respectively). Intriguingly, whereas this binding site overlaps with that for Ins(1,3,4,5)P 4 , consistent with the constitutive plasma membrane association of GAP1 IP4BP resulting from its PH domain-binding PtdIns(4,5)P 2 , we show that in vivo depletion of PtdIns(4,5)P 2 , but not PtdIns(3,4,5)P 3 , results in dissociation of GAP1 IP4BP from this membrane. Thus, the Ins(1,3,4,5)P 4 -binding PH domain from GAP1 IP4BP defines a novel class of group I PH domains that constitutively targets the protein to the plasma membrane and may allow GAP1 IP4BP to be regulated in vivo by Ins(1,3,4,5)P 4 rather than PtdIns(3,4,5)P 3 .