PIP2 and PIP3 Complex Roles at the Cell Surface

of pleckstrin homology (PH) domains that bind selectively to these phosphoinositides (Kavran et al., 1998). Worcester, Massachusetts 01605 PH domains have been identified in over 100 proteins, many of which are involved in regulating the actin cy-toskeleton and signaling events at the plasma mem-Phosphatidylinositol (4,5)-bisphosphate (PIP2) and phos-brane. PH domains are about 120 amino acid residues phatidylinositol (3,4,5)-trisphosphate (PIP3) represent long, contain an invariant tryptophan, and form a struc-less than 1% of membrane phospholipids, yet they func-ture consisting of seven beta sheets with connecting tion in a remarkable number of crucial cellular pro-loops, representing a superfamily or superfold that also cesses. These low-abundance polyphosphoinositides includes ligand-binding protein domains with little se-direct two major independent signaling cascades. PIP3 quence similarity. In a survey of PH domains, most were is the effector of multiple downstream targets of the found to bind phosphoinositides with high affinity but phosphoinositide 3 kinase (PI3K) pathway (Rameh and low selectivity (Kavran et al., 1998). Two exceptions were Cantley, 1999); PIP2 is the precursor of the mediators found; the PH domain of PLC␦ and the PH domain of diacylglycerol and inositol(1,4,5)P3 following its hydroly-the ARF protein exchange factor GRP1, which bind, sis by hormone-sensitive phospholipase C (PLC) en-respectively, PIP2 and PIP3 with high affinity and selec-zymes. New experiments are now revealing yet another tivity. These PH domains have become powerful probes signaling mode controlled by PIP2 (Toker, 1998; Honda for determining the subcellular localization of these et al., 1999; Raucher et al., 2000). This novel cascade lipids. depends on intact PIP2 rather than products of its hydro-In order to selectively recognize PIP3 in living cells, lysis. Recent work demonstrates not only new signaling a PH domain must exhibit an affinity for PIP3 at least functions of PIP2 but also intricate regulation of mem-one to two orders of magnitude higher than for PIP2, brane phospholipids. New insights on these events high-because, even in stimulated cells, the abundance of light the cell surface membrane as a major site of action PIP2 is much greater than PIP3. This requirement is of both PIP2 and PIP3 and reveal unexpected cross-uniquely satisfied by the PH domain of GRP1, which talk between these polyphosphoinositides. exhibits an affinity for PIP3 that is two to three orders PIP3 and other PI3K products control many processes of magnitude greater than for PIP2 (Kavran et al., 1998). at the plasma membrane, including phagocytosis, pino-Importantly, the GRP1 PH domain, …