Phosphatidylinositol 4,5-bisphosphate or PtdInsP2, also known simply as PIP2 or PIP2, is a minor phospholipid component of cell membranes. PtdInsP2 is enriched at the plasma membrane where it is a substrate for a number of important signaling proteins. PIP2 is formed primarily by the type Iphosphatidylinositol 4-phosphate 5-kinases from PIP. In metazoans, PIP2 can also be formed by type II phosphatidylinositol 5-phosphate 4-kinases from PIP. The fatty acids of PIP2 are variable in different species and tissues, but the most common fatty acids are stearic in position 1 and arachidonic in 2.
Signaling pathways
PIP2 is a part of many cellular signaling pathways, including PIP2 cycle, PI3K signalling, and PI5P metabolism. Recently, it has been found in the nucleus with unknown function.
Functions
Cytoskeleton dynamics near membranes
PIP2 regulates the organization, polymerization, and branching of filamentous actin via direct binding to F-actin regulatory proteins.
Endocytosis and exocytosis
The first evidence that indicated phosphoinositides are important during the exocytosis process was in 1990. Emberhard et al. found that the application of PI-specific phospholipase C into digitonin-permeablized chromaffin cells decreased PI levels, and inhibited calcium-triggered exocytosis. This exocytosis inhibition was preferential for an ATP-dependent stage, indicating PI function was required for secretion. Later studies identified associated proteins necessary during this stage, such as phosphatidylinositol transfer protein , and phosphoinositol-4-monophosphatase 5 kinase type Iγ , which mediates PIP2 restoration in permeable cell incubation in an ATP-dependent way. In these later studies, PIP2 specific antibodies strongly inhibited exocytosis, thus providing direct evidence that PIP2 plays a pivotal role during the LDCV exocytosis process. Through the use of PI-specific kinase/phosphatase identification and PI antibody/drug/blocker discovery, the role of PI in secretion regulation was extensively investigated. Studies utilizing PHPLCδ1 domain over-expression , PIPKIγ knockout in chromaffin cell and in central nerve system , PIPKIγ knockdown in beta cell lines , and over-expression of membrane-tethered inositol 5-phosphatase domain of synaptojanin 1 , all suggested vesicle secretion were severely impaired after PIP2 depletion or blockage. Moreover, some studies showed an impaired/reduced RRP of those vesicles, though the docked vesicle number were not altered after PIP2 depletion, indicating a defect at a pre-fusion stage. Follow-up studies indicated that PIP2 interactions with CAPS, Munc13 and synaptotagmin1 are likely to play a role in this PIP2 dependent priming defect.
IP3/DAG pathway
PIP2 functions as an intermediate in the , which is initiated by ligands binding to G protein-coupled receptors activating the Gq alpha subunit. PtdInsP2 is a substrate for hydrolysis by phospholipase C, a membrane-bound enzyme activated through protein receptors such as α1 adrenergic receptors. PIP2 regulates the function of many membrane proteins and ion channels, such as the M-channel. The products of the PLC catalyzation of PIP2 are inositol 1,4,5-trisphosphate and diacylglycerol, both of which function as second messengers. In this cascade, DAG remains on the cell membrane and activates the signal cascade by activating protein kinase C. PKC in turn activates other cytosolic proteins by phosphorylating them. The effect of PKC could be reversed by phosphatases. IP3 enters the cytoplasm and activates IP3 receptors on the smooth endoplasmic reticulum, which opens calcium channels on the smooth ER, allowing mobilization of calcium ions through specific Ca2+ channels into the cytosol. Calcium participates in the cascade by activating other proteins.
Docking phospholipids
phosphorylate PtdInsP2 forming phosphatidylinositol -trisphosphate and PtdInsP2 can be converted from PtdIns4P. PtdIns4P, PtdInsP3 and PtdInsP2 not only act as substrates for enzymes but also serve as docking phospholipids that bind specific domains that promote the recruitment of proteins to the plasma membrane and subsequent activation of signaling cascades.
Examples of proteins activated by PtdInsP3 are AKT, PDPK1, Btk1.
PtdInsP2 has been shown to stabilize the active states of Class A G protein-coupled receptors via direct binding, and enhance their selectivity toward certain G proteins.
PIP2 is regulated by many different components. One emerging hypothesis is that PIP2 concentration is maintained locally. Some of the factors involved in PIP2 regulation are:
