|Molar mass||1042.05 g/mol|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Phosphatidylinositol 4,5-bisphosphate or PtdIns(4,5)P2, also known simply as PIP2, is a minor phospholipid component of cell membranes. PtdIns(4,5)P2 is enriched at the plasma membrane where it is a substrate for a number of important signaling proteins.
PtdIns(4,5)P2 is formed primarily by the type I phosphatidylinositol 4 phosphate 5 kinases from PI(4)P.
PtdIns(4,5)P2 functions as an intermediate in the IP3/DAG pathway, which is initiated by ligands binding to G protein-coupled receptors activating the Gq alpha subunit. PtdIns(4,5)P2 is a substrate for hydrolysis by phospholipase C (PLC), 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 (InsP3; IP3) and diacylglycerol (DAG), 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). 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 (ER), 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.
Class I PI 3-kinases phosphorylate PtdIns(4,5)P2 forming phosphatidylinositol (3,4,5)-trisphosphate (PtdIns(3,4,5)P3). Both PtdIns(3,4,5)P3 and PtdIns(4,5)P2 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 PtdIns(3,4,5)P3 are AKT, PDPK1, Btk1.
- One mechanism for direct effect of PtdIns(4,5)P2 is opening of Na+ channels as a minor function in growth hormone release by growth hormone-releasing hormone.
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
- Lipid kinases, Lipid Phosphatase
- Lipid Transfer Proteins
- Growth Factors, Small GTPases
- Cell Attachment
- Cell-Cell Interaction
- Change in cell volume
- Cell differentiation state
- Cell stress
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