Phosphatidylinositol 4,5-bisphosphate
Phosphatidylinositol 4,5-bisphosphate or PtdIns(4,5)P2, also known simply as PIP2 or PI(4,5)P2, 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. PIP2 also forms lipid clusters that sort proteins. PIP2 is formed primarily by the type I phosphatidylinositol 4-phosphate 5-kinases from PI(4)P. In metazoans, PIP2 can also be formed by type II phosphatidylinositol 5-phosphate 4-kinases from PI(5)P. 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 (F-actin) via direct binding to F-actin regulatory proteins. Endocytosis and exocytosis The first evidence that indicated phosphoinositides(PIs) (especially PI(4,5)P2) are important during the exocytosis process was in 1990. Emberhard et al. found that the application of PI-specific phospholipase C into digitonin-permeabilized chromaffin cells decreased PI levels, and inhibited calcium-triggered exocytosis. This exocytosis inhibition was