Phosphatidylinositol, generally abbreviated as PI or PtdIns, is a glycerophospholipid that has myo-inositol (one of nine stereoisomers of hexahydroxycyclohexane) as the polar head group. It is found in the membranes of eukaryotes and some eubacteria. Because of the anionic phosphodiester group, it contributes to the negative charge of the phospholipid bilayer. The free hydroxyl groups of PI can be phosphorylated or glycosylated to produce a wide variety of inositol phospholipids with specific functions. Two distinct types of inositol phospholipid have particularly important functions in eukaryotes that are described in separate entries:
1. Phosphatidylinositol phosphates are key components of several intracellular signaling pathways, either as the intact molecules or after they are cleaved by phospholipase C to 1,2-diacylglycerol and an inositol phosphate.
2. Glycosylphosphatidylinositols (or GPIs) can be covalently attached to the C-terminus of many proteins and are responsible for anchoring them to the membrane (see Membrane Anchors and GPI Anchor). GPIs may also occur in a "free form" without attached protein. Although the role of these free GPIs in mammalian cells is uncertain, they are an essential part of te protective coat used by many protozoal parasites.
