Practice question

Phosphatidylinositol is quantitatively minor, about 5-8% of total phospholipid, but disproportionately important for intracellular signaling because its inositol ring can be phosphorylated at positions 3, 4, and 5 by dedicated kinases to generate combinatorial phosphoinositide codes. PI 4,5-bisphosphate resides in plasma membrane inner leaflet where phospholipase C, activated by Gq-coupled receptors or receptor tyrosine kinases, hydrolyzes it into two second messengers: diacylglycerol that remains membrane bound to recruit and activate conventional protein kinase C via C1 domain, and inositol 1,4,5-trisphosphate that diffuses rapidly to bind IP3 receptors on endoplasmic reticulum triggering calcium release from stores activating calmodulin pathways. PI 3-kinase phosphorylates PIP2 to PIP3, creating docking site for Akt/PKB via PH domain to activate mTOR survival, growth, and glucose transporter trafficking. Other species like PI3P marks early endosomes recruiting FYVE domain proteins for sorting. Thus phosphoinositide metabolism links extracellular cues to cytoskeletal remodeling, proliferation, and metabolism, a role cholesterol and neutral glycosphingolipids do not perform in membranes.