What is the role of phosphatidylinositol in the membrane?
Phosphatidylinositol's inositol ring protruding into cytosol can be phosphorylated to produce phosphoinositides that act as transient second messenger precursors. Upon activation of GPCR coupling to Gq or RTK activating PLC-gamma, phospholipase C cleaves PIP2 into diacylglycerol which stays embedded to recruit C1 domain of protein kinase C and Munc13 for exocytosis, and soluble IP3 that binds tetrameric IP3 receptor calcium channels on ER releasing calcium that activates calmodulin and NFAT. Concurrently class I PI3K phosphorylates PIP2 to PIP3 which accumulates transiently and recruits proteins containing PH domain such as Akt, PDK1, and BTK to plasma membrane, triggering Akt-mTOR pathway controlling survival, protein synthesis, and glucose metabolism. Dephosphorylation by PTEN and SHIP phosphatases terminates signal. Additional roles include PI4P at Golgi recruiting adaptors for vesicle budding and PIP2 binding to actin regulators like profilin. Thus PI cycle provides rapid, reversible production of lipid and soluble messengers, distinguishing it from structural roles or ion conduction.
Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 15: Phosphoinositide Signaling Cascade.