Phosphatidylinositol (PI) is a minor yet pivotal anionic glycerophospholipid, constituting 5-10% of total membrane phospholipids. It features a diacylglycerol backbone esterified at sn-3 to a phosphate-linked myo-inositol ring, which serves as the precursor to seven polyphosphorylated signaling derivatives. Extending the phosphatidylserine discussion, PI's cyclohexanehexol headgroup—with hydroxyls at positions 2-6 available for phosphorylation—endows it with remarkable regulatory versatility, localizing primarily to the cytosolic leaflet where kinases generate PI(3)P, PI(4)P, PI(4,5)P2, and PI(3,4,5)P3 to orchestrate trafficking, cytoskeletal dynamics, and survival signaling.

Molecular Structure

PI follows the sn-glycerol-3-phosphate scaffold with sn-1 saturated (typically 18:0) and sn-2 monounsaturated/polyunsaturated acyls (18:1/20:4), e.g., 18:0/20:4-PI. The myo-inositol adopts a chair conformation with axial 2-OH and equatorial phosphates at 3/4/5 positions in derivatives. Its net negative charge (pKa ~6.7-7.7 for phosphomonoesters) promotes Ca²⁺/Mg²⁺ bridging and nanodomain formation.

Biophysical Properties

The cylindrical geometry of PI (head area ~60-70 Ų) supports bilayer formation, but phosphorylation tilts the headgroup (100° for PI(4,5)P2), favoring flat membrane orientation and protein recruitment. PI forms stable nanodomains with physiological cations, unlike PS. Its melting temperature (Tm ~0-14°C depending on acyl chains) enhances membrane curvature, facilitating processes such as fission.