Combined electrostatics and hydrogen bonding determine intermolecular interactions between polyphosphoinositides

Membrane lipids are active contributors to cell function as key mediators in signaling pathways controlling cell functions including inflammation, apoptosis, migration, and proliferation. Recent work on multimolecular lipid structures suggests a critical role for lipid organization in regulating the function of both lipids and proteins. Of particular interest in this context are the polyphosphoinositides (PPI's), especially phosphatidylinositol (4,5) bisphosphate (PIP₂). The cellular functions of PIP₂ are numerous but the organization of PIP₂ in the inner leaflet of the plasma membrane, as well as the factors controlling targeting of PIP₂ to specific proteins, remains poorly understood. To analyze the organization of PIP₂ in a simplified planar system, we used Langmuir monolayers to study the effects of subphase conditions on monolayers of purified naturally derived PIP₂ and other anionic or zwitterionic phospholipids. We report a significant molecular area expanding effect of subphase monovalent salts on PIP₂ at biologically relevant surface densities. This effect is shown to be specific to PIP₂ and independent of subphase pH. Chaotropic agents (e.g., salts, trehalose, urea, temperature) that disrupt water structure and the ability of water to mediate intermolecular hydrogen bonding also specifically expanded PIP₂ monolayers. These results suggest a combination of water-mediated hydrogen bonding and headgroup repulsion in determining the organization of PIP₂, and may contribute to an explanation for the unique functionality of PIP₂ compared to other anionic phospholipids.