Divalent cation-dependent formation of electrostatic PIP₂ clusters in lipid monolayers

Polyphosphoinositides are among the most highly charged molecules in the cell membrane, and the most common polyphosphoinositide, phosphatidylinositol-4, 5-bisphosphate (PIP₂), is involved in many mechanical and biochemical processes in the cell membrane. Divalent cations such as calcium can cause clustering of the polyanionic PIP₂, but the origin and strength of the effective attractions leading to clustering has been unclear. In addition, the question of whether the ion-mediated attractions could be strong enough to alter the mechanical properties of the membrane, to our knowledge, has not been addressed. We study phase separation in mixed monolayers of neutral and highly negatively charged lipids, induced by the addition of divalent positively charged counterions, both experimentally and numerically. We find good agreement between experiments on mixtures of PIP₂ and 1-stearoyl-2-oleoyl phosphatidylcholine and simulations of a simplified model in which only the essential electrostatic interactions are retained. In addition, we find numerically that under certain conditions the effective attractions can rigidify the resulting clusters. Our results support an interpretation of PIP ₂ clustering as governed primarily by electrostatic interactions. At physiological pH, the simulations suggest that the effective attractions are strong enough to give nearly pure clusters of PIP₂ even at small overall concentrations of PIP₂.