Divalent cation-induced cluster formation by polyphosphoinositides in model membranes

Polyphosphoinositides (PPIs) and in particular phosphatidylinositol-(4,5)- bisphosphate (PI4,5P2), control many cellular events and bind with variable levels of specificity to hundreds of intracellular proteins in vitro. The much more restricted targeting of proteins to PPIs in cell membranes is thought to result in part from the formation of spatially distinct PIP2 pools, but the mechanisms that cause formation and maintenance of PIP2 clusters are still under debate. The hypothesis that PIP2 forms submicrometer-sized clusters in the membrane by electrostatic interactions with intracellular divalent cations is tested here using lipid monolayer and bilayer model membranes. Competitive binding between Ca ²⁺ and Mg ²⁺ to PIP2 is quantified by surface pressure measurements and analyzed by a Langmuir competitive adsorption model. The physical chemical differences among three PIP2 isomers are also investigated. Addition of Ca ²⁺ but not Mg ²⁺, Zn ²⁺, or polyamines to PIP2-containing monolayers induces surface pressure drops coincident with the formation of PIP2 clusters visualized by fluorescence, atomic force, and electron microscopy. Studies of bilayer membranes using steady-state probe-partitioning fluorescence resonance energy transfer (SP-FRET) and fluorescence correlation spectroscopy (FCS) also reveal divalent metal ion (Me ²⁺)-induced cluster formation or diffusion retardation, which follows the trend: Ca ²⁺ ≫ Mg ²⁺ > Zn ²⁺, and polyamines have minimal effects. These results suggest that divalent metal ions have substantial effects on PIP2 lateral organization at physiological concentrations, and local fluxes in their cytoplasmic levels can contribute to regulating protein-PIP2 interactions.