Divalent cation-induced cluster formation by polyphosphoinositides in model membranes.

Area of Science:

• Cellular Biology
• Biophysics
• Membrane Biophysics

Background:

• Polyphosphoinositides (PPIs), especially phosphatidylinositol-(4,5)-bisphosphate (PI4,5P2), regulate numerous cellular processes.
• PI4,5P2 interacts with hundreds of intracellular proteins, but its precise targeting in cell membranes is not fully understood.
• The formation and maintenance of PI4,5P2 clusters are critical for selective protein recruitment but remain mechanistically debated.

Purpose of the Study:

• To investigate the hypothesis that PI4,5P2 forms submicrometer clusters in membranes via electrostatic interactions with intracellular divalent cations.
• To quantify the competitive binding of Ca(2+) and Mg(2+) to PI4,5P2.
• To explore the physical chemical differences among three PI4,5P2 isomers.

Main Methods:

• Lipid monolayer and bilayer model membranes were utilized.
• Surface pressure measurements and Langmuir competitive adsorption models were employed to quantify ion binding.
• Techniques including fluorescence, atomic force microscopy (AFM), electron microscopy, steady-state probe-partitioning fluorescence resonance energy transfer (SP-FRET), and fluorescence correlation spectroscopy (FCS) were used.

Main Results:

• Calcium ions (Ca(2+)), but not Mg(2+), Zn(2+), or polyamines, induced PI4,5P2 cluster formation in lipid monolayers, visualized by microscopy.
• Bilayer studies revealed that divalent metal ions (Me(2+)) induce cluster formation or diffusion retardation, with an efficacy trend of Ca(2+) ≫ Mg(2+) > Zn(2+).
• Polyamines showed minimal effects on PI4,5P2 organization.

Conclusions:

• Divalent metal ions significantly impact PI4,5P2 lateral organization in membranes at physiological concentrations.
• Local cytoplasmic fluctuations in divalent cation levels can modulate protein-PI4,5P2 interactions.
• Ca(2+) plays a key role in driving PI4,5P2 clustering, suggesting a mechanism for regulating protein targeting to specific membrane domains.