Counterion-mediated cluster formation by polyphosphoinositides

Abstract

Polyphosphoinositides (PPI) and in particular PI(4,5)P2, are among the most highly charged molecules in cell membranes, are important in many cellular signaling pathways, and are frequently targeted by peripheral polybasic proteins for anchoring through electrostatic interactions. Such interactions between PIP2 and proteins containing polybasic stretches depend on the physical state and the lateral distribution of PIP2 within the inner leaflet of the cell's lipid bilayer. The physical chemical properties of PIP2 such as pH-dependent changes in headgroup ionization and area per molecule as determined by experiments together with molecular simulations that predict headgroup conformations at various ionization states have revealed the electrostatic properties and phase behavior of PIP2-containing membranes. This review focuses on recent experimental and computational developments in defining the physical chemistry of PIP2 and its interactions with counterions. Ca2+-induced changes in PIP2 charge, conformation, and lateral structure within the membrane are documented by numerous experimental and computational studies. A simplified electrostatic model successfully predicts the Ca2+-driven formation of PIP2 clusters but cannot account for the different effects of Ca2+ and Mg2+ on PIP2-containing membranes. A more recent computational study is able to see the difference between Ca2+ and Mg2+ binding to PIP2 in the absence of a membrane and without cluster formation. Spectroscopic studies suggest that divalent cation- and multivalent polyamine-induced changes in the PIP2 lateral distribution in model membrane are also different, and not simply related to the net charge of the counterion. Among these differences is the capacity of Ca2+ but not other polycations to induce nm scale clusters of PIP2 in fluid membranes. Recent super resolution optical studies show that PIP2 forms nanoclusters in the inner leaflet of a plasma membrane with a similar size distribution as those induced by Ca2+ in model membranes. The mechanisms by which PIP2 forms nanoclusters and other structures inside a cell remain to be determined, but the unique electrostatic properties of PIP2 and its interactions with multivalent counterions might have particular physiological relevance.

PI(4,5)P2-binding effector proteins for vesicle exocytosis

“…Substantial experimental work suggests that PI(4,5)P 2 in synthetic membranes can self-associate in microdomains via H-bonding interactions especially when electrostatic repulsive interactions are neutralized with cations such as Ca 2+ [54, 55]. PI(4,5)P 2 clustering can also be achieved by electrostatic interactions between proteins with basic charge regions and highly anionic PI(4,5)P 2 (−4 charge at pH 7.0) [56].…”

“…PI(4,5)P 2 in membrane domains at high concentrations is expected to have a pronounced effect on membrane phase behavior, the occurrence of hydrophobic defects, and local membrane curvature [17, 54, 70]. However the impact of direct lipid effects on vesicle exocytosis has been difficult to assess.…”

Cellular and molecular interactions of phosphoinositides and peripheral proteins

“…The effects of lipid packing density by PIPs may influence interactions at the membrane interface as electrostatic interactions between peripheral proteins and loosely packed lipids have been shown to be stronger than that with more densely packed lipids (Bigay et al 2012; Mesmin et al 2007). When PIPs are mixed with other lipids in cellular membranes they can form PIP-enriched regions (Levental et al 2008; Redfern and Gericke 2004; Wang et al 2014) that can be further stabilized by peripheral proteins that cluster PIPs (McLaughlin and Murray 2005; Saarikangas et al 2009). Clustering of PIPs may provide areas of the membrane interface to attract polycationic proteins and proteins with PI-binding domains.…”

Biophysical methods for the characterization of PTEN/lipid bilayer interactions

“…Lipid demixing (domain formation), whether inherent to the respective lipid system or due to the interaction with bivalent cations or positively charged proteins like PTEN, complicates the experimental analysis for all techniques that do not generate spatially resolved data. A range of papers have discussed the influence of bivalent cations [38, 63-65], lipids with hydrogen bonding capabilities [24] and/or cholesterol [17, 63, 66] on phosphoinositide domain formation. In cases where domain formation is observed, one may be advised to conduct a series of experiments, each mimicking the composition and physical properties of one spatial component of the complex system.…”