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2014 ; 182
(ä): 38-51
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Counterion-mediated cluster formation by polyphosphoinositides
#MMPMID24440472
Wang YH
; Slochower DR
; Janmey PA
Chem Phys Lipids
2014[Sep]; 182
(ä): 38-51
PMID24440472
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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 and 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. Ca(2+)-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 Ca(2+)-driven formation of PIP2
clusters but cannot account for the different effects of Ca(2+) and Mg(2+) on
PIP2-containing membranes. A more recent computational study is able to see the
difference between Ca(2+) and Mg(2+) 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 Ca(2+)
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 Ca(2+) 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.
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