SALT-DEPENDENT PHASE-BEHAVIOR OF THE PHOSPHATIDYLINOSITOL 4,5-DIPHOSPHATE-WATER SYSTEM

We have studied the effect of mono- and di-valent cations on the struc ture of the phosphatidylinositol 4,5-diphosphate (PIP2)-biological buf fer system at pH 7.2. We found that the PIP2 aggregates form prolate e llipsoidal micelles at pH 7.2 and that the addition of a small amount of Ca2+ ions even below 5 mmol dm(-3) ([Ca2+] : [PIP2] = 5/7) induces a drastic structural phase transition from prolate micelles to disorde red lamellae. Mg2+ ions induced a similar phase transition around 10 m mol dm(-3) which, however, occurred less sensitively. In contrast to t he case of divalent cations, the presence of Na+ ions did not affect t he PIP2 micellar structures up to 0.1 mol dm(-3) ([Na+] : [PIP2] = 100 /7), indicating that the above structural change of the PIP2 aggregate s is not attributable to the elevation of the electrostatic shielding effect between the polar heads. Modelling analysis suggests that the o bserved phase transition results from an increase of the critical pack ing parameter of the PIP2 molecule accompanying the dehydration of the PIP2 polar-head region.