In recent years there has beeen widespread interest in the agonist-stimulated breakdown and turnover of phosphatidylinositol lipids (or polyphosphoinositides). Although these lipids comprise < 10 % of the total cellular phospholipid, changes in the synthesis or distribution of poly-phosphoinositides appear to be involved in the signalling pathways that trigger a wide range of cellular processes.
Since the inositides are only present in very low concentrations in the membrane and furthermore since certain forms (in particular 3-phosphorylized inositides) are only present for a very short time, it is assumed that the inositides are not homogeneously distributed in the membrane but rather that they are compartimentalized [1]. Therefore, the structure of mixed monolayers of phosphatidylcholine (PC), the major lipid component of plasma membranes, and phosphatidylinositol (PI) and the possibility of phase separation in these films has been studied using surface pressure-area isotherms, grazing incidence X-ray diffraction, Brewster angle microscopy and scanning force microscopy [2].
The mixed (up to 20 mol% PI) monolayer isotherms exhibited a plateau region corresponding to the collapse pressure of PI. The phase separation of the mixed monolayers into a DSPC condensed phase and a PI fluid phase was confirmed by BAM and SFM measurements. DSPC forms a condensed monolayer with centred rectangular structure and tilt towards nearest neighbours (NN) while PI formed a fluid monolayer with no lateral structure at all pressures investigated. Mixed DSPC/PI monolayers containing 20 mol% PI also exhibited a centred rectangular lattice with NN tilt. At surface pressures below the collapse of the fluid, PI phase, the addition of 20 mol% PI has very little effect on the structure of the condensed phase indicating that the phases almost completely demixed.
References
[1] P. Gascard, M. Sauvage, J.-C. Sulpice and F. Giraud Biochemistry, 32 5941-5948 (1993).
[2] C. DeWolf, S. Leporatti, C. Kirsch, R. Klinger , G. Brezesinski Chem. Phys of Lip. (1998) in press.