Orientation of the Headgroup of Phosphatidylinositol in a Model Biomembrane As Determined by Neutron Diffraction

Abstract

Derivatives of the sodium salt of dimyristoylphosphatidylinositol (DMPI) have been synthesized specifically deuterated in the headgroup. A 50:50 (molar) mixture of DMPI with dimyristoylphosphatidylcholine (DMPC) hydrated to the level of 16 waters/lipid gives a biomembrane-like Lalpha phase at 50 degrees C. Comparison of the neutron diffraction scattering profiles for deuterated and undeuterated membranes allowed the depth of each deuterium (hydrogen) within the bilayer to be determined to +/-0.5 A. This gave the orientation of the inositol ring which lies more-or-less along the bilayer normal projecting directly out into the water. This orientation is similar to that of the sugar residue in glycolipids and confirms previous models for PI. On the assumption that the (P)O-DAG bond is more-or-less parallel to the bilayer normal, it is consistent with a roughly trans, trans, trans, gauche- conformation for the glyceryl-phosphate-inositol link. In the case of DMPI, it is the C4-hydroxy group which is most fully extended into the water layer, but when this is phosphorylated, the inositol ring turns over and tilts so that the C5-hydroxy group is now the one furthest extended into the water layer. Hence, at each stage in the pathway PI --> PI-4P --> PI-4,5-P2, it is the hydroxy position most exposed to the water which undergoes phosphorylation. Whereas the orientation of the inositol ring in DMPI can be seen simply as maximizing its hydration, the tilt of the ring in DMPI-4P cannot be explained in this way. It is suggested that it is due to an electrostatic interaction.

Structural and mechanistic insights into the association of PKCα-C2 domain to PtdIns(4,5)P 2

“…Several studies have demonstrated that the unphosphorylated inositol ring is oriented perpendicular to the bilayer surface (26), whereas the double phosphorylation in C4 and C5 of the inositol induces a bend toward the bilayer surface (26,27). This orientation would be similar to the one found in the crystal structure presented here and thus would be compatible with a perfect docking of the C2 domain in the membrane interface being PtdIns(4,5)P 2 , the target molecule.…”

Structural investigation on the adsorption of the MARCKS peptide on anionic lipid monolayers – effects beyond electrostatic

“…It was found by Dietrich et al (2009), that the polyvalent PIP 2 molecules were arranged in a uniform distribution in the disordered phase of the mixed monolayer, which is consistent with the literature (DeWolf et al, 1999;Foster and Janmey, 2001;Fernandes et al, 2006). This lateral distribution was presumably caused by the perpendicular orientation of the PI(4,5) group to the membrane interface (Zhou et al, 1997;Bradshaw et al, 1999) and their repulsive interaction (Leventhal et al, 2008). The distance of the secondary phosphate group to the level of the phosphodiester groups of the membrane increased with increasing lateral pressure.…”

“…In contrast to the primary phosphate group z ph1 , the position of the secondary phosphate group z ph2 showed here a pronounced dependency on the presence of MARCKS (151-175). In the absence of the peptide, the PI(4,5) group evidently elongated in direction of the surface normal, in agreement with existing literature (Zhou et al, 1997;Bradshaw et al, 1999). The interaction of the acidic lipid with the MARCKS effector domain yielded a tilt of the PI(4,5) towards the membrane interface -confirmed by the decrease of the distance of the secondary phosphate group z ph2 to the interface, compared to the monolayer without the MARCKS peptide.…”

Lateral Sequestration of Phosphatidylinositol 4,5-Bisphosphate by the Basic Effector Domain of Myristoylated Alanine-rich C Kinase Substrate Is Due to Nonspecific Electrostatic Interactions

“…A model for the PIP 2 head group was taken from the structure of the pleckstrin homology domain of PLC-␦ 1 bound to IP 3 (Protein Data Bank code 1MAI) (65). We assumed that the head group of PIP 2 is perpendicular to the surface, similar to the orientation of phosphatidylinositol (66,67). We derived a detailed partial charge distribution for its atoms from similar functional groups in the CHARMM parameterization set.…”