Unravelling the orientation of the inositol-biphosphate ring and its dependence on phosphatidylinositol 4,5-bisphosphate cluster formation in model membranes

Abstract

Hypothesis: Inositol phospholipids are well known to form clusters in the cytoplasmic leaflet of the plasma membrane that are responsible for the interaction and recruitment of proteins involved in key biological processes like endocytosis, ion channel activation and secondary messenger production. Although their phosphorylated inositol ring headgroup plays an important role in protein binding, its orientation with respect to the plane of the membrane and its lateral packing density has not been previously described experimentally.

Experiments: Here, we study phosphatidylinositol 4,5-bisphosphate (PIP₂) planar model membranes in the form of Langmuir monolayers by surface pressure-area isotherms, Brewster angle microscopy and neutron reflectometry to elucidate the relation between lateral (in-plane) and perpendicular (out-of-plane) molecular organization of PIP₂.

Findings: Different surface areas were explored through monolayer compression, allowing us to correlate the formation of transient PIP₂ clusters with the change in orientation of the inositol-biphosphate headgroup, which was experimentally determined by neutron reflectometry.

Keywords: Air/water interface; Brewster angle microscopy; Lipid monolayer; Neutron reflectometry; PIP(2) cluster formation; PIP(2) headgroup orientation; Phosphatidylinositol 4,5-bisphosphate (PIP(2)).