Abstract
Glycosylphosphatidylinositols (GPIs), natural complex glycolipids essential for a range of biological functions, are poorly understood with regard to their interactions and arrangements in cellular membranes. To evaluate the role of the head group in the structure formation in 2D model membranes (monolayers formed at the soft air/liquid interface), we employed the highly surface sensitive grazing incidence X-ray diffraction technique to investigate three GPI-fragments bearing the same hydrophobic part but different head groups. Condensed monolayers of simple GPI fragments are defined only by ordered alkyl chains. The monolayers of more complex fragments are additionally characterized by highly ordered head groups. Due to the strong H-bond network formed by the head groups, GPI-fragment both segregates and induces order into a model membrane phospholipid (POPC) that mimics the liquid-disordered phase of cell membranes. Here, we show that the strong van der Waals interactions between hydrophobic chains overcome the head group interactions and dominate the structure formation in mixtures of GPI-fragment with lipids that form liquid-condensed phases. This behaviour can be linked to the GPIs affinity for the lipid rafts.
Highlights
To evaluate the role of the head group in the structure formation in 2D model membranes, we employed the highly surface sensitive grazing incidence X-ray diffraction technique to investigate three GPI-fragments bearing the same hydrophobic part but different head groups
We show that the strong van der Waals interactions between hydrophobic chains overcome the head group interactions and dominate the structure formation in mixtures of GPI-fragment 3 with lipids that form liquid-condensed phases
A comparative analysis of the structural arrangement in a series of 2D model membranes of three GPI-fragments demonstrates that increase in the size of the head groups from 1 to 2 and 3 results in respective increase in the in-plane area per molecule that causes increase in the tilt of the alkyl chains and increase in surface pressure required for the transition to a non-tilted phase
Summary
Glycosylphosphatidylinositols (GPIs) are natural complex glycolipids present on the outer leaflet of eukaryotic cell membranes.[1,2,3] Existing as free GPIs or as part of GPI-anchored proteins (GPI-APs), they are involved in a range of vital biological functions such as signal transduction, intermembrane transport, protein sorting and trafficking.[4,5,6,7,8] Their affinity for temporarily existent liquidordered membrane microdomains (lipid rafts), not well understood, is postulated to play a role in important biological processes.[9,10,11,12] Details about the structural arrangement of GPIs in cellular membranes could provide a better understanding of the chemical structure/structural membrane arrangement/biological function relationship for GPI glycolipids and the contributions of GPIs to the function of the attached protein that could have wide biomedical implications.Paper provide a description of the molecular ordering in investigated model membranes with angstrom-level resolution and offer insights into interactions between structurally different lipids. We show that the strong van der Waals interactions between hydrophobic chains overcome the head group interactions and dominate the structure formation in mixtures of GPI-fragment 3 with lipids that form liquid-condensed phases.
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