The GM1 Ganglioside Forms GM1-Rich Gel Phase Microdomains within Lipid Rafts

Abstract

Mercury-supported, self-assembled monolayers (SAMs) of the sole dioleoylphosphatidylcholine (DOPC) and of a raft-forming mixture of DOPC, cholesterol (Chol) and palmitoylsphingomyelin (PSM) of (59:26:15) mol% composition, were investigated by electrochemical impedance spectroscopy (EIS), both in the absence and in the presence of the monosialoganglioside GM1. The impedance spectra of these four SAMs were fitted by a series of parallel combinations of a resistance and a capacitance (RC meshes) and displayed on plots of ωZ′ against −ωZ″, where Z′ and Z″ are the in-phase and quadrature components of the impedance and ω is the angular frequency. A comparison among these different impedance spectra points to the formation of GM1-rich gel phase microdomains within the lipid rafts of the DOPC/Chol/PSM mixture, thanks to the unique molecular-level smooth support provided by mercury, which allows EIS to detect the protruding gel phase microdomains by averaging them over a macroscopically large area.

Nanoscale Membrane Budding Induced by CTxB and Detected via Polarized Localization Microscopy

“…To minimize the possibility of a spontaneous GM1 clustering in the bilayers, a low GM1 concentration was used here (0.3 mol %); however, GM1-rich gel phases have been observed in otherwise fluid bilayers (46). For a binary system of POPC and GM1, a GM1-rich gel phase is expected to form at low temperatures and high GM1 concentrations (>1% GM1).…”

“…The steric pressure between the crowded CTxB within a nanoscale area could inherently encourage membrane bending if there was an attractive force on the GM1 to counter the steric repulsion between CTxB and provide a local membrane torque (48). An attraction between GM1 is plausible considering the strong liquid-ordered phase preference of GM1 in ternary lipid mixtures, the affinity of GM1 to self-cluster (46,49,51), and the possibility of GM1 unbound to CTxB further accumulating around the clusters of CTxB-GM1. A leaflet asymmetry in GM1-GM1 clustering and CTxB binding could encourage a mismatch of composition between the leaflets and encourage curvature.…”