How GM1 Affects the Phase State and Mechanical Properties of Phospholipid Membranes

Abstract

Even though glycolipids are present at increased concentrations in the outer leaflet of eukaryotic biomembranes, their influence on the mechanical properties of the membrane has not been studied in much detail. In this work, we investigate the effect of GM1, a prominent example among glycolipids, on the physical characteristics such as phase state and bending rigidity of membranes. Both giant vesicles and large unilamellar vesicles made of palmitoyloleoylphosphatidylcholine (POPC) are explored. We find that for GM1 fractions above ∼5 mol%, the membranes are phase separated at room temperature and exhibit GM1-rich microdomains with gel-like nature as observed by fluorescent microscopy. However, cholera toxin B, which is conventionally used as a GM1 marker, is found to be excluded from these domains. So is the fluorescently labelled conjugate Bodipy-GM1. We also explore the influence of GM1 on the membrane bending rigidity, which determines how pliable the membrane to deformations is. Results obtained from fluctuation analysis of giant vesicles and from the method of vesicle electrodeformation show a significant decrease in the membrane bending rigidity with increasing GM1 fractions when approaching the 5 mol% region. Micropipette aspiration measurements show that GM1 also leads to a decrease in the stretching elasticity modulus suggesting stronger coupling of the two leaflets of the membrane and probably also thickening of the bilayer. Our results are relevant to understanding the plasticity of neurons and their protrusions.