"Biophysical factors underlying Myelin-associated Glycoprotein (MAG) Binding to Gangliosides"
Gangliosides are a subgroup of glycosphingolipids that populate the outer cell surface and are an integral part of the membrane lipid bilayer; they have a hydrophilic head group containing various sugars and sialic acid, a strategic position that enables them to interact with various extracellular molecules, such as pathogens, toxins, and antibodies, as well as other cells. As a result, they mediate several cellular processes, including membrane trafficking, signal transduction, endocytosis, cell recognition, and cell death. Within the cell membrane, the gangliosides cluster with cholesterol in membrane microdomains. Unlike the surrounding disordered phospholipid bilayer enriched in unsaturated phosphatidylcholine (PC), the molecules in the microdomains are more ordered and tightly packed because they contain saturated PC, sphingomyelin, cholesterol, and gangliosides. Because gangliosides self-associate with cholesterol, this raises the question of whether that association affects their availability for subsequent protein interactions. The literature shows that a cholesterol-rich environment increases influenza virus interaction with ganglioside GD1a, while the same environment reduces cholera toxin subunit B interactions with ganglioside GM1. In addition to the fact that these results contrast, these studies involve exogenous agents (toxins and viruses) rather than endogenous ganglioside-binding partners. The overall theme of my presentation is to describe how cholesterol or a cholesterol-rich environment influences ganglioside interactions with myelin-associated glycoprotein (MAG), an endogenous ganglioside-binding partner found exclusively in the nervous system. I will describe the development of a cell-membrane-mimicking system to quantitatively investigate how cholesterol and membrane phase separation affect binding interactions between MAG and GD1a and GT1b gangliosides. My experimental findings show that cholesterol reduces binding, and that membrane phase also affects the interaction; the larger the membrane phase, the higher the fraction of gangliosides available to bind MAG. In another project, I also found that lipid compositional mismatch can be used to dissolve the membrane phase.