Evidence for cholera aggregation on GM1-decorated lipid bilayers

Abstract

The binding properties of cholera toxin B (CTB) oligomer to substrate supported membrane bilayer, containing physiologically relevant concentrations of receptor glycolipids, viz. monosialoganglioside (GM1), have been extensively studied by the atomic force microscopy (AFM). Two distinct classes of GM1 containing membrane-mimetic surfaces were prepared: supported lipid bilayer membranes (sBLMs) on freshly cleaved mica and hybrid lipid bilayer membranes (hBLMs) on octadecyltrichlorosilane (OTS) derivatized silicon substrates. On sBLMs, aggregates with a well-defined ordered arrangement of individual CTB molecules were observed at all GM1 and cholera concentrations studied. In sharp contrast, features consistent with randomly distributed adsorbed individual CTB molecules were seen on a bare mica surface. On the hBLMs, the aggregate structures were only observed when the bilayer was formed onto ordered OTS surfaces, offering continuous and defect-free lipid membrane for the lateral diffusion of GM1. Ill-packed and disordered OTS monolayers yielded a random distribution of adsorbed proteins comparable to that observed for CTB binding on bare mica substrates. These observations strongly support that the aggregation of CTB–GM1 complex is a result of the specific interaction of CTB molecules with GM1 receptors in the fluid membrane bilayers. The high mobility of GM1 allows lateral diffusion of the complex to form ordered aggregates.