Nanomechanical recognition of sphingomyelin-rich membrane domains by atomic force microscopy

Sphingomyelin (SM) is a reservoir of signaling lipids and forms specific lipid domains in biomembranes together with cholesterol. In this study, atomic force microscopy (AFM) and force measurement were applied to investigate the interaction of SM-binding protein toxin, lysenin, with N-palmitoyl-D-erythro-sphingosylphosphorylcholine (palmitoyl sphingomyelin, PSM) bilayer spread over a mica substrate, in an aqueous buffer solution. Lysenin molecules were grafted on a silicon nitride tip for AFM by siloxane-thiol-amide coupling. The bilayers were prepared by the Langmuir-Blodgett (LB)/Langmuir-Schaefer (LS) method. By repeating cycles of tip approach/retraction motion, single-molecular adhesion motions were observed on the force curve, characterized as "fishing curves". The addition of cholesterol and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) did not alter the peak force but increased the peak extension. Mixtures of PSM/DOPC/cholesterol exhibited 2-dimensional two-phase domain separation. The characteristic fishing curves were observed exclusively in one of the phases, indicating the selective interaction of the lysenin tip to PSM-rich membrane domains. Our results indicate that the AFM tips conjugated with lysenin are useful to detect the surface distribution of SM-rich membrane domains as well as the nanomechanical properties of the domains.