Dynamics of Bicelle Model Membrane Promote Domain Formation

Abstract

We have created a magnetically-aligned bicelle model membrane system (containing POPC, DMPC, and cholesterol) that laterally separates to form lipid domains. Lipid domains have been observed in a number of ternary systems (cholesterol, saturated lipid, unsaturated lipid), but rarely in systems containing POPC and never in systems containing POPC and DMPC. Using pulsed-field gradient NMR techniques, we measured the lateral diffusion constant for the lipids as a function of diffusion time. The time dependence of the diffusion constant showed the presence of domains ∼1um in size at 295K. The domains increase in size with temperature, until an isotropic membrane structure appears, indicating that the domains are liquid-disordered phase. Variable temperature 1H-MAS NMR of the domains supports this assignment. Bicelles are magnetically-aligned, highly perforated bilayers often used in solid state NMR for membrane protein structure determination. The formation of POPC/DMPC/cholesterol domains may result from the rapid (τc∼10-6 s) undulations of the bicelle bilayer surface. The domain-forming bicelles are also suitable for NMR structure determination of raft-associated membrane proteins.