Viscosity Landscape of Phase-Separated Lipid Membrane Estimated from Fluid Velocity Field

In cell membranes, the functional constituents such as peptides, proteins, and polysaccharides diffuse in a sea of lipids as single molecules and molecular aggregates. Thus, the fluidity of the heterogeneous multicomponent membrane is important for understanding the roles of the membrane in cell functionality. Recently, Henle and Levine described the hydrodynamics of molecular diffusion in a spherical membrane. A tangential point force at the north pole induces a pair of vortices whose centers lie on a line perpendicular to the point force and are symmetrical with respect to the point force. The position of the vortex center depends on η_m/Rη_w, where R is the radius of the spherical membrane, and η_m and η_w are the viscosities of the membrane and the surrounding medium, respectively. Based on this theoretical prediction, we applied a point force to a phase-separated spherical vesicle composed of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/1,2-dioleoyl-sn-glycero-3-phosphocholine/cholesterol by means of a microinjection technique. The pathlines were visualized by trajectories of microdomains. We determined the position of the vortex center and estimated the membrane viscosity using the dependence of the position of the vortex center on η_m/Rη_w. The obtained apparent membrane viscosities for various compositions are mapped on the phase diagram. The membrane viscosity is almost constant in the range of 0 <ϕ_Lo ≤ 0.5 (ϕ_Lo: area fraction of the liquid ordered phase), whereas that in the range of 0.5 ≤ ϕ_Lo < 1.0 exponentially increases with increase of ϕ_Lo. The obtained viscosity landscape provides a basic understanding of the fluidity of heterogeneous multicomponent membranes.