Temperature Dependence of Membrane Viscosity of Ternary Lipid GUV with Lo Domains.

Abstract

In the cell membrane, it is considered that saturated lipids and cholesterol organize liquid-ordered (L_o) domains in a sea of liquid-disordered (L_d) phase, and proteins relevant to cellular functions are localized in the L_o domains. Since the diffusion of transmembrane proteins is regulated by the membrane viscosity, we investigate the temperature dependence of the membrane viscosity of the ternary GUV composed of the saturated lipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), the unsaturated lipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and cholesterol (CHOL) to understand the effect of the phase separation on the membrane viscosity using microinjection technique. In the microinjection method, membrane viscosity is estimated by comparing the flow pattern induced on a spherical membrane with a hydrodynamic model. For phase-separated GUVs, the flow pattern is visualized by the motion of the domains. In this study, we developed a method to visualize the flow patterns of homogeneous GUVs above the phase separation temperature by using beads attached to the GUVs. We succeed to measure the membrane viscosity of ternary GUVs both above phase separation temperature and in the phase-separated region and find that the membrane viscosity decreases dramatically by the phase separation. In the phase-separated region, i.e., GUV with L_o domains, the membrane viscosity is determined by that of the L_d phase, η_Ld, and show weak temperature dependence compared with that of the DOPC single component GUV, which is a main component of the L_d phase. We revealed that the Moelwyn-Hughest (MH) model which takes into account the effects of the membrane composition, viscosity of pure component and interaction between components well describes the obtained membrane viscosity of the ternary GUV both above phase separation temperature and in the phase-separated region. The drastic decrease of the membrane viscosity by the phase separation plays an important role in regulating the mobility of constituents in multi-component membranes.