Phys. Rev. Lett. 112, 188101 (2014)

Cell membrane components can recognize and interact with one another because the underlying lipid membrane is a two-dimensional fluid. Dependent on membrane viscosity, phospholipids and membrane-bound proteins exhibit lateral and rotational diffusion behavior that alters a membrane's physiological properties and activities. Current strategies to measure membrane viscosity using routine lipid- and protein-diffusion coefficient measurements are inadequate because they involve problematic assumptions and difficulty in controlling the generation of probes. To overcome these issues, Hormel et al. developed a model membrane system using planar bilayers that are probed with fluorescent microspheres that can be visualized by fluorescence microscopy to determine the orientation and position of the beads. Viewed in the context of two mathematical models describing diffusion in a planar membrane, the data provide rotational and translational diffusion coefficients of the tracers and tracer radii as well as measurements of viscosity. The viscosity measurements were approximately ten times larger than what has been reported for other fluid membranes, which may relate to the membrane geometry used here. The authors also used their methodology to show that Sar1p, a small GTPase that can induce membrane curvature during vesicle traffic, markedly increases the viscosity of the bilayers, which should be sufficient to affect vesicle trafficking dynamics.