Measurements of Reductionist Membranes that Beautifully Fit Physics Theories

Abstract

Micron-scale liquid domains appear in lipid membranes containing three lipid types (lipids with high melting temperature, lipids with low melting temperature, and cholesterol or a similar sterol) when the membrane is below a miscibility transition. When this transition occurs at a critical point, large fluctuations appear within the membranes. The fluctuations are described by beautiful physics: the critical exponents for correlation length and for the difference in composition between the two phases are consistent with the universality class of the 2-dimensional Ising model (Honerkamp-Smith et al., BJ, 2008). Complex mixtures of lipids and proteins derived from cell membranes in GPMVs (giant plasma membrane vesicles) exhibit the same critical behavior (Veatch et al., ACS Chem. Biol., 2008). Recently, we measured the effective dynamic critical exponent relating the decay time of membrane composition fluctuations to the wavenumber (an inverse length). We find that at temperatures far from the critical point, the exponent is 2, as expected from diffusion. As the temperature approaches the critical point, the exponent increases. We find that submicron membrane fluctuations corresponding to a wavenumber of 1/(50nm) persist for at least 0.8±0.3ms, on the order of times required for changes in protein configuration (e.g. 1ms). Therefore, similar and longer-lived fluctuations in cell membranes can potentially alter protein function.