Membrane Bending Modulus for Ternary Mixture Models of the Cell Plasma Membrane

Abstract

Ternary mixtures of high-melting lipid, low-melting lipid, and cholesterol exhibit a region of liquid-ordered + liquid-disordered phase coexistence analogous to raft + non-raft behavior in cells. These coexisting phases manifest domain sizes that range from a few nanometers to many microns, depending strongly on the nature of the low-melting lipid. When POPC, which gives rise to nanodomains, is replaced by DOPC, which yields macrodomains, an intermediate region is observed of patterned, or modulated phases. This domain morphology can be explained as a competition between line tension and bending energies with patterns occurring when the two are nearly balanced. Necessary for testing this model are measurements of line tension, and bending moduli for both phases. Here we report the bending moduli of coexisting Lo and Ld phases from mixtures that produce domains ranging in size from nanoscopic to macroscopic. Measurements were made by shape analysis of giant unilamellar vesicles with both fluorescence and phase contrast microscopy. Vesicles of a single phase were made by the gentle hydration method to obtain a more narrow distribution of vesicle tensions than is obtained by electroformation. A transmembrane helical peptide, WALP23, strongly partitions to the liquid disordered phase, and changes the size of coexisting Lo + Ld domains. We are currently investigating the effects of WALP23 on membrane mechanical properties and line tension.