Tuning Length Scales of Small Domains in Cell-Derived Membranes and Synthetic Model Membranes

Abstract

Micron-scale, coexisting liquid-ordered (L) and liquid-disordered (L) phases are straightforward to observe in giant unilamellar vesicles (GUVs) composed of ternary lipid mixtures. Experimentally, uniform membranes undergo demixing when temperature is decreased: domains subsequently nucleate, diffuse, collide, and coalesce until only one domain of each phase remains. The sizes of these two domains are limited only by the size of the system. Under different conditions, vesicles exhibit smaller-scale domains of fixed sizes, leading to the question of what sets the length scale. In membranes with excess area, small domains are expected when coarsening is hindered or when a microemulsion or modulated phase arises. Here, we test predictions of how the size, morphology, and fluorescence levels of small domains vary with the membrane's temperature, tension, and composition. Using GUVs and cell-derived giant plasma membrane vesicles, we find that 1) the characteristic size of domains decreases when temperature is increased or membrane tension is decreased, 2) stripes are favored over circular domains for lipid compositions with low energy per unit interface, 3) fluorescence levels are consistent with domain registration across both monolayer leaflets of the bilayer, and 4) small domains form in GUVs composed of lipids both with and without ester-linked lipids. Our experimental results are consistent with several elements of current theories for microemulsions and modulated phases and inconsistent with others, suggesting a motivation to modify or enhance current theories.

Model Plasma Membrane exhibits a Microemulsion in both Leaves providing a Foundation for “Rafts”

“…The experiments of ref. (26) are particularly illuminating as they show that the behavior of the characteristic length with surface tension is quite consistent with the theory (27). To observe a microemulsion in the plasma membrane with a characteristic size of 100 nm would require a neutron scattering experiment.…”

“…Indeed some of its predictions have been confirmed, such as the behavior of the characteristic domain size with temperature and surface tension. However whereas domains in a symmetric membrane would, by this theory that has no direct coupling between leaves (44, 45) be predicted to be anti-correlated, domains were reported to be corrrelated (26). There is clearly a need for additional experimental tests.…”

“…where the five terms are given in Eqs. (25)(26)(27)(28)(29) It is convenient at this point to express all fluctuations in terms of the Fourier transforms…”

“…This was cited as a reason that this mechanism could, indeed, account for the origin of rafts. Since then, microemulsions have been observed in model membranes as well as in cell-derived ones (24)(25)(26). They have not always been identified as such, however (24,25).…”

Recent Experiments Support a Microemulsion Origin of Plasma Membrane Domains: Dependence of Domain Size on Physical Parameters

“…The domain size diverges at small surface tension, decreases as the tension increases, and then increases with increasing tension as the Lifshitz line is approached. The increase of domain size with tension for large tensions has been experimentally observed both in giant unilamellar vesicles and in GPMVs [ 37 ]. We note that this observation, like that of the sequence of transitions observed in reference [ 22 ] argues that the GPMVs are also weakly coupled systems, i.e., that is not large.…”