Vesicles with multiple membrane domains

Vesicles with two types of intramembrane domains, both of which are in the fluid state, are studied theoretically using free energy minimization and Monte Carlo simulations. Specific examples are provided by liquid-ordered and liquid-disorderd domains, which are formed by phase separation within multi-component membranes. Multi-domain morphologies, which arise from the interplay between the bending energies of the domains and the line energy of the domain boundaries, are found for a wide range of these elastic parameters and can be further stabilized by the constraint of constant volume, by spontaneous curvatures, and/or by a difference in the Gaussian curvature moduli of the two types of domains. In the latter case, the vesicles attain stable multi-domain morphologies with up to six liquid-disordered domains. Morphologies with three and four such domains have also been observed experimentally, in qualitative agreement with our simulations. For sufficiently small line tension, "lipid rafts" with multiple liquid-ordered domains are also found to be stable if the liquid-ordered domains have a positive spontaneous curvature. The vesicles are found to undergo a variety of transitions between different multi-domain morphologies. Presumably the simplest way to explore these morphological transitions experimentally is by changing the vesicle volume via osmotic deflation.