Membrane-adhesion-induced phase separation of two species of junctions

A theory of membrane-adhesion-induced phase separation of two species of ligand-receptor complexes (i.e., junctions) is presented. Different species of junctions are assumed to have different natural heights and flexibilities. It is shown that the equilibrium properties of the system are equivalent to a membrane under an effective external potential, and for given junction flexibility difference phase separation occurs at sufficiently large junction height difference. The phase coexistence curve shows two distinct regions. (i) When junction height difference is large, the system is far from the mean-field critical point. Because of the higher entropy associated with softer junctions, phase coexistence occurs when the harder junctions have higher effective binding energy (free energy released due to the formation of a junction). (ii) When junction height difference is small such that the system is near the mean-field critical point, the contribution of the binding energy of the softer junctions to the free energy of the state with intermembrane distance close to the natural height of the harder junctions is not negligible. Therefore phase coexistence occurs when the harder junctions have smaller effective binding energy. Monte Carlo simulation that studies the effect of non-Gaussian fluctuations on the critical point indicates that the situation described in (ii) can be observed in typical biological systems.