Sorting mechanisms and communication in phase-separating coupled monolayers.

A continuous model of two coupled monolayers constituting a fluid bilayer membrane is presented. The model is based on the minimization of a membrane free energy considering in both monolayer leaflets two different molecule types, undergoing lateral phase separation. Differences in the mechanical properties of the molecules, such as shape, stiffness, and length are accounted explicitly by the model. In the presented model, coupling between monolayers is realized via an energy-based model depending on the local distance between the two monolayers as well as the lengths of molecules constituting the local monolayer region. We numerically study different passive mechanisms for molecule sorting and correlation across the bilayer induced by first-order mechanical constraints. Here, we focus on three aspects: First, we find that stretching of the two monolayers in the normal direction yields a sorting of molecules according to their length. Furthermore, we show that the length of molecules can be used to synchronize phases across the bilayer membrane. Moreover, we find that generating curvature in one layer (induced by different curvature creating mechanisms) sorts molecules of the other layer according to their shape and stiffness. Many recent experimental data indicate the importance of specific lipid-protein interactions and the role of the bilayer thickness in membrane protein function and sorting. The presented model proposes different mechanisms leading to a colocalization of different components in different monolayers at the same place at the same time.