# Interactions between Phase-Separated Liquids and Membrane Surfaces
## 4 Phase Separation Mediated by Membranes Can Reshape and Laterally Reorganize Membranes
### Phase-Separated Protein Domains Modify the Properties of Membranes and Reshape Them  
The phase separation of proteins restricted to a lipid bilayer can create spontaneous curvature by modifying the physical properties of a leaflet, leading to the reshaping of membranes. Phase separation of integral proteins and aggregation of peripheral proteins into domains on one surface of a membrane is expected to generate spontaneous curvature that can bend membranes [48]. Spontaneous curvature in membrane nanotubes has been shown to generate undulating and eventually pearling morphologies [49; 50]. Simulation of a phase-separated protein domain on the exterior of a nanotube demonstrated that this kind of morphological change can be driven by spontaneous curvature and by a difference in membrane stiffness introduced locally by a protein domain [51]. A recent in vitro study of proteins forming liquid-like domains on membranes lends support to these models: Protein domains on the exterior surface of vesicles tubulated the membrane inwards due to a compressive force--originating from the protein-protein interactions that led to phase separation--that reduced the outside leaflet area. Spontaneous curvature modelled the difference in leaflet area well in accompanying simulations, and the leading of the nanotubes could be controlled by modulating the stiffness of the protein domain [52]. These domains bent the vesicle membrane inward and coated the interior of the nanotube, however, unlike the aforementioned simulations of domains on the exterior of nanotubes.  
This lipid bilayer leaflet compression force exhibited by a biomolecular condensate reflects the fact that membrane-mediated biomolecular condensates have the ability to laterally reorganize lipids and other bilayer components. The clearest case of this phenomenon is when a lipid is covalently bound to a phase-separating macromolecule. When cholesterol-bound polyU (chol-polyU) and poly-L-lysine were triggered to phase separate within a vesicle, the resulting biomolecular condensate remained close to the membrane, both chol-polyU and fluorescent Rh-PE lipids were concentrated in the membrane region in contact with the biomolecular condensate, and the liposome experienced a sudden and permanent decrease in size. It should be noted that this decrease in size, thought to coincide with biomolecular condensate nucleation, could have many causes including loss of lipids to the condensate [53].  
Lipid phase separation into heterogeneous membrane domains is a lateral reorganization phenomenon that is observable in giant vesicles [54; 55; 56]; it is mostly beyond the scope of this review. However, lipid and protein phase separation can affect each other: The two phenomena can promote one another, even to the point that homogenous but nearly phase-separated membranes treated with membrane-localized proteins below the phase separation concentration threshold could undergo dual phase separation, where both the lipid and protein layers phase separate [46]. One recent study proposes that the promotion of lipid phase separation by a signaling-associated membrane-bound biomolecular condensate could play a role in mediating downstream signaling [57]. Proteins in the membrane can be concentrated within (or excluded from) membrane-mediated biomolecular condensates, similar to proteins and biomolecular condensates free in solution. We will discuss this phenomenon and its functional consequences later.