Two- Dimensional Macroscopic Protein Domains Induced by the Interplay between Lipid- Protein and Protein- Protein Interactions

It has been suggested that lipids and proteins are not homogeneously distributed in cell membranes; they can segregate into dynamic micro/ nanodomains, serving as centers for signal transduction, membrane trafficking, and cytoskeletal organization. Here we ask the question whether two- dimensional protein domains can be created by the interactions between membrane- anchored multivalent proteins. Utilizing the binding pairs, SH3 (SRC homology 3) and PRM (proline-rich motif), which were recently reported to form phase-separated micro-droplets in solution [Li et al., 2012, Nature, 483, 336-340], with histidine tags allowing efficient binding to lipid membranes containing nitrilotriacetic acid (NTA) lipids, we demonstrated that macroscopic protein domains appeared in both giant unilamellar vesicles (GUVs) and Langmuir monolayers. In GUVs, these domains remained circular over a large range of temperatures and protein concentration ratios. In Langmuir monolayers, domains showed reversible transitions from circular shapes to fractal ones depending on surface pressures. Overall, we have demonstrated that the interplay between lipid- protein and protein- protein interactions can induce phase separation of proteins on model membranes.