Protein Crowding Buffers Membrane Fluidity

Like cholesterol, protein crowding has been shown to alter the temperature dependence of lateral diffusion in biomembranes, eventually eliminating the distinct liquid-gel phase transition. Here, we resolve the molecular origin of this protein effect with coarse-grained and all-atom simulations of protein-crowded phosphatidylcholine bilayers, which give access to lipid structure and dynamics on both sides of the transition temperature. Unlike cholesterol, which is known to induce a continuous liquid ordered phase by having opposite effects on the two phases, we find that membrane proteins decrease lipid order at all temperatures studied. The disordering is stronger at low temperatures, so that once the proteins cover roughly half of the membrane area the low- and high-temperature phases become alike and the transition is lost. Our results suggest that in addition to local cholesterol concentration, local protein concentration may also help dictate membrane fluidity and especially its response to temperature change.