Water Controls Membrane Protein Stability

Through the insertion of nonpolar side chains into the bilayer, the hydrophobic effect has long been accepted as a driving force for membrane protein folding. However, how the changing chemical composition of the bilayer affects the magnitude side chain transfer free energies [Formula] has historically not been well understood. A particularly challenging region for experimental interrogation is the bilayer interfacial region that is characterized by a steep polarity gradient. In this study we have determined the [Formula] for nonpolar side chains as a function of bilayer position using a combination of experiment and simulation. We discovered an empirical correlation between the surface area of nonpolar side chain, the transfer free energies, and the local water concentration in the membrane that allows for [Formula] to be accurately estimated at any location in the bilayer. Using these water-to-bilayer [Formula] values, we calculated the interface-to-bilayer transfer free energy [Formula]. We find that the [Formula] are similar to the "biological", translocon-based transfer free energies, indicating that the translocon energetically mimics the bilayer interface. Together these findings can be applied to increase the accuracy of computational workflows used to identify and design membrane proteins, as well as bring greater insight into our understanding of how disease-causing mutations affect membrane protein folding and function.