Molecular Dynamics of Proteorhodopsin in Lipid Bilayers by Solid-State NMR

Environmental factors such as temperature, hydration, and lipid bilayer properties are tightly coupled to the dynamics of membrane proteins. So far, site-resolved data visualizing the protein's response to alterations in these factors are rare, and conclusions had to be drawn from dynamic data averaged over the whole protein structure. In the current study, high-resolution solid-state NMR at high magnetic field was used to investigate their effects on the molecular dynamics of green proteorhodopsin, a bacterial light-driven proton pump. Through-space and through-bond correlation experiments were employed to identify and characterize highly mobile and motionally restricted regions of proteorhodopsin. Our data show that hydration water plays an essential role for enhancing molecular dynamics of residues in tails and interhelical loops, while it is found less important for residues in transmembrane domains or rigid, structured loop segments. In contrast, switching the lipids from the gel to their liquid crystalline phase enhances molecular fluctuations mainly in transmembrane helices on a time scale of 10 but has little effect on loop and tail residues. Increased mobility is especially observed in helices C, F, and G, but also in the EF loop. Fluctuations in those regions are relevant to structural dynamics during the photocycle of proteorhodopsin. Our data are important for the functional understanding of proteorhodopsin, but also offer an important contribution to the general understanding of site-resolved effects of water and lipid bilayets onto the dynamic properties of membrane proteins.