Membrane Proteins Have Distinct Fast Internal Motion and Residual Conformational Entropy

For a variety of reasons, the internal motions of integral membrane proteins have largely eluded comprehensive experiential characterization. Here, the fast side chain dynamics of the 7-transmembrane helix protein sensory rhodopsin II and the beta-barrel bacterial outer membrane channel protein W have been characterized in lipid bilayers and detergent micelles by solution NMR relaxation techniques. Though of quite different topologies, both proteins are found to have a similar and striking distribution of methyl-bearing amino acid side chain motion that is independent of membrane mimetic. The methyl-bearing side chains of both proteins, on average, are more dynamic in the ps-ns time regime than any soluble protein characterized to date. Approximately one third of methyl-bearing side chains exhibit extreme rotameric averaging on this timescale. Accordingly, both proteins retain an extraordinary residual conformational entropy in the folded state, which provides a counterbalance to the absence of the hydrophobic effect that normally stabilizes the folded state of water-soluble proteins. Furthermore, the large reservoir of conformational entropy that is observed provides the potential to greatly influence the thermodynamics underlying a plethora of membrane protein functions including ligand binding, allostery and signaling.