Conformational Dynamics in a Seven Transmembrane Protein Anabaena Sensory Rhodopsin Probed by Solid State NMR

Abstract

Solid state NMR spectroscopy can be used to probe internal motions in membrane proteins in a lipid environment. In this study the site-specific measurements of the dipolar order parameters are reported and combined with the spin-lattice in the rotating frame (R1ρ) relaxation measurements carried out under fast magic angle spinning conditions in a seven transmembrane (TM) protein Anabaena Sensory Rhodopsin (ASR). Surprisingly, it was found that both the well-defined transmembrane regions and the mainly unstructured intramembrane loops and turns undergo restricted submicrosecond time scale motions corresponding to order parameters between 0.9 and 1.0. In contrast, the experimentally determined R1ρ relaxation rates vary by an order of magnitude between the TM and exposed fragments, indicating the presence of intermediate time scale motions that dominate the relaxation pathways for the exposed regions. Using a simple model that assumes a single exponential autocorrelation function, the dominant timescales of stochastic motions are estimated. Motions were found to be on the order of few nanoseconds for the TM helices, and tens to hundreds of nanoseconds for the extracellular B-C and F-G loops. These slower time scales may be associated with collective motions in these regions.