Structure determination of the seven-helix transmembrane receptor sensory rhodopsin II by solution NMR spectroscopy

Seven-helix membrane proteins represent a challenge for structural biology. Here we report the first NMR structure determination of a detergent-solubilized seven-helix transmembrane (7TM) protein, the phototaxis receptor sensory rhodopsin II (pSRII) from Natronomonas pharaonis, as a proof of principle. The overall quality of the structure ensemble is good (backbone r.m.s. deviation of 0.48 angstrom) and agrees well with previously determined X-ray structures. Furthermore, measurements in more native-like small phospholipid bicelles indicate that the protein structure is the same as in detergent micelles, suggesting that environment-specific effects are minimal when using mild detergents. We use our case study as a platform to discuss the feasibility of similar solution NMR studies for other 7TM proteins, including members of the family of G protein-coupled receptors.