• Xuelian Zhao, Jose Manuel Perez-Aguilar, Felipe Matsunaga, Mitchell Lerner, Jin Xi, Bernard Selling, A T Charlie Johnson, Jeffery G Saven, and Renyu Liu.
• From the Department of Anesthesiology and Critical Care (X.Z., F.M., J.X., R.L.), Department of Chemistry (J.M.P.-A., J.G.S.), and Department of Physics and Astronomy (M.L., A.T.C.J.), University of Pennsylvania, Philadelphia, Pennsylvania; and Impact Biologicals Inc., Wallingford, Pennsylvania (B.S.). Current addresses: Department of Anesthesiology, the Fourth Hospital of Hebei Medical University, Shijiazhuang, Hebei, China (X.Z.); and Department of Physiology and Biophysics, Weill Cornell Medical College, New York, New York (J.M.P.-A.).
• Anesthesiology. 2014 Oct 1; 121 (4): 866-75.

BackgroundThe recent X-ray crystal structure of the murine μ-opioid receptor (MUR) allowed the authors to reengineer a previously designed water-soluble variant of the transmembrane portion of the human MUR (wsMUR-TM).MethodsThe new variant of water-soluble MUR (wsMUR-TM_v2) was engineered based on the murine MUR crystal structure. This novel variant was expressed in Escherichia coli and purified. The properties of the receptor were characterized and compared with those of wsMUR-TM.ResultsSeven residues originally included for mutation in the design of the wsMUR-TM were reverted to their native identities. wsMUR-TM_v2 contains 16% mutations of the total sequence. It was overexpressed and purified with high yield. Although dimers and higher oligomers were observed to form over time, the wsMUR-TM_v2 stayed predominantly monomeric at concentrations as high as 7.5 mg/ml in buffer within a 2-month period. Its secondary structure was predominantly helical and comparable with those of both the original wsMUR-TM variant and the native MUR. The binding affinity of wsMUR-TM_v2 for naltrexone (K(d) approximately 70 nM) was in close agreement with that for wsMUR-TM. The helical content of wsMUR-TM_v2 decreased cooperatively with increasing temperature, and the introduction of sucrose was able to stabilize the protein.ConclusionsA novel functional wsMUR-TM_v2 with only 16% mutations was successfully engineered, expressed in E. coli, and purified based on information from the crystal structure of murine MUR. This not only provides a novel alternative tool for MUR studies in solution conditions but also offers valuable information for protein engineering and structure-function relations.

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