NMR and Modeling Studies of a Synthetic Extracellular Loop II of the κ Opioid Receptor in a DPC Micelle

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Abstract
This paper provides the first direct experimental evidence for the secondary structural features of the putative second extracellular loop (ECL II) of the kappa opioid receptor through a synthetic peptide mimic in a DPC micelle environment. These studies indicate that residues V(6)-A(15) of the ECL II peptide adopt a well-defined helical structure analogous to that formed by V(201)-C(210) of the native receptor. Moreover, a beta-turn around the D(22) (D(217)) and D(23) (D(218)) residues represents another feature of the ECL II. The NMR and fluorescent data also suggest the location of the two helical turns of TM V and the approximate location of the C-terminal end of the TM IV of the kappa opioid receptor. We modeled the kappa opioid receptor including the extracellular region of the receptor. The model of the ECL II utilized the information obtained from the NMR structural analysis of the ECL II peptide in a DPC micelle solution and the molecular dynamic simulations in a biphasic membrane environment. Our discovery of this amphiphilic helical region in the ECL II peptide by NMR and molecular modeling studies provides direct evidence that the sequence of residues V(201)-C(210) is likely to be the helical region that interacts with Dynorphin (Dyn) A [Paterlini, G., Portoghese, P. S., and Ferguson, D. M. (1997) J. Med. Chem. 40, 3254-3262]. We believe that this work offers further insight into the structural characteristics of the extracellular portions of the seven-TM kappa opioid receptor.