Supra-Molecular Organization of the Crystallographic Conformations of Inactive and Activated μ-Opioid Receptor in a Multi-Component Plasma Membrane Model

Abstract

While oligomerization of several G Protein-Coupled Receptor systems, including the μ-opioid receptor (MOR), is an accepted phenomenon, its role in receptor signaling is the subject of much debate as are the specific interfaces formed and the time-scales of their formation and disruption. Crystallographic information of the antagonist-bound, inactive MOR suggested formation of an extensive dimer interface involving transmembrane (TM) helices TM5 and TM6 (TM5,6) on each protomer and a less compact symmetric interface involving TM1, TM2, and helix 8 (TM1,2,H8). The recent agonist-bound, activated MOR crystal structure confirmed that the TM5,6 interface is incompatible with conformational changes occurring upon activation, arguing against its physiological relevance.In this work, several independent, multi-microsecond, coarse-grained molecular dynamics simulations of arrays of 16 MORs in the inactive or activated conformations were performed to elucidate possible differences in the supra-molecular organization of the receptors in the Martini force field plasma membrane model composed of 63 different lipid types. Unlike the crystallographic TM1,2,H8 interface, which was observed for both inactive and activated conformations of MOR, the TM5,6 dimer interface was not seen in these simulations. In general, interfaces involving TM5 in either dimeric or oligomeric arrangements were formed much less frequently in the case of the activated receptor. While both the inactive and activated states of MOR formed dimers on the same timescale, higher-order oligomers (trimers and tetramers) were observed much more quickly for the inactive system than the activated system. It appeared as if the ability of the inactive state to associate through TM5 allowed for easier formation of higher order oligomers, although other factors (e.g., specific lipids) may have played a role, and are currently under investigation.