Internal Sodium in GPCRs Strongly Responds to Transmembrane Voltage Changes

Abstract

G protein-coupled receptors (GPCRs) are the largest superfamily of membrane proteins in the human genome, mediating the propagation of extracellular ligand binding information into intracellular signal transduction cascades. Crystal structures have revealed a water-filled hydrophilic internal pocket within their transmembrane domain, extending from the orthosteric ligand-binding site to regions near the G protein binding site. Recent high-resolution structures have identified a sodium ion near the base of this pocket, coordinated by highly conserved residues (1,2). Owing to this conservation level, sodium binding may play a role for most rhodopsin-like GPCRs (3). Recently, functional or conformational effects have been shown to be elicited by physiological transmembrane voltage in several GPCRs, and voltage-clamp recordings have determined a gating charge of ∼0.85 e in the M1 and M2 muscarinic receptors (4). However, the nature of the voltage sensor has remained enigmatic. Here we show by MD and computational electrophysiology simulations (5) that the internal sodium ion in the delta-opioid and muscarinic receptors is highly mobile under the influence of transmembrane voltage changes. By calculating the energetics of ion movement along the pocket axis, we find that the free energy barriers to migration can be overcome by the physiological voltage range. Furthermore, we demonstrate that the motion along the axis creates a gating charge in excellent agreement with the experimentally measured gating currents. Thus, voltage-induced movements of Na+ in GPCRs may represent an attractive mechanism for voltage-regulation of GPCRs.1. Liu, W. et al., Science 337: 232-236, 2012.2. Fenalti, G. et al., Nature 506: 191-196, 2014.3.Katritch, V. et al., TiBS 39: 233-244, 2014.4. Ben-Chaim, Y. et al., Nature 444: 106-9, 2006.5. Kutzner, C. et al., Biophysical Journal 101: 809-817, 2011.