G protein-coupled receptors of class A harness the energy of membrane potential to increase their sensitivity and selectivity

Abstract

The human genome contains about 700 genes of G protein-coupled receptors (GPCRs) of class A; these seven-helical membrane proteins are the targets of almost half of all known drugs. In the middle of the helix bundle, crystal structures revealed a highly conserved sodium-binding site, which is connected with the extracellular side by a water-filled tunnel. This binding site contains a sodium ion in GPCRs crystallized in their inactive conformations, but not in GPCRs that were trapped in agonist-bound active conformations. The escape route of the sodium ion upon the inactive-to-active transition and its very direction, either into the cytoplasm or back outside the cell, have remained obscure. By modeling the available experimental data we show that the sodium gradient over the cell membrane would increase the sensitivity of GPCRs if their activation were thermodynamically coupled to the sodium ion translocation into the cytoplasm, but decrease it if the sodium ion retreats into the extracellular space upon receptor activation. The model quantitatively describes the available data on both activation and suppression of distinct GPCRs by membrane voltage. The model also predicts selective amplification of the signal from (endogenous) agonists if only they, but not their (partial) analogs, could induce sodium translocation. Comparative structure and sequence analyses of sodium-binding GPCRs indicate a *

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“…Changes in membrane potential are known to directly modulate activation of GPCRs, especially class A GPCRs (50,54,55). Although the mechanisms by which membrane potential modulates GPCRs are not fully understood, recent studies have shown that allosteric effects of Na + ion on class A GPCRs modulate GPCR activation and GPCR-mediated signaling (54)(55)(56). Binding of voltage-driven translocated Na + to the highly conserved aspartic acid residue in transmembrane domain 2 (Asp 2.50 ) in class A GPCRs, combined with the effects of the Na + -coordinating residues, modulates GPCR agonist binding and activation.…”

Ion-water coupling controls class A GPCR signal transduction pathways

“…A network of ionizable residues extending through GPCRs is thought to be involved in GPCR signal transduction and may represent an evolutionary link to microbial rhodopsins (9, 12, 24, 49, 50). Our findings therefore suggest that the long-range information pathway we identified may couple the protonation states of the distal conserved Asp 2.50 and Arg 3.50 /Asp 3.49 .…”

“…In many cases, ligand binding alone is thought to provide insufficient energy for receptors to transition between inactive and active conformations (3, 9). However, GPCRs are able to harness energy from the membrane potential gradient, potentially through ion and proton transfer (9, 12, 50, 53). Even in the presence of bound antagonists, we observed conformational rearrangements within highly conserved microswitches in the DRY and NPxxY motifs upon protonation of three receptors, which were reminiscent of the transition between inactive and active state crystal structures.…”

“…Our analysis was based on all-atom molecular dynamics simulations that we started from three inactive-state, antagonist-bound class A GPCRs: the A 2A -adenosine receptor (A 2A AR), the µ-opioid receptor (µOR), and the δ-opioid receptor (δOR). Sodium is believed to reduce the thermal fluctuation level of the protein, thereby stabilising inactive conformations of microswitches (10,(27)(28)(29), but the mechanisms by which this is achieved are unknown. Therefore, for each receptor, we collected simulation data on 1.7 µs timescales performed in three different receptor states of the main ion binding residue: D2.50 in a charged state with sodium bound (state Na + /D − ), D2.50 in a charged state with sodium removed (state 0/D − ), and D2.50 in a protonated state without sodium (state 0/D n ).…”

“…By contrast, structures of receptor active states reveal a collapsed sodium binding site, unable to accommodate the ion (3,4). Consequently, a wide range of studies have suggested a role for sodium in GPCR activation (4)(5)(6)(7)(8)(9)(10). The absence of sodium is likely to trigger protonation of Asp 2.50 (6,11,12), while the sodium binding site is further embedded in a conserved network of water molecules that connect polar residues and undergo significant re-positioning 4dkl) as example.…”