A Role for a Specific Cholesterol Interaction in Stabilizing the Apo Configuration of the Human A 2A Adenosine Receptor

Abstract

SUMMARY The function of G-protein coupled receptors is tightly modulated by the lipid environment. Long timescale molecular dynamics simulations (totaling ~3 microsec) of the A2A receptor in cholesterol-free bilayers, with and without the antagonist ZM241385 bound, demonstrate an instability of helix II in the apo receptor in cholesterol-poor membrane regions. We directly observe that the effect of cholesterol binding is to stabilize helix II against a buckling type deformation, perhaps rationalizing the observation that the A2A receptor couples to G-protein only in the presence of cholesterol (Zezula and Freissmuth, 2008). The results suggest a mechanism by which the A2A receptor may function as a coincidence detector, activating only in the presence of both cholesterol and agonist. We also observed a previously hypothesized conformation of the tryptophan “rotameric switch” on helix VI in which a phenylalanine on helix V positions the tryptophan out of the ligand binding pocket.

Identification of Cholesterol Binding Sites in the Serotonin1A Receptor

“…Interestingly, recent molecular dynamics simulations reviewed in ref 36 have shown that membrane cholesterol specifically interacts with transmembrane domains of GPCRs, such as rhodopsin 37 and human A 2A adenosine receptor. 38 In this work, we show that membrane cholesterol binds preferentially to certain sites on the serotonin 1A receptor using multiple long time scale coarse-grain simulations. It should be noted here that the binding of cholesterol to membrane proteins in general and GPCRs in particular is currently being extensively explored and it appears that no consensus model has emerged yet.…”

Molecular mechanisms in signal transduction at the membrane

“…Cholesterol interaction apparently stabilizes this helix and has been proposed as a possible explanation for the observation that the A 2A receptor couples to G protein only in the presence of cholesterol 37 . Interactions of membrane lipids with ion channels are also of substantial interest 38,39 and could be influenced by membrane demixing effects 39,40 .…”

The Significance of G Protein-Coupled Receptor Crystallography for Drug Discovery

“…This observation led to the speculation that the actual lock might be unique to rhodopsin, so although the motif was present in most GPCRs, it cannot actually act as a functional "lock" Rasmussen et al, 2007). This supposition prompted a series of molecular dynamics experiments for each of these receptors, all of which revealed that the ionic lock, even if absent (disrupted) in the crystal structure, quickly reforms once restraints imposed by the T4-lysozyme fusion and crystal lattice are removed (Dror et al, 2009;Lyman et al, 2009;Vanni et al, 2009;Jojart et al, 2010;Romo et al, 2010;Fanelli and Felline, 2011). This does not mean that the observed disruption of the ionic lock is an entirely artificially induced state; partial occupancy of a disrupted ionic lock state might explain the agonist-independent activation observed for these receptors (Fig.…”