Structural Basis of Lipid Effects on G-Protein-Coupled Receptor (GPCR) Activation

Abstract

Several new crystal structures published in the past year give insight into the activation mechanism of G-protein-coupled receptors (GPCRs). The inactive receptor is stabilized by interactions of TM-3 with TM-7 in the ligand binding pocket on the extracellular side and with TM-6 on the cytoplasmic side. Both interactions are weakened upon activation. The helix movement model of receptor activation suggests that conformational changes in the ligand-binding pocket are transmitted to the cytoplasmic surface. The model is consistent with structural changes from the inverse agonist-bound receptor ground state (rhodopsin) to the G-protein-interacting conformation (‘activated’ opsin). We demonstrate analogous changes in the TM-3/TM-7 interaction from long molecular dynamics simulations (>600 ns) of β2-adrenergic receptor (AR) in two forms, bound to carazolol (inverse agonist) and to adrenaline/epinephrine (agonist), respectively.[1] The activated opsin structure exhibits movement (tilt and rotation) of TM-6, which generates the G-protein-binding site and disrupts stabilizing (“ionic lock”) interactions of E247(6:30) with the (D/E)RY motif on TM-3. Movement of TM-6 is independent of a broken ionic lock as seen in inverse agonist-bound β1/2-AR. On the other hand, movement of TM-6 appears to be the structural basis for several lipid effects on receptor activation. We have shown that in bilayer membranes receptor activation is facilitated by non-lamellar phase-promoting (phospho-) lipids with small head-groups and/or bulky acyl chains. Moreover, mismatch of bilayer hydrophobic thickness with the receptor results in oligomerization and/or local molecular crowding, which in turn inhibits receptor activation. [1] T. Huber, S. Menon, and T.P. Sakmar. 2008. Current Topics. Structural Basis for Ligand Binding and Specificity in Adrenergic Receptors: Implications for GPCR-targeted Drug Discovery. Biochemistry 47, in press.