Hydrophobic amino acids at the cytoplasmic ends of helices 3 and 6 of rhodopsin conjointly modulate transducin activation

Functional elements of the gastric inhibitory polypeptide receptor: Comparison between secretin- and rhodopsin-like G protein-coupled receptors

Innovative crystallographic techniques have resulted in an exponential growth in the number of solved G-protein coupled receptor (GPCR) structures and a better understanding of the mechanisms of class A receptor activation and G protein binding. The recent release of the type 1 receptor for the corticotropin-releasing factor and the glucagon receptor structures, two members of the secretin-like family, gives the opportunity to understand these mechanisms of activation in this family of GPCRs. Here, we addressed the comparison of the functional elements of class A and secretin-like GPCRs, using the glucose-dependent insulinotropic polypeptide receptor (GIPR) as a model receptor. Inactive and active models of GIPR permitted to select, by structural homology with class A GPCRs, several residues that may form key interactions presumably involved in receptor activation and Gs coupling, for pharmacological evaluation. Mutants on these amino acids were expressed in HEKT 293 cells and characterized in terms of GIP-induced cAMP production. We identified various functional domains spanning from the peptide-binding to the G protein pockets: including: a network linking the extracellular part of transmembrane (TM) 6 with TMs 2 and 7; a polar lock that resembles the ionic-lock in class A GPCRs; an interaction between TMs 3 and 7 that favors activation; and two clusters of polar/charged and of hydrophobic residues that interact with the C-terminus of the Gα. The results show that despite the low degree of sequence similarity between rhodopsin- and secretin-like GPCRs, the two families share conserved elements in their mechanisms of activation and G protein binding.

The hydrophobic amino acid cluster at the cytoplasmic end of transmembrane helix III modulates the coupling of the β(1)-adrenergic receptor to G(s)

Abstract A cluster of hydrophobic amino acids at the cytoplasmic end of trans-membranal helix III (TM-III) is a common feature among class-A of G protein-coupled receptors (GPCR). We mutagenized alanine 159(3.53) to glutamic acid and isoleucine160(3.54) to arginine (A159E/I160R) in TM-III of the human β(1)-adrenergic receptor (β(1)-AR) to disrupt the function of the hydrophobic cluster. Structurally, the combined mutations of A159E/I160R caused an almost 90° tilt in the rotation of Arg156(3.50) in the E/DRY motif of TM-III and displaced Tyr166(3.60) in intracellular loop 2. The A159E/I160R β(1)-AR was uncoupled from G(s) as determined by cyclic AMP/adenylyl cyclase assays and by FRET-based proximity measurements between the β(1)-AR and G(s)α. Isoproterenol induced β-arrestin trafficking in cells expressing both the wild-type β(1)-AR and the A159E/I160R β(1)-AR. Isoproterenol markedly increased the phosphorylation of ERK1/2 in cells expressing the WT β(1)-AR and this effect was dependent on the activation of the G(s)-cyclic AMP-dependent protein kinase → Rap → B-raf axis. However, in cells bearing the A159E/I160R β(1)-AR, isoproterenol failed to increase the phosphorylation of ERK(1/2). These results indicate that mutations in the G(s)α-binding pocket of the GPCR interfered with receptor coupling to G(s) and with its downstream signaling cascades.