Crystal structure of oligomeric β1-adrenergic G protein–coupled receptors in ligand-free basal state

Abstract

G protein-coupled receptors (GPCRs) mediate transmembrane signaling. Before ligand binding, GPCRs exist in a basal state. Crystal structures of several GPCRs bound with antagonists or agonists have been solved. However, the crystal structure of the ligand-free basal state of a GPCR, the starting point of GPCR activation and function, has not been determined. Here we report the X-ray crystal structure of the first ligand-free basal state of a GPCR in a lipid membrane-like environment. Oligomeric turkey β1-adrenergic receptors display two alternating dimer interfaces. One interface involves the transmembrane domain (TM) 1, TM2, the C-terminal H8, and the extracellular loop 1. The other interface engages residues from TM4, TM5, the intracellular loop 2 and the extracellular loop 2. Structural comparisons show that this ligand-free state is in an inactive conformation. This provides the structural information regarding GPCR dimerization and oligomerization.

Heteromeric Dopamine Receptor Signaling Complexes: Emerging Neurobiology and Disease Relevance

“…What has become increasingly evident is that the interface(s) between the receptors in homomeric complexes likely involves residues located in transmembrane domains (TM), such as TM4 and TM5, as has been shown for the D2 receptor (D2R) (Guo et al, 2003;Lee et al, 2003), the alpha(1b)-adrenoceptor (López-Giménez et al, 2007), the 5HT1A receptor (Gorinski et al, 2012), and the 5HT2C receptor (Mancia et al, 2008). These observations have also been supported by the recent crystal structure reported for the beta1-adrenergic receptor dimer (Huang et al, 2013) in a lipid membrane-like environment which showed two dimer interfaces, one involving TM1, TM2, helix 8 and extracellular loop 1, and the second involving TM4, TM5, intracellular loop 2, and extracellular loop 2. The analysis of the crystal structure of the chemokine CXCR4 receptor dimer (Wu et al, 2010) reported receptor interfaces at TM5 and TM6.…”

Heteromerization of chemokine (C-X-C motif) receptor 4 with α1A/B-adrenergic receptors controls α1-adrenergic receptor function

“…The main interface of the CXCR4 homodimer is localized at TM5 and TM6, whereas several other GPCR homodimers form interfaces that also include TM2 (46)(47)(48)(49)(50). Thus, a TM2 contact site in CXCR4 could permit receptor heteromerization without interfering with the constitutive CXCR4 homodimerization (46).…”

“…CXCR4 silencing reduced α 1A/B -AR:CXCR4 heteromeric complexes in VSMC and abolished phenylephrine-induced Ca 2+ fluxes and MLC2 phosphorylation. Treatment of rats with CXCR4 agonists (CXCL12, ubiquitin) reduced the EC 50 of the phenylephrineinduced blood pressure response three-to fourfold. These observations suggest that disruption of the quaternary structure of α 1A/B -AR:CXCR4 heteromeric complexes by targeting transmembrane helix 2 of CXCR4 and depletion of the heteromeric receptor complexes by CXCR4 knockdown inhibit α 1 -AR-mediated function in VSMC and that activation of CXCR4 enhances the potency of α 1 -AR agonists.…”

Allosteric Activation of a G Protein-coupled Receptor with Cell-penetrating Receptor Mimetics

“…Although the exact structure and relative orientation of the homodimer are difficult to determine with certainty, the direct binding data presented here indicate that a stable homodimer may form with each i3 loop interacting with an H8 helix across the dimer interface. Several crystal structures indi-cate that the H8 helix plays a prominent role for homomeric interactions in lipid membrane environments, including ␤ 2 -adrenergic receptor (63), (rhod)opsin in its G protein-interacting state (49), ␤ 1 -adrenergic receptor (64), and human -opioid receptor (65).…”