Structural Basis for Allosteric Modulation of Class B G Protein-Coupled Receptors

Abstract

Recent advances in our understanding of the structure and function of class B G protein-coupled receptors (GPCRs) provide multiple opportunities for targeted development of allosteric modulators. Given the pleiotropic signaling patterns emanating from these receptors in response to a variety of natural agonist ligands, modulators have the potential to sculpt the responses to meet distinct needs of different groups of patients. In this review, we provide insights into how this family of GPCRs differs from the rest of the superfamily, how orthosteric agonists bind and activate these receptors, the potential for allosteric modulators to interact with various regions of these targets, and the allosteric influence of endogenous proteins on the pharmacology of these receptors, all of which are important considerations when developing new therapies.

Keywords: GPCRs; RAMPs; allosteric modulation; biased agonism; structure function.

Figure 1.. Structural characterization of class B GPCRs – two relatively stable domains with variable relative orientations.

(A) Structural domains of class B GPCRs using the GLP-1R structure as an exemplar. The amino-terminal extracellular domain (ECD) of the class B GPCRs contains two central anti-parallel β-sheets (with the peptide-binding cleft in between) and an amino-terminal α-helix connected by a series of loops and stabilized by three disulfide bonds. The core transmembrane helical bundle domain is more open toward the extracellular side of the membrane than other families of GPCRs, and lacks distinct small molecule docking sites. The cytosolic side of the helical bundle is identified as the transducer binding site, shown here as binding the heterotrimeric G protein, GαSβγ. The orthosteric natural peptide ligand binding begins with the carboxyl terminus of the peptide occupying the ECD cleft, thereby directing the biologically active amino terminus of the peptide toward the junctional domain (J-domain); this comprises the two-site hypothesis with two distinct stages separated spatially and temporally. (B) Peptide agonist-bound active structures solved to date. Shown are structures of the CT family of receptors (CT receptor and CLR) binding CT or CGRP and peptides with GLP-1 agonist activity bound to the GLP-1 receptor. Note the differences in orientations of the receptor amino-terminal domain (ECD) relative to the core transmembrane helical bundle domain. (C) Peptide-bound inactive structures solved to date. Shown are structures of the PTH-1 receptor and glucagon receptor (GCGR) with peptide agonists bound, but stabilized in inactive receptor conformations, again highlighting the differences in orientation of the two key peptide-binding domains.

Figure 2.. Class B GPCR ligand binding sites.

(A) Ligand-binding sites confirmed in existing structures. Shown are sites for the orthosteric peptide ligands, a well as an antibody partially directed to that site. Also shown are the location of allosteric ligand binding sites deep within the helical bundle and outside of this domain, as well as sites for G protein association, RAMP interaction, and sites of antagonists that recognize the RAMP:receptor ECD complex. (B) Broad theoretical classes of ligand-binding sites. Based on existing structures and what is currently understood about dynamics and functional domains of class B GPCRs, this illustrates potential sites of modulation by various types of ligands. In addition to what is shown, lipids could act anywhere around the intramembranous helical bundle and different RAMPs could interact differently than the prototype illustrated in panel A and could possess unique allosteric sites that could be targeted. Also, other endogenous molecules, such as ions, have potential sites of action too numerous to display.