Structural Basis for Receptor Activity-Modifying Protein-Dependent Selective Peptide Recognition by a G Protein-Coupled Receptor

Abstract

Association of receptor activity-modifying proteins (RAMP1-3) with the G protein-coupled receptor (GPCR) calcitonin receptor-like receptor (CLR) enables selective recognition of the peptides calcitonin gene-related peptide (CGRP) and adrenomedullin (AM) that have diverse functions in the cardiovascular and lymphatic systems. How peptides selectively bind GPCR:RAMP complexes is unknown. We report crystal structures of CGRP analog-bound CLR:RAMP1 and AM-bound CLR:RAMP2 extracellular domain heterodimers at 2.5 and 1.8 Å resolutions, respectively. The peptides similarly occupy a shared binding site on CLR with conformations characterized by a β-turn structure near their C termini rather than the α-helical structure common to peptides that bind related GPCRs. The RAMPs augment the binding site with distinct contacts to the variable C-terminal peptide residues and elicit subtly different CLR conformations. The structures and accompanying pharmacology data reveal how a class of accessory membrane proteins modulate ligand binding of a GPCR and may inform drug development targeting CLR:RAMP complexes.

Graphical abstract

Figure 1

Peptide-Bound CGRP and AM₁ Receptor ECD Heterodimer Structures (A and B) CGRPmut- and AM-bound complexes in cartoon representation with disulfide bonds as sticks and secondary structure elements labeled. Peptide, CLR, and RAMP terminal residues are numbered. Peptide side chains are shown as sticks in the left images but are omitted in the right images. MBP is not shown. The color scheme is consistent throughout the figures with carbon atoms varied in color to distinguish the proteins/peptides and oxygen, nitrogen, and sulfur atoms in CPK colors. (C) Superpositions of the peptide-bound complexes with the PTH:PTH1R ECD (PDB: 3C4M) and CRF:CRFR1 ECD (PDB: 3EHU) structures. The receptors are shown as Cα traces and the peptides as cartoons. (D) Amino acid sequences of the C-terminal regions of the human CGRP, CGRPmut, AM, and AM2 peptides. The turn structure region is highlighted in gray and Pro residues are in bold. See also Figures S1 and S2.

Figure 2

CGRPmut and AM Interactions with Their Receptor ECD Complexes (A and C) The peptide-bound structures viewed with the receptor complexes in molecular surface representation and the peptides as cartoons with side chains as sticks. (B and D) Detailed views with the receptors in cartoon representation and selected receptor residues and the peptides as sticks. Red dashes are hydrogen bonds. Peptide residues are labeled in (A) and (C) and receptor residues are labeled in (B) and (D). In (B), CGRPmut residues F27 and V28 are shown as lines for visualization of the CGRPmut T30-CLR D94 interaction. See also Tables S1 and S2.

Figure 3

Comparisons of the CGRPmut- and AM-Bound Structures to Ligand-free and Small Molecule Antagonist-Bound Structures (A) CGRPmut-bound complexes from Mol A and Mol C were aligned with four independent ligand-free CLR:RAMP1 complexes (PDB: 3N7P) based on the CLR positions. Mol B of the CGRPmut-bound structure was omitted because crystal packing altered its conformation. Receptors are shown as Cα traces. Arrows denote the directions of loop movements upon CGRPmut binding. The double-headed arrow highlights variability in the RAMP1 position relative to CLR. (B) Detailed view of differences between the CGRPmut-bound and ligand-free states of the CLR:RAMP1 ECD complex. CGRPmut F27 and V28 are omitted for clarity. (C and D) Superposition of CGRPmut-bound and olcegepant-bound (PDB: 3N7S) (C) or telcagepant-bound (PDB: 3N7R) (D) structures aligned based on the CLR positions. The peptide and small molecules are shown as sticks. (E) Superpositions of the CGRPmut-bound structures with two independent olcegepant-bound complexes and a single telcagepant-bound complex based on the CLR positions. The arrow indicates the direction of movement of RAMP1 from the small molecule antagonist-bound to CGRPmut-bound states. (F) Superposition of the AM-bound and ligand-free (PDB: 3AQF) CLR:RAMP2 ECD structures aligned based on the CLR positions. Receptors are shown as Cα traces and selected receptor and peptide residues as sticks. Arrows indicate directions of movement from ligand-free to AM-bound states. (G) Putative dimer of ligand-free CLR:RAMP2 ECD heterodimers with the AM-bound CLR:RAMP2 ECD [L106R] heterodimer superimposed based on the CLR positions. The receptors and peptide are in cartoon representation and residues L/R106 are in space-filling representation. The arrow indicates the shift of RAMP2 in the AM-bound structure as compared to the ligand-free state. The top image is oriented similar to that in (F), right image.

Figure 4

Validation of the CGRPmut-Bound ECD Heterodimer Structure for the Intact CGRP Receptor Transiently Expressed in COS-7 Cells Concentration-response curves for each of the CLR alanine substitution mutants tested with hαCGRP in cAMP signaling assays. Data are represented as mean ± SEM. See also Table S3.

Figure 5

Validation of the AM-Bound ECD Heterodimer Structure for the Intact AM₁ Receptor Transiently Expressed in COS-7 Cells Concentration-response curves for each of the CLR (A) or RAMP2 (B) alanine substitution mutants tested with hAM in cAMP signaling assays. Data are represented as mean ± SEM. See also Table S4.

Figure 6

Peptide Selectivity Determinants for CLR:RAMP1/2 Complexes (A) Superposition of the CGRPmut- and AM-bound ECD heterodimers aligned based on the CLR positions. The receptors and peptides are in cartoon representation with selected residues as sticks in the left image. In the right image, the receptors are Cα traces and the peptide cartoons were omitted for clarity. Arrows indicate directions of movement of CLR α1 and loop 2 and RAMP α2 and α3 from the CGRPmut/RAMP1-bound state to the AM/RAMP2-bound state. Red dashes are hydrogen bonds. (B) Detailed view of the aligned CGRPmut- and AM-bound complexes with selected residues as sticks and the receptors as Cα traces. The arrow highlights the shift of the C-terminal region of CLR α1 from the CGRPmut/RAMP1- to AM/RAMP2-bound states. (C and D) Concentration-response curves for the CGRP and AM₁ receptors with the RAMP1 (A70R, W74E, F83G, W84F) and RAMP2 (R97A, E101W, G110F, F111W) quadruple “swap” mutants tested with hαCGRP and hAM in cAMP assays in COS-7 cells. The cell surface expression was: RAMP1 quad 116.4 ± 5.25 (n = 4) % WT, RAMP2 quad 73.7 ± 9.52 (n = 4) % WT p < 0.01 by one-way ANOVA followed by Dunnett’s test for the RAMP2 quad mutant. (E–J) Competition AlphaScreen assays with purified receptor ECD heterodimer proteins and the indicated competitor “swap” peptides. (E)–(G) are for the MBP-RAMP1-(GSA)₃-CLR-H₆ protein with biotin-CGRP (100 nM each) and (H)–(J) are for the MBP-RAMP2[L106R]-(GSA)₃-CLR-H₆ protein with biotin-AM (100 nM each). The binding data are representative of at least three independent experiments each performed in duplicate. The error bars represent the SEM of the experiment. Determinable pIC₅₀ values were as follows: (F), CGRPmut 6.79 ± 0.10 and CGRPmut [F37Y] 6.24 ± 0.09; (G), CGRP(8-37) 5.01 ± 0.04; (H), AM(22-52) 4.92 ± 0.16; (I), AM(22-52) 4.84 ± 0.09 and CGRPmut [F37Y] 4.79 ± 0.07; (J), AM(37-52) 5.00 ± 0.11.

Figure 7

Summary of Peptide Recognition and Selectivity Determinants for CLR:RAMP1-3 Complexes (A and B) Structures of the CGRPmut- and AM-bound CLR:RAMP1/2 ECD heterodimers with the surface of receptor residues colored according to their effect on CGRP (A) or AM (B) signaling potency when mutated to alanine. Color coding signifies the extent of reduced signaling potency at intact receptor complexes in cells compared to wild-type as indicated by the inset legend. RAMP1 W84A data are from Moore et al. (2010) (34-fold reduced potency). (C) Model for RAMP3 binding site augmentation. A homology model of the AM-bound CLR:RAMP3 ECD complex (Supplemental Experimental Procedures) was superimposed with the CGRPmut-bound CLR:RAMP1 and AM-bound CLR:RAMP2 ECD structures based on the CLR positions. Only the RAMP3 subunit from the homology model is shown and RAMP2 is omitted. The receptors are shown as Cα traces and selected residues as sticks. (D) Amino acid sequence alignments for RAMP1-3 from the indicated species showing the α2 and α2-α3 loop regions that augment the peptide-binding site.