Amylin receptors respond to a 37-amino acid endocrine peptide, amylin (Amy). The activation of AMYRs provides a prandial satiety signal with many physiological effects including reduced food intake, decreased adiposity and increased energy expenditure1. Thus, AMYRs are being pursued as a therapeutic target for obesity and related metabolic diseases2.
AMYRs are heterodimers of the calcitonin receptor (CTR) and one of 3 receptor activity-modifying proteins (RAMPs) to generate AMY1R (CTR-RAMP1), AMY2R (CTR-RAMP1) or AMY3R (CTR-RAMP3), respectively. Expressed alone, CTRs exhibit high potency for human calcitonin (hCT) but a low potency for Amy in functional assays. In contrast, salmon calcitonin (sCT) has high affinity and potency for both CTRs and AMYRs3. The complexity of the receptor-ligand system has complicated physiological and pharmacological analysis of AMYR function. Thus, it is of prime importance for future drug discovery to comprehend the structural basis for activation by, and selectivity for peptides that bind AMYRs and CTR.
We determined cryogenic electron microscopy (cryo-EM) structures of each of AMY1R, AMY2R, AMY3R bound to rat Amy (rAmy), the AMY2R bound to sCT, and the CTR bound to either hCT or sCT, all in complex with the predominant G protein transducer, Gs. These near-atomic resolution structures of AMYR subtypes provided insight into the allosteric modulation of CTR by RAMPs to yield AMYR phenotype and the molecular details of binding for Amy and related peptides. Amy peptides exhibited a “bypass” motif of rAmy (S19-P25) that contributes to a distinct conformation of, and selective interactions with, AMYRs. In contrast sCT bound to a AMYR conformation, distinct from that bound to rAmy, which mimicked the conformation of the CT bound CTR. Our work identifies different mechanisms of activation of AMYRs by Amy or CT peptides with implications for the rational design of selective or non-selective derivatives using CT and Amy template peptides.