Fig. 1. Comparison of RAMP sequences. GenBank accession numbers are given for reference. Dark grey shading: sequence identity amongst all RAMPs. Light grey shading: sequence identity within 1 RAMP family. (TN RAMP has been compared with both RAMP1 and RAMP2). Italics, putative signal peptides. Ma, marmoset (partial); H, human; C, chimp; RM, Rhesus monkey (partial); GP, guinea pig; R, rat; M, mouse; P, pig; D, dog; GF, guinea fowl; ZF, zebra fish; TN, Tetraodon nigroviridis. R1, RAMP1; R2, RAMP; R3, RAMP3.

Fig. 2. Structural features of human RAMPs. Dark shading, gylcosylation sites and conserved cysteines; light shading, regions implicated in ligand binding; italics, putative signal peptides.

Fig. 3. CGRP₁ receptor-specific small molecule antagonists. The small molecule antagonist BIBN4096 BS (brown) is a specific antagonist of the CGRP₁ receptor, acting at the interface between RAMP1 and the CL receptor to inhibit CGRP action. At least part of the binding affinity for BIBN4096 BS arises from interaction with Trp74 (red) of RAMP1. In contrast, antagonists that bind principally to the CL component of the complex will not discriminate between different CL/RAMP complexes.

Fig. 4. The broadening spectrum of RAMP–receptor interactions. RAMPs can interact with multiple receptor partners. All RAMPs interact with the calcitonin receptor-like receptor (CL-R), the calcitonin receptor (CTR), and the VPAC1 receptor, while the glucagon and PTH1 receptors interact with RAMP2, the PTH2 receptor with RAMP3, and the calcium sensing receptor (CalS-R) with RAMP1 or RAMP3. The consequence of RAMP interaction varies. For the CL and CalS receptors, RAMPs play a chaperone role, allowing cell surface expression (1). For the CL and calcitonin receptors, RAMP interaction leads to novel receptor binding phenotypes (2). There is also evidence that RAMP interaction will modify signaling, and this has been seen for the VPAC1–RAMP2 heterodimer and for calcitonin receptor/RAMP complexes (3). In many instances, however, the consequence of RAMP interaction has yet to be defined (4).

Fig. 5. RAMP2 modulation of VPAC1 receptor signaling. The VPAC1 receptor couples to multiple signaling pathways. Vasoactive intestinal polypeptide (VIP) causes a potent stimulation of cAMP accumulation and a weaker activation of phosphoinositide (PI) hydrolysis. The coexpression of the receptor with RAMP2 leads to a specific enhancement of agonist-mediated PI hydrolysis without modulating cAMP production.

Fig. 6. Modulation of receptor recycling through RAMP-specific interaction with accessory proteins. RAMP2/CL and RAMP3/CL generate specific AM receptors (AM₁ and AM₂, respectively). Upon agonist (AM) binding, AM₁ receptors are rapidly internalized and targeted to lysosomal degradation. In contrast, the internalization and recycling of AM₂ receptors can be modified by the interaction of RAMP3 with accessory proteins that bind to its C-terminal PDZ domain. Illustrated is the potential interaction of the AM₂ receptor with N-ethylmaleimide-sensitive factor (NSF) leading to rapid recycling of the receptor complex.

Pharmacological profile summary for human CGRP, AM, AMY and calcitonin receptors

AM, adrenomedullin; AMY, amylin; CGRP, calcitonin gene-related peptide; CL, calcitonin receptor like receptor; CT, calcitonin; h, human; ND, not determined; RAMP, receptor activity modifying protein; r, rat; s, salmon.

Correlation of calcitonin peptide family pharmacology with RAMPs, CL, and calcitonin receptors in tissues and cells

 = present; × = absent; ? = weak signal, inconclusive.

AM, adrenomedullin; AMY, amylin; CGRP, calcitonin gene-related peptide; CL, calcitonin receptor like receptor; CT, calcitonin; CT(a) or (b), calcitonin receptor isoforms; gp, guinea-pig; h, human; ND, not determined; r, rat; s, salmon; Sp, species.