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The calcitonin receptor-like receptor (CRLR), a class B GPCR, forms a heterodimer with receptor activity-modifying protein 2 (RAMP2), and serves as the adrenomedullin (AM) receptor to control neovascularization, while CRLR and RAMP1 form the calcitonin gene-related peptide (CGRP) receptor. Here, we report the crystal structures of the RAMP2 extracellular domain alone and in the complex with the CRLR extracellular domain. The CRLR-RAMP2 complex exhibits several intermolecular interactions that were not observed in the previously reported CRLR-RAMP1 complex, and thus the shape of the putative ligand-binding pocket of CRLR-RAMP2 is distinct from that of CRLR-RAMP1. The CRLR-RAMP2 interactions were confirmed for the full-length proteins on the cell surface by site-specific photo-crosslinking. Mutagenesis revealed that AM binding requires RAMP2 residues that are not conserved in RAMP1. Therefore, the differences in both the shapes and the key residues of the binding pocket are essential for the ligand specificity.

Receptor activity-modifying protein (RAMP) 1 forms a heterodimer with calcitonin receptor-like receptor (CRLR) and regulates its transport to the cell surface. The CRLRÁRAMP1 heterodimer functions as a specific receptor for calcitonin gene-related peptide (CGRP). Here, we report the crystal structure of the human RAMP1 extracellular domain. The RAMP1 structure is a three-helix bundle that is stabilized by three disulfide bonds. The RAMP1 residues important for cell-surface expression of the CRLRÁRAMP1 heterodimer are clustered to form a hydrophobic patch on the molecular surface. The hydrophobic patch is located near the tryptophan residue essential for binding of the CGRP antagonist, BIBN4096BS. These results suggest that the hydrophobic patch participates in the interaction with CRLR and the formation of the ligand-binding pocket when it forms the CRLRÁRAMP1 heterodimer.

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