Functions of the extracellular histidine residues of receptor activity-modifying proteins vary within adrenomedullin receptors

Receptor activity-modifying protein (RAMP)-2 and -3 chaperone calcitonin receptor-like receptor (CRLR) to the plasma membrane, where together they form heterodimeric adrenomedullin (AM) receptors. We investigated the contributions made by His residues situated in the RAMP extracellular domain to AM receptor trafficking and receptor signaling by co-expressing hCRLR and V5-tagged-hRAMP2 or -3 mutants in which a His residue was substituted with Ala in HEK-293 cells. Flow cytometric analysis revealed that hRAMP2-H71A mediated normal hCRLR surface delivery, but the resultant heterodimers showed significantly diminished ¹²⁵I]AM binding and AM-evoked cAMP production. Expression of hRAMP2-H124A and -H127A impaired surface delivery of hCRLR, which impaired or abolishing AM binding and receptor signaling. Although hRAMP3-H97A mediated full surface delivery of hCRLR, the resultant heterodimers showed impaired AM binding and signaling. Other His residues appeared uninvolved in hCRLR-related functions. Thus, the His residues of hRAMP2 and -3 differentially govern AM receptor function.