Receptor activity-modifying proteins differentially modulate the G protein-coupling efficiency of amylin receptors

Receptor activity modifying proteins (RAMPs) 1, 2 and 3 are prototypic G protein-coupled receptor (GPCR) accessory proteins that can alter not only receptor trafficking but also receptor phenotype. Specific RAMP interaction with the calcitonin receptor (CTR) generates novel and distinct receptors for the peptide amylin, however, the role of RAMPs in receptor signalling is not understood. The current study demonstrates that RAMP interaction with the CTRa in COS-7 or HEK-293 cells leads to selective modulation of signalling pathways activated by the receptor complex. There was a 20-30 fold induction in amylin potency at CTR/RAMP1 (AMY1) and CTR/RAMP3 (AMY3) receptors, compared with CTR alone, for formation of the second messenger cAMP that parallels an increase in amylin binding affinity. In contrast, only 2-5 fold induction of amylin potency was seen for mobilization of intracellular Ca(++) or activation of ERK1/2. In addition, in COS-7 cells, the increase in amylin potency for Ca(++) mobilization was 2 fold greater for AMY3 receptors compared with AMY1 receptors and this paralleled the relative capacity of over-expression of Galphaq proteins to augment induction of high affinity (125)I-amylin binding. These data demonstrate that RAMP-complexed receptors have a different signalling profile to CTRs expressed in the absence of RAMPs and this is likely due to direct effects of the RAMP on G protein-coupling efficiency.