At least one of three receptor activity-modifying proteins (RAMP1, RAMP2 and RAMP3) can interact with 10
G protein-coupled receptors (GPCRs; nine Family B GPCRs and a Family C GPCR). All three RAMPs interact with the
calcitonin (CT) receptor (CTR), the CTR-like receptor (CLR), the vasoactive intestinal peptide (VIP)/pituitary adenylate
cyclase-activating polypeptide (PACAP) 1 (VPAC1) and the VPAC2 receptor, which are all Family B GPCRs. Three
RAMPs enable CTR to function as three heterodimeric receptors for amylin, which is a feeding suppression peptide.
These RAMPs also transport the CLR to the cell surface, where they function as a CT gene-related peptide (CGRP) receptor
(CLR/RAMP1 heterodimer) and two adrenomedullin (AM) receptors (CLR/RAMP2 and CLR/RAMP3 heterodimers).
CGRP and AM are potent hypotensive peptides that exert powerful protective effects against multi-organ damage. We recently
reported that the third extracellular loop (ECL3) of CLR governs the activation of AM, but not CGRP, signaling in
the three CLR/RAMP heterodimers. Furthermore, we showed that in the presence of RAMP2, the eighth helix (helix 8) in
the proximal portion of the cytoplasmic C-terminal tail of the CLR, which is thought to be present in all family B GPCRs,
participates in receptor signaling. In addition, we demonstrated that overexpression of GPCR kinase (GRK) 2, GRK3 and
GRK4 enhances the AM-induced internalization of the CLR/RAMP2 heterodimer. In this review, we describe these studies
and consider their implications for other Family B GPCRs that can interact with RAMPs.
Keywords: Receptor activity-modifying protein, calcitonin receptor-like receptor, calcitonin gene-related peptide, adrenomedullin,
receptor activation, receptor internalization, G protein-coupled receptor kinase.
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