Ligand binding and activation of the CGRP receptor

The CGRP receptor is an atypical G-protein coupled receptor (GPCR), consisting of at least three proteins; a Family-B GPCR (calcitonin receptor-like receptor; CLR or CRLR), receptor activity modifying protein 1 (RAMP1) and receptor component protein (RCP). The extracellular domain of RAMP1 is tri-helical and possibly interacts with the extreme N-terminus of CLR to form the functional receptor. CGRP binding probably follows a two-step model of activation. The C-terminus of CGRP interacts with the N-terminus of the CLR/RAMP1 complex and its N-terminus interacts with the extracellular loops and of CLR to cause activation. The second and third extracellular loops are particularly important. During receptor activation TM helices 3 and 6 probably move apart. P343 in TM 6 is particularly important; E233 in TM3 and R173 and/or H178 in TM2 may form intermolecular interactions that may mirror the function of the DRY motif found in Family A GPCRs. Upon receptor activation the intracellular loops move to create a G_s-protein binding pocket.