The amino terminus of receptor activity modifying proteins is a critical determinant of glycosylation state and ligand binding of calcitonin receptor-like receptor

The calcitonin receptor-like receptor (CRLR) can function as either a recep tor for calcitonin gene-related peptide (CGRP) or for adrenomedullin (ADM), depending upon the coexpression of a novel family of single transmembrane p roteins, which we have called receptor activity modifying proteins or RAMPs . RAMPs 1, 2, and 3 transport CRLR to the plasma membrane with similar effi ciencies, however RAMP1 presents CRLR as a terminally glycosylated, mature glycoprotein and a CORP receptor, whereas RAMPs2 and 3 present CRLR as an i mmature, core glycosylated ADM receptor. Characterization of the RAMP2/CRLR and RAMP3/CRLR receptors in HEK293T cells by radioligand binding (I-125-AD M as radioligand), functional assay (cAMP measurement), or biochemical anal ysis (SDS-polyacrylamide gel electrophoresis) revealed them to be indisting uishable, even though RAMPs 2 and 3 share only 30% identity. Chimeric prote ins were created with the transmembrane and cytosolic portions of RAMP1 ass ociated with the amino terminus of RAMP2 (RAMP2/1) and vice verse (RAMP1/ 2 ). Coexpression of RAMP2/1 with CRLR formed a core glycosylated ADM recepto r, whereas the RAMP1/2 chimera generated both core glycosylated and mature forms of CRLR and enabled both ADM and CGRP receptor binding. Hence, the gl ycosylation state of CRLR appears to correlate with its pharmacology.