When co-expressed with receptor activity-modifying protein (RAMP) 1, calcitonin receptor-like receptor (CRLR) can function as a receptor for both calcitonin gene-related peptide (CGRP) and adrenomedullin (AM). To investigate the structural determinants of ligand binding specificity, we examined the extracellular domain of human (h) RAMP1 using various deletion mutants. Co-expression of the hRAMP1 mutants with hCRLR in HEK-293 cells revealed that deletion of residues 91-94, 96 -100, or 101-103 blocked [ 125 I]CGRP binding and completely abolished intracellular cAMP accumulation normally elicited by CGRP or AM. On the other hand, the deletion of residues 78 -80 or 88 -90 significantly attenuated only AM-evoked responses. In all of these cases, the receptor heterodimers were fully expressed at the cell surface. Substituting alanine for residues 91-103 one at a time had little effect on CGRPinduced responses, indicating that although this segment is essential for high affinity agonist binding to the receptors, none of the residues directly interacts with either CGRP or AM. This finding suggests that RAMPs probably determine ligand specificity by contributing to the structure of the ligand-binding pocket or by allosteric modulation of the conformation of the receptor. Interestingly, the L94A mutant up-regulated surface expression of the receptor heterodimer to a greater degree than wild-type hRAMP1, thereby increasing CGRP binding and signaling. L94A also significantly increased cell surface expression of the hRAMP1 deletion mutant D101-103 when co-transfected with hCRLR, and expression of a L94A/D101-103 double mutant markedly attenuated the activity of endogenous RAMP1 in HEK-293T cells.CGRP 1 and AM belong to the calcitonin family of regulatory peptides and are both highly potent vasodilators (1, 2). Although they share very little sequence identity, both contain ring structures comprised of six amino acids linked by a disulfide bridge and an amidated C terminus that are required for biological activity (3). Both of these peptides and their specific or common receptors are widely distributed among peripheral tissues and in the central nervous system, enabling them to exert a wide variety of biological effects (4, 5).RAMPs are a recently identified group of single transmembrane domain accessory proteins that serve to transport CRLR to the cell surface where they form functional CGRP and AM receptors (6). The three RAMP isoforms (RAMP1, RAMP2, and RAMP3), which share only 30% sequence identity and differ in their tissue distributions, are all comprised of ϳ160 amino acids that make up a large extracellular N-terminal domain, a single membrane-spanning domain, and a very short cytoplasmic domain (6, 7). Co-expression of CRLR with RAMP1 leads to both proteins being presented at the plasma membrane as a heterodimeric CGRP receptor, whereas co-expression of CRLR with RAMP2 or RAMP3 enables the resultant heterodimer to function as a AM receptor (6,8,9). Upon binding their respective agonist, both receptors mediate a rise...