PMC:4504005 / 1931-7347 JSONTXT

{"project":"TEST0","denotations":[{"id":"25982113-176-184-8318979","span":{"begin":399,"end":403},"obj":"[\"15808821\", \"15808821\", \"15808821\"]"},{"id":"25982113-113-121-8318980","span":{"begin":798,"end":802},"obj":"[\"21722971\", \"21722971\", \"21722971\"]"},{"id":"25982113-124-132-8318981","span":{"begin":1092,"end":1096},"obj":"[\"20966082\", \"20966082\", \"20966082\"]"},{"id":"25982113-147-155-8318982","span":{"begin":1115,"end":1119},"obj":"[\"17715056\", \"17715056\", \"17715056\"]"},{"id":"25982113-169-177-8318983","span":{"begin":1137,"end":1141},"obj":"[\"18801728\", \"18801728\", \"18801728\"]"},{"id":"25982113-175-183-8318984","span":{"begin":1143,"end":1147},"obj":"[\"19674967\", \"19674967\", \"19674967\"]"},{"id":"25982113-197-205-8318985","span":{"begin":1165,"end":1169},"obj":"[\"18375760\", \"18375760\", \"18375760\"]"},{"id":"25982113-217-225-8318986","span":{"begin":1185,"end":1189},"obj":"[\"18287102\", \"18287102\", \"18287102\"]"},{"id":"25982113-233-241-8318987","span":{"begin":1209,"end":1213},"obj":"[\"19861722\", \"19861722\", \"19861722\"]"},{"id":"25982113-235-243-8318988","span":{"begin":1938,"end":1942},"obj":"[\"21658025\", \"21658025\", \"21658025\"]"},{"id":"25982113-230-238-8318989","span":{"begin":1959,"end":1963},"obj":"[\"12037140\", \"12037140\", \"12037140\"]"},{"id":"25982113-150-158-8318990","span":{"begin":2400,"end":2404},"obj":"[\"20433208\", \"20433208\", \"20433208\"]"},{"id":"25982113-84-92-8318991","span":{"begin":2515,"end":2519},"obj":"[\"18998755\", \"18998755\", \"18998755\"]"},{"id":"25982113-156-164-8318992","span":{"begin":2587,"end":2591},"obj":"[\"12529940\", \"12529940\", \"12529940\"]"},{"id":"25982113-231-239-8318993","span":{"begin":2886,"end":2890},"obj":"[\"21658025\", \"21658025\", \"21658025\"]"},{"id":"25982113-226-234-8318994","span":{"begin":2910,"end":2914},"obj":"[\"9620797\", \"9620797\", \"9620797\"]"},{"id":"25982113-95-103-8318995","span":{"begin":3012,"end":3016},"obj":"[\"17010614\", \"17010614\", \"17010614\"]"},{"id":"25982113-186-194-8318996","span":{"begin":3395,"end":3399},"obj":"[\"10385705\", \"10385705\", \"10385705\"]"},{"id":"25982113-207-215-8318997","span":{"begin":3416,"end":3420},"obj":"[\"12037140\", \"12037140\", \"12037140\"]"},{"id":"25982113-234-242-8318998","span":{"begin":3812,"end":3816},"obj":"[\"16188935\", \"16188935\", \"16188935\"]"},{"id":"25982113-228-236-8318999","span":{"begin":3834,"end":3838},"obj":"[\"23674134\", \"23674134\", \"23674134\"]"},{"id":"25982113-231-239-8319000","span":{"begin":3856,"end":3860},"obj":"[\"20691959\", \"20691959\", \"20691959\"]"},{"id":"25982113-225-233-8319001","span":{"begin":4220,"end":4224},"obj":"[\"22102369\", \"22102369\", \"22102369\"]"},{"id":"25982113-234-242-8319002","span":{"begin":4243,"end":4247},"obj":"[\"20826335\", \"20826335\", \"20826335\"]"},{"id":"25982113-48-56-8319003","span":{"begin":4298,"end":4302},"obj":"[\"20015292\", \"20015292\", \"20015292\"]"},{"id":"25982113-227-235-8319004","span":{"begin":4477,"end":4481},"obj":"[\"20188075\", \"20188075\", \"20188075\"]"},{"id":"25982113-227-235-8319005","span":{"begin":4499,"end":4503},"obj":"[\"24199627\", \"24199627\", \"24199627\"]"},{"id":"25982113-232-240-8319006","span":{"begin":4869,"end":4873},"obj":"[\"1988044\", \"1988044\", \"1988044\"]"},{"id":"25982113-228-236-8319007","span":{"begin":4893,"end":4897},"obj":"[\"11444978\", \"11444978\", \"11444978\"]"},{"id":"25982113-232-240-8319008","span":{"begin":4922,"end":4926},"obj":"[\"21830197\", \"21830197\", \"21830197\"]"}],"text":"Introduction\nG protein-coupled receptors (GPCRs) are a large family of cell surface receptors that regulate a multitude of biological processes in response to a diverse array of stimuli and they are important drug targets. The class B/Secretin family GPCRs in humans include 15 receptors that are activated by diverse neuropeptides, peptide paracrine factors, and peptide endocrine hormones (Hoare, 2005). These receptors are less well understood than the larger class A/Rhodopsin family, despite their physiological and clinical importance. Class B GPCRs comprise an extracellular domain (ECD) of about 120 amino acids in addition to the 7-transmembrane (7TM) domain in the membrane. The ECD has an N-terminal α-helix and a set of β sheets held together by three disulfide bonds (Archbold et al., 2011). Peptides bind class B GPCRs via a “two-domain” model whereby their C-terminal region binds the ECD and their N-terminal region binds and activates the 7TM domain. Crystal structures are available for class B GPCR ECDs with bound peptides related to PTH, CRF, GIP, and GLP-1 (Pal et al., 2010; Parthier et al., 2007; Pioszak et al., 2008, 2009; Pioszak and Xu, 2008; Runge et al., 2008; Underwood et al., 2010) and a consensus has emerged from these studies. The peptides bind as extended α helices to the same region of the receptor, in a groove between the N and C termini of the isolated ECDs. For PTH, GIP, and GLP-1 families, the peptides are closest to the N terminus; for the CRF-related peptides, they are displaced to be closer to the C terminus.\nAlthough this model of binding is valid for several class B GPCRs, it cannot apply to all class B receptors. In particular, there are problems understanding the binding of members of the calcitonin (CT) family of peptides; calcitonin gene-related peptides alpha and beta (αCGRP, βCGRP), adrenomedullin (AM), adrenomedullin 2/intermedin (AM2), amylin (Amy), and CT (Hong et al., 2012; Poyner et al., 2002). These C terminally amidated peptides have a range of actions including neurogenic inflammation (CGRP), vasodilation/cardioprotection (CGRP, AM, and AM2), and regulation of blood and lymphatic vascular development (AM), nutrient intake and blood glucose (Amy), and bone turnover (CT). CGRP antagonists showed promise for the treatment of migraine and AM may be of value for the treatment of cardiovascular disorders (Durham and Vause, 2010; Karpinich et al., 2011). An Amy analog is used to treat insulin-dependent diabetes patients (Edelman et al., 2008) and CT has been long used to treat bone disorders (Purdue et al., 2002).\nCGRP, AM, and AM2 binding to their cognate class B receptor, the calcitonin receptor-like receptor (CLR), is dependent on association of CLR with one of three accessory membrane proteins that determine ligand selectivity; receptor activity-modifying proteins (RAMPs) 1, 2, or 3 (Hong et al., 2012; McLatchie et al., 1998). RAMPs have an ECD of about 100 amino acids and a single TM segment (Parameswaran and Spielman, 2006). CLR:RAMP1 is a CGRP receptor, CLR:RAMP2 preferentially recognizes AM and is called the AM1 receptor, and CLR:RAMP3 binds both AM and AM2 with high affinities and is called the AM2 receptor. Amy by itself has a low affinity for the class B CT receptor (CTR); however, when CTR associates with any of the RAMPs, its affinity for Amy is markedly increased (Christopoulos et al., 1999; Poyner et al., 2002). CTR alone is the receptor for CT. Thus, the RAMPs profoundly alter the behavior of CLR and CTR. Although RAMPs are best characterized for their effects on CLR/CTR, they also interact with several other class B GPCRs and with certain class A/Rhodopsin and class C/Glutamate family GPCRs, making it particularly important to understand the molecular basis for RAMP actions (Bouschet et al., 2005; Lenhart et al., 2013; Wootten et al., 2010). RAMPs provide an excellent opportunity to explore how accessory membrane proteins can modulate GPCR pharmacology.\nCrystal structures are available for ligand-free and small molecule antagonist-bound CLR:RAMP1 and ligand-free CLR:RAMP2 ECD complexes, but these provide little insight into how peptides bind or how RAMPs determine selectivity (Kusano et al., 2012; ter Haar et al., 2010). Extensive mutagenesis on the RAMPs (Qi and Hay, 2010) only provided clear evidence for the involvement of one RAMP residue (RAMP1 W84) in the binding of CGRP and two residues (RAMP2 F111 and E101) for AM binding (Moore et al., 2010; Watkins et al., 2014). It has not been possible to interpret these data mechanistically. A further complication is that it appears unlikely that CGRP and AM bind as extended helices as seen with other class B peptide ligands; there is evidence that only a small portion of these peptides form α helices and that at their C termini, there are one or more turn structures (Breeze et al., 1991; Carpenter et al., 2001; Pérez-Castells et al., 2012; Watkins et al., 2013b). Consequently, the mechanism of RAMP action and the mode of binding of CT family peptides remain unknown. Here, we describe high-resolution crystal structures of CGRP analog-bound CLR:RAMP1 and AM-bound CLR:RAMP2 ECD heterodimers that reveal bound peptide conformations starkly different from other class B GPCR peptide ligands, explain how RAMPs determine peptide selectivity, and provide molecular templates to guide drug development targeting CLR:RAMP complexes."}

{"project":"2_test","denotations":[{"id":"25982113-15808821-71076097","span":{"begin":399,"end":403},"obj":"15808821"},{"id":"25982113-15808821-71076097","span":{"begin":399,"end":403},"obj":"15808821"},{"id":"25982113-21722971-118216083","span":{"begin":798,"end":802},"obj":"21722971"},{"id":"25982113-21722971-118216083","span":{"begin":798,"end":802},"obj":"21722971"},{"id":"25982113-20966082-118216084","span":{"begin":1092,"end":1096},"obj":"20966082"},{"id":"25982113-20966082-118216084","span":{"begin":1092,"end":1096},"obj":"20966082"},{"id":"25982113-17715056-118216085","span":{"begin":1115,"end":1119},"obj":"17715056"},{"id":"25982113-17715056-118216085","span":{"begin":1115,"end":1119},"obj":"17715056"},{"id":"25982113-18801728-118216086","span":{"begin":1137,"end":1141},"obj":"18801728"},{"id":"25982113-18801728-118216086","span":{"begin":1137,"end":1141},"obj":"18801728"},{"id":"25982113-19674967-118216087","span":{"begin":1143,"end":1147},"obj":"19674967"},{"id":"25982113-19674967-118216087","span":{"begin":1143,"end":1147},"obj":"19674967"},{"id":"25982113-18375760-118216088","span":{"begin":1165,"end":1169},"obj":"18375760"},{"id":"25982113-18375760-118216088","span":{"begin":1165,"end":1169},"obj":"18375760"},{"id":"25982113-18287102-118216089","span":{"begin":1185,"end":1189},"obj":"18287102"},{"id":"25982113-18287102-118216089","span":{"begin":1185,"end":1189},"obj":"18287102"},{"id":"25982113-19861722-118216090","span":{"begin":1209,"end":1213},"obj":"19861722"},{"id":"25982113-19861722-118216090","span":{"begin":1209,"end":1213},"obj":"19861722"},{"id":"25982113-21658025-118216091","span":{"begin":1938,"end":1942},"obj":"21658025"},{"id":"25982113-21658025-118216091","span":{"begin":1938,"end":1942},"obj":"21658025"},{"id":"25982113-12037140-118216092","span":{"begin":1959,"end":1963},"obj":"12037140"},{"id":"25982113-12037140-118216092","span":{"begin":1959,"end":1963},"obj":"12037140"},{"id":"25982113-20433208-118216093","span":{"begin":2400,"end":2404},"obj":"20433208"},{"id":"25982113-20433208-118216093","span":{"begin":2400,"end":2404},"obj":"20433208"},{"id":"25982113-18998755-118216094","span":{"begin":2515,"end":2519},"obj":"18998755"},{"id":"25982113-18998755-118216094","span":{"begin":2515,"end":2519},"obj":"18998755"},{"id":"25982113-12529940-118216095","span":{"begin":2587,"end":2591},"obj":"12529940"},{"id":"25982113-12529940-118216095","span":{"begin":2587,"end":2591},"obj":"12529940"},{"id":"25982113-21658025-118216096","span":{"begin":2886,"end":2890},"obj":"21658025"},{"id":"25982113-21658025-118216096","span":{"begin":2886,"end":2890},"obj":"21658025"},{"id":"25982113-9620797-118216097","span":{"begin":2910,"end":2914},"obj":"9620797"},{"id":"25982113-9620797-118216097","span":{"begin":2910,"end":2914},"obj":"9620797"},{"id":"25982113-17010614-71076105","span":{"begin":3012,"end":3016},"obj":"17010614"},{"id":"25982113-17010614-71076105","span":{"begin":3012,"end":3016},"obj":"17010614"},{"id":"25982113-10385705-118216098","span":{"begin":3395,"end":3399},"obj":"10385705"},{"id":"25982113-10385705-118216098","span":{"begin":3395,"end":3399},"obj":"10385705"},{"id":"25982113-12037140-118216099","span":{"begin":3416,"end":3420},"obj":"12037140"},{"id":"25982113-12037140-118216099","span":{"begin":3416,"end":3420},"obj":"12037140"},{"id":"25982113-16188935-118216100","span":{"begin":3812,"end":3816},"obj":"16188935"},{"id":"25982113-16188935-118216100","span":{"begin":3812,"end":3816},"obj":"16188935"},{"id":"25982113-23674134-118216101","span":{"begin":3834,"end":3838},"obj":"23674134"},{"id":"25982113-23674134-118216101","span":{"begin":3834,"end":3838},"obj":"23674134"},{"id":"25982113-20691959-118216102","span":{"begin":3856,"end":3860},"obj":"20691959"},{"id":"25982113-20691959-118216102","span":{"begin":3856,"end":3860},"obj":"20691959"},{"id":"25982113-22102369-118216103","span":{"begin":4220,"end":4224},"obj":"22102369"},{"id":"25982113-22102369-118216103","span":{"begin":4220,"end":4224},"obj":"22102369"},{"id":"25982113-20826335-118216104","span":{"begin":4243,"end":4247},"obj":"20826335"},{"id":"25982113-20826335-118216104","span":{"begin":4243,"end":4247},"obj":"20826335"},{"id":"25982113-20015292-118216105","span":{"begin":4298,"end":4302},"obj":"20015292"},{"id":"25982113-20015292-118216105","span":{"begin":4298,"end":4302},"obj":"20015292"},{"id":"25982113-20188075-118216106","span":{"begin":4477,"end":4481},"obj":"20188075"},{"id":"25982113-20188075-118216106","span":{"begin":4477,"end":4481},"obj":"20188075"},{"id":"25982113-24199627-118216107","span":{"begin":4499,"end":4503},"obj":"24199627"},{"id":"25982113-24199627-118216107","span":{"begin":4499,"end":4503},"obj":"24199627"},{"id":"25982113-1988044-118216108","span":{"begin":4869,"end":4873},"obj":"1988044"},{"id":"25982113-1988044-118216108","span":{"begin":4869,"end":4873},"obj":"1988044"},{"id":"25982113-11444978-118216109","span":{"begin":4893,"end":4897},"obj":"11444978"},{"id":"25982113-11444978-118216109","span":{"begin":4893,"end":4897},"obj":"11444978"},{"id":"25982113-21830197-118216110","span":{"begin":4922,"end":4926},"obj":"21830197"},{"id":"25982113-21830197-118216110","span":{"begin":4922,"end":4926},"obj":"21830197"},{"id":"T55104","span":{"begin":399,"end":403},"obj":"15808821"},{"id":"T68601","span":{"begin":399,"end":403},"obj":"15808821"},{"id":"25982113-21722971-71076098","span":{"begin":798,"end":802},"obj":"21722971"},{"id":"25982113-21722971-71076098","span":{"begin":798,"end":802},"obj":"21722971"},{"id":"25982113-20966082-71076099","span":{"begin":1209,"end":1213},"obj":"20966082"},{"id":"25982113-20966082-71076099","span":{"begin":1209,"end":1213},"obj":"20966082"},{"id":"25982113-17715056-71076099","span":{"begin":1209,"end":1213},"obj":"17715056"},{"id":"25982113-17715056-71076099","span":{"begin":1209,"end":1213},"obj":"17715056"},{"id":"25982113-18801728-71076099","span":{"begin":1209,"end":1213},"obj":"18801728"},{"id":"25982113-18801728-71076099","span":{"begin":1209,"end":1213},"obj":"18801728"},{"id":"25982113-19674967-71076099","span":{"begin":1209,"end":1213},"obj":"19674967"},{"id":"25982113-19674967-71076099","span":{"begin":1209,"end":1213},"obj":"19674967"},{"id":"25982113-18375760-71076099","span":{"begin":1209,"end":1213},"obj":"18375760"},{"id":"25982113-18375760-71076099","span":{"begin":1209,"end":1213},"obj":"18375760"},{"id":"25982113-18287102-71076099","span":{"begin":1209,"end":1213},"obj":"18287102"},{"id":"25982113-18287102-71076099","span":{"begin":1209,"end":1213},"obj":"18287102"},{"id":"25982113-19861722-71076099","span":{"begin":1209,"end":1213},"obj":"19861722"},{"id":"25982113-19861722-71076099","span":{"begin":1209,"end":1213},"obj":"19861722"},{"id":"25982113-21658025-71076100","span":{"begin":1959,"end":1963},"obj":"21658025"},{"id":"25982113-21658025-71076100","span":{"begin":1959,"end":1963},"obj":"21658025"},{"id":"25982113-12037140-71076100","span":{"begin":1959,"end":1963},"obj":"12037140"},{"id":"25982113-12037140-71076100","span":{"begin":1959,"end":1963},"obj":"12037140"},{"id":"25982113-20433208-71076101","span":{"begin":2424,"end":2428},"obj":"20433208"},{"id":"25982113-20433208-71076101","span":{"begin":2424,"end":2428},"obj":"20433208"},{"id":"25982113-18998755-71076102","span":{"begin":2515,"end":2519},"obj":"18998755"},{"id":"25982113-18998755-71076102","span":{"begin":2515,"end":2519},"obj":"18998755"},{"id":"25982113-12529940-71076103","span":{"begin":2587,"end":2591},"obj":"12529940"},{"id":"25982113-12529940-71076103","span":{"begin":2587,"end":2591},"obj":"12529940"},{"id":"25982113-21658025-71076104","span":{"begin":2910,"end":2914},"obj":"21658025"},{"id":"25982113-21658025-71076104","span":{"begin":2910,"end":2914},"obj":"21658025"},{"id":"25982113-9620797-71076104","span":{"begin":2910,"end":2914},"obj":"9620797"},{"id":"25982113-9620797-71076104","span":{"begin":2910,"end":2914},"obj":"9620797"},{"id":"T92713","span":{"begin":3012,"end":3016},"obj":"17010614"},{"id":"T86754","span":{"begin":3012,"end":3016},"obj":"17010614"},{"id":"25982113-10385705-71076106","span":{"begin":3416,"end":3420},"obj":"10385705"},{"id":"25982113-10385705-71076106","span":{"begin":3416,"end":3420},"obj":"10385705"},{"id":"25982113-12037140-71076106","span":{"begin":3416,"end":3420},"obj":"12037140"},{"id":"25982113-12037140-71076106","span":{"begin":3416,"end":3420},"obj":"12037140"},{"id":"25982113-16188935-71076107","span":{"begin":3856,"end":3860},"obj":"16188935"},{"id":"25982113-16188935-71076107","span":{"begin":3856,"end":3860},"obj":"16188935"},{"id":"25982113-23674134-71076107","span":{"begin":3856,"end":3860},"obj":"23674134"},{"id":"25982113-23674134-71076107","span":{"begin":3856,"end":3860},"obj":"23674134"},{"id":"25982113-20691959-71076107","span":{"begin":3856,"end":3860},"obj":"20691959"},{"id":"25982113-20691959-71076107","span":{"begin":3856,"end":3860},"obj":"20691959"},{"id":"25982113-22102369-71076108","span":{"begin":4243,"end":4247},"obj":"22102369"},{"id":"25982113-22102369-71076108","span":{"begin":4243,"end":4247},"obj":"22102369"},{"id":"25982113-20826335-71076108","span":{"begin":4243,"end":4247},"obj":"20826335"},{"id":"25982113-20826335-71076108","span":{"begin":4243,"end":4247},"obj":"20826335"},{"id":"25982113-20015292-71076109","span":{"begin":4298,"end":4302},"obj":"20015292"},{"id":"25982113-20015292-71076109","span":{"begin":4298,"end":4302},"obj":"20015292"},{"id":"25982113-20188075-71076110","span":{"begin":4499,"end":4503},"obj":"20188075"},{"id":"25982113-20188075-71076110","span":{"begin":4499,"end":4503},"obj":"20188075"},{"id":"25982113-24199627-71076110","span":{"begin":4499,"end":4503},"obj":"24199627"},{"id":"25982113-24199627-71076110","span":{"begin":4499,"end":4503},"obj":"24199627"}],"text":"Introduction\nG protein-coupled receptors (GPCRs) are a large family of cell surface receptors that regulate a multitude of biological processes in response to a diverse array of stimuli and they are important drug targets. The class B/Secretin family GPCRs in humans include 15 receptors that are activated by diverse neuropeptides, peptide paracrine factors, and peptide endocrine hormones (Hoare, 2005). These receptors are less well understood than the larger class A/Rhodopsin family, despite their physiological and clinical importance. Class B GPCRs comprise an extracellular domain (ECD) of about 120 amino acids in addition to the 7-transmembrane (7TM) domain in the membrane. The ECD has an N-terminal α-helix and a set of β sheets held together by three disulfide bonds (Archbold et al., 2011). Peptides bind class B GPCRs via a “two-domain” model whereby their C-terminal region binds the ECD and their N-terminal region binds and activates the 7TM domain. Crystal structures are available for class B GPCR ECDs with bound peptides related to PTH, CRF, GIP, and GLP-1 (Pal et al., 2010; Parthier et al., 2007; Pioszak et al., 2008, 2009; Pioszak and Xu, 2008; Runge et al., 2008; Underwood et al., 2010) and a consensus has emerged from these studies. The peptides bind as extended α helices to the same region of the receptor, in a groove between the N and C termini of the isolated ECDs. For PTH, GIP, and GLP-1 families, the peptides are closest to the N terminus; for the CRF-related peptides, they are displaced to be closer to the C terminus.\nAlthough this model of binding is valid for several class B GPCRs, it cannot apply to all class B receptors. In particular, there are problems understanding the binding of members of the calcitonin (CT) family of peptides; calcitonin gene-related peptides alpha and beta (αCGRP, βCGRP), adrenomedullin (AM), adrenomedullin 2/intermedin (AM2), amylin (Amy), and CT (Hong et al., 2012; Poyner et al., 2002). These C terminally amidated peptides have a range of actions including neurogenic inflammation (CGRP), vasodilation/cardioprotection (CGRP, AM, and AM2), and regulation of blood and lymphatic vascular development (AM), nutrient intake and blood glucose (Amy), and bone turnover (CT). CGRP antagonists showed promise for the treatment of migraine and AM may be of value for the treatment of cardiovascular disorders (Durham and Vause, 2010; Karpinich et al., 2011). An Amy analog is used to treat insulin-dependent diabetes patients (Edelman et al., 2008) and CT has been long used to treat bone disorders (Purdue et al., 2002).\nCGRP, AM, and AM2 binding to their cognate class B receptor, the calcitonin receptor-like receptor (CLR), is dependent on association of CLR with one of three accessory membrane proteins that determine ligand selectivity; receptor activity-modifying proteins (RAMPs) 1, 2, or 3 (Hong et al., 2012; McLatchie et al., 1998). RAMPs have an ECD of about 100 amino acids and a single TM segment (Parameswaran and Spielman, 2006). CLR:RAMP1 is a CGRP receptor, CLR:RAMP2 preferentially recognizes AM and is called the AM1 receptor, and CLR:RAMP3 binds both AM and AM2 with high affinities and is called the AM2 receptor. Amy by itself has a low affinity for the class B CT receptor (CTR); however, when CTR associates with any of the RAMPs, its affinity for Amy is markedly increased (Christopoulos et al., 1999; Poyner et al., 2002). CTR alone is the receptor for CT. Thus, the RAMPs profoundly alter the behavior of CLR and CTR. Although RAMPs are best characterized for their effects on CLR/CTR, they also interact with several other class B GPCRs and with certain class A/Rhodopsin and class C/Glutamate family GPCRs, making it particularly important to understand the molecular basis for RAMP actions (Bouschet et al., 2005; Lenhart et al., 2013; Wootten et al., 2010). RAMPs provide an excellent opportunity to explore how accessory membrane proteins can modulate GPCR pharmacology.\nCrystal structures are available for ligand-free and small molecule antagonist-bound CLR:RAMP1 and ligand-free CLR:RAMP2 ECD complexes, but these provide little insight into how peptides bind or how RAMPs determine selectivity (Kusano et al., 2012; ter Haar et al., 2010). Extensive mutagenesis on the RAMPs (Qi and Hay, 2010) only provided clear evidence for the involvement of one RAMP residue (RAMP1 W84) in the binding of CGRP and two residues (RAMP2 F111 and E101) for AM binding (Moore et al., 2010; Watkins et al., 2014). It has not been possible to interpret these data mechanistically. A further complication is that it appears unlikely that CGRP and AM bind as extended helices as seen with other class B peptide ligands; there is evidence that only a small portion of these peptides form α helices and that at their C termini, there are one or more turn structures (Breeze et al., 1991; Carpenter et al., 2001; Pérez-Castells et al., 2012; Watkins et al., 2013b). Consequently, the mechanism of RAMP action and the mode of binding of CT family peptides remain unknown. Here, we describe high-resolution crystal structures of CGRP analog-bound CLR:RAMP1 and AM-bound CLR:RAMP2 ECD heterodimers that reveal bound peptide conformations starkly different from other class B GPCR peptide ligands, explain how RAMPs determine peptide selectivity, and provide molecular templates to guide drug development targeting CLR:RAMP complexes."}