{"gene":"MRAP2","run_date":"2026-06-10T02:59:51","timeline":{"discoveries":[{"year":2009,"finding":"MRAP2 interacts with all five melanocortin receptors (MC1R–MC5R). This interaction enables MC2R surface expression and signaling, while reducing MC1R, MC3R, MC4R, and MC5R responsiveness to NDP-MSH, establishing MRAP2 as a bidirectional regulator of the MCR family.","method":"Co-immunoprecipitation, cell surface expression assays, cAMP signaling assays in heterologous cells","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP and functional signaling assays across multiple receptors, replicated in multiple follow-up studies","pmids":["19329486"],"is_preprint":false},{"year":2013,"finding":"In zebrafish, MRAP2a binds MC4R and reduces its ability to bind α-MSH, blocking MC4R function and stimulating larval growth; a paralog MRAP2b also binds MC4R but increases ligand sensitivity, demonstrating developmental isoform-specific control of MC4R activity.","method":"Cell culture binding assays, signaling assays, zebrafish genetic model","journal":"Science (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo zebrafish model combined with in vitro binding/signaling assays, published in high-impact journal","pmids":["23869017"],"is_preprint":false},{"year":2017,"finding":"MRAP2 interacts with the ghrelin receptor GHSR1a and potentiates ghrelin-stimulated G-protein signaling both in vitro and in vivo. In the absence of MRAP2, fasting fails to activate AgRP neurons and the orexigenic effect of ghrelin is lost in mice.","method":"Co-immunoprecipitation, in vitro signaling assays, mouse knockout model with neuronal activity readouts","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal Co-IP plus in vivo mouse KO with defined neuronal and behavioral phenotype","pmids":["28959025"],"is_preprint":false},{"year":2017,"finding":"MRAP2 inhibits the trafficking and signaling of prokineticin receptor 1 (PKR1) and orexin receptor 1 (OX1R). Specific regions of MRAP2 are required for regulation of OX1R and PKR1, establishing that different MRAP2 domains mediate regulation of distinct GPCRs.","method":"Cell-based trafficking assays, signaling assays, MRAP2 deletion/truncation mutants","journal":"Biochimica et biophysica acta. Molecular cell research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — domain mapping with multiple deletion mutants, single lab, two orthogonal readouts (trafficking + signaling)","pmids":["28939058"],"is_preprint":false},{"year":2020,"finding":"MRAP2 inhibits GHSR1a constitutive activity, enhances G-protein-dependent signaling in response to ghrelin, and blocks β-arrestin recruitment and signaling. The effects on Gαq and β-arrestin pathways are independent and involve distinct regions of MRAP2, establishing that MRAP2 biases GHSR1a signaling.","method":"In vitro signaling assays (cAMP, IP1, β-arrestin recruitment assays), MRAP2 domain deletion constructs","journal":"Science signaling","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal signaling assays plus domain-mapping mutagenesis, mechanistically detailed","pmids":["31911434"],"is_preprint":false},{"year":2019,"finding":"Loss-of-function MRAP2 variants in humans are pathogenic for monogenic hyperphagic obesity, hyperglycemia, and hypertension, with functional assessment demonstrating these variants impair MRAP2-regulated GPCR function in relevant tissues including pancreatic islets.","method":"Large-scale sequencing of 9,418 individuals, functional assessment of each variant","journal":"Nature medicine","confidence":"High","confidence_rationale":"Tier 2 / Strong — large cohort with functional validation of each variant, multiple independent loss-of-function mutations confirmed","pmids":["31700171"],"is_preprint":false},{"year":2023,"finding":"MRAP2 is critical for the ciliary localization of MC4R in neurons. Loss of MRAP2 abolishes MC4R targeting to neuronal primary cilia, and disruption of ciliary MC4R localization impairs long-term energy homeostasis regulation.","method":"Fluorescence microscopy for ciliary localization, mouse genetic models (MRAP2 KO), functional energy homeostasis measurements","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 2 / Strong — direct localization experiment tied to functional consequence, in vivo mouse model with defined phenotype","pmids":["36692018"],"is_preprint":false},{"year":2022,"finding":"MRAP2 inhibits β-arrestin recruitment to GHSR1a by blocking GRK2 and PKC interaction with the receptor, thereby preventing phosphorylation of Ser252 and Thr261 in the third intracellular loop of GHSR1a that are required for β-arrestin recruitment.","method":"Phosphorylation site mutagenesis, GRK2/PKC interaction assays, β-arrestin recruitment assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Moderate — active-site mutagenesis identifying specific phosphorylation residues and blocking kinase interactions, single lab with multiple orthogonal methods","pmids":["35605660"],"is_preprint":false},{"year":2020,"finding":"MRAP2 is expressed in pancreatic islet δ cells and is required for ghrelin to elicit a calcium response in those cells. Global and δ-cell-targeted deletion of MRAP2 abrogates the insulinostatic effect of ghrelin, establishing MRAP2 as a regulator of insulin secretion via δ cell GHSR1a signaling.","method":"Calcium imaging in δ cells, conditional and global MRAP2 knockout mice, insulin secretion assays","journal":"iScience","confidence":"High","confidence_rationale":"Tier 2 / Strong — cell-type-specific conditional KO with mechanistic readouts (calcium response and insulin secretion), multiple genetic models tested","pmids":["32535024"],"is_preprint":false},{"year":2018,"finding":"Overexpression of MRAP2 specifically in PVN MC4R-expressing neurons reduces food intake, increases energy expenditure and core body temperature, and increases neuronal activation in response to melanocortin agonist MTII, demonstrating a site-specific role for MRAP2 in potentiating MC4R neuronal activation.","method":"Stereotaxic AAV-mediated MRAP2 overexpression in Mc4r-cre mice, energy homeostasis measurements, glucose/insulin profiling, c-Fos neuronal activation","journal":"Molecular metabolism","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — targeted in vivo overexpression with multiple functional readouts, single lab","pmids":["30352741"],"is_preprint":false},{"year":2025,"finding":"MRAP2 co-expression with MC4R enhances G-protein-mediated signaling, impairs β-arrestin2 recruitment and internalization, and disrupts MC4R oligomers by increasing the fraction of MC4R monomers. A structural homology model suggests MRAP2 contacts MC4R at transmembrane helices 5 and 6.","method":"cAMP and β-arrestin signaling assays, single-molecule imaging for oligomerization state, structural homology modeling","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal functional assays plus oligomerization measurement, single lab, structural model not experimentally validated","pmids":["40998819"],"is_preprint":false},{"year":2025,"finding":"MRAP2 directly interacts with MC3R with a preferential 1:1 stoichiometry, enhancing MC3R cAMP signaling, impairing β-arrestin recruitment, and reducing internalization. Five MRAP2 and two MC3R transmembrane residues identified by structural homology modeling are required for the interaction, and obesity-associated MRAP2 variants fail to enhance MC3R signaling.","method":"Single-molecule pull-down (SiMPull), fluorescence photobleaching stoichiometry, cAMP assays, β-arrestin recruitment assays, alanine mutagenesis","journal":"Science signaling","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — single-molecule interaction assay with stoichiometry determination, multiple orthogonal functional assays, and mutagenesis validating predicted contact residues","pmids":["41401256"],"is_preprint":false},{"year":2025,"finding":"MRAP2 regulates MC4R, MC3R, and GHSR via shared molecular mechanisms: all three GPCRs preferentially form 1:1 complexes with MRAP2, MRAP2 binding disrupts GPCR homodimerization, MRAP2 interacts with the same transmembrane regions across receptors, and deletion of the MRAP2 cytoplasmic region impairs GPCR signaling by modulating constitutive activity. Obesity-associated MRAP2 variants alter constitutive activity of all three GPCRs.","method":"Single-molecule pull-down, β-arrestin recruitment assays, constitutive activity measurements, MRAP2 domain deletion constructs, functional variant analysis","journal":"Cell reports","confidence":"High","confidence_rationale":"Tier 2 / Strong — convergent evidence across three GPCRs with multiple orthogonal methods including single-molecule interaction assays and domain deletion mutagenesis","pmids":["41722048"],"is_preprint":false},{"year":2025,"finding":"Twelve obesity-associated MRAP2 variants impair MC4R function across multiple signaling pathways: seven impair cAMP signaling and nine impair IP3 signaling when expressed at equal concentrations; four C-terminal mutations affect MC4R internalization. Structural models predict MRAP2 interacts with MC4R transmembrane helices 5 and 6, and mutagenesis of two putative contact residues impaired MRAP2 facilitation of MC4R signaling.","method":"cAMP signaling assays, IP3 signaling assays, internalization assays, MRAP2 variant expression at equal concentrations, alanine mutagenesis of predicted contact sites","journal":"Human molecular genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple signaling pathway readouts and mutagenesis, single lab","pmids":["39807633"],"is_preprint":false},{"year":2022,"finding":"MRAP2 exists as a symmetric antiparallel homodimer on the plasma membrane. The arginine at position 125 (R125) is essential for MRAP2 function, affecting protein conformation, dimer formation, and PKR2 binding. Human obesity-associated mutations R125H and R125C impair these properties.","method":"MRAP2 mutant expression, dimerization assays, PKR2 binding assays, comparison with mouse MRAP2 (where position 125 is naturally histidine)","journal":"International journal of molecular sciences","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — site-directed mutagenesis with multiple functional readouts, single lab","pmids":["36077245"],"is_preprint":false},{"year":2021,"finding":"MRAP2 forms symmetric antiparallel homodimers on the plasma membrane; inversion of N-terminal, transmembrane, and C-terminal domains in six MRAP2 conformational variants showed that proper antiparallel homodimer assembly is maintained but regulatory profiles on MC4R surface expression and cAMP signaling are altered, revealing the functional importance of domain orientation.","method":"Domain inversion mutagenesis, dimerization assays, MC4R surface expression assays, cAMP signaling assays","journal":"Frontiers in endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — systematic domain inversion mutagenesis with multiple functional readouts, single lab","pmids":["34759891"],"is_preprint":false},{"year":2024,"finding":"MRAP2 inhibits β-arrestin-2 recruitment to prokineticin receptor 2 (PKR2) and modulates PKR2-mediated β-arrestin signaling.","method":"β-arrestin-2 recruitment assays, PKR2/MRAP2 co-expression in cells","journal":"Current issues in molecular biology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single method described in abstract, limited detail on mechanism","pmids":["38392222"],"is_preprint":false},{"year":2022,"finding":"MRAP2 interacts with melanin-concentrating hormone receptor 1 (MCHR1) and inhibits MCHR1 signaling in vitro. The C-terminal domain of MRAP2 is required for pharmacological modulation of intracellular Ca2+-coupled cascades and membrane transport of MCHR1.","method":"Co-immunoprecipitation, bimolecular fluorescence complementation (BiFC), Ca2+ signaling assays, MRAP2 truncation mutants","journal":"Frontiers in endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal Co-IP plus BiFC plus functional domain truncation, single lab","pmids":["35311242"],"is_preprint":false},{"year":2016,"finding":"Brain-specific and global deletion of Mrap2 in mice causes severe obesity with increased fat and lean mass. Transcriptomic analysis of Mrap2-deficient hypothalamic PVN revealed significantly decreased expression of Sim1, Trh, Oxt, and Crh, and circulating HDL and LDL were significantly elevated, indicating defective central melanocortin signaling downstream of MRAP2 loss.","method":"Two independently derived Mrap2-deficient mouse lines, body composition analysis, transcriptomic analysis of PVN, plasma lipid measurements","journal":"The Journal of endocrinology","confidence":"High","confidence_rationale":"Tier 2 / Strong — two independent KO lines with molecular and metabolic phenotyping, pathway placement via downstream gene expression","pmids":["27106110"],"is_preprint":false},{"year":2025,"finding":"MRAP2 interacts with melatonin receptors MTNR1A and MTNR1B (shown by Co-IP and BiFC), inhibits their constitutive activity, enhances maximal agonist potency, suppresses membrane trafficking of MTNR1A, and promotes surface trafficking of MTNR1B.","method":"Co-immunoprecipitation, BiFC, GloSensor luminescence assay (Gi signaling), fixed-cell ELISA for surface trafficking","journal":"Frontiers in endocrinology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, BiFC, functional assay, trafficking assay), single lab","pmids":["40510472"],"is_preprint":false}],"current_model":"MRAP2 is a single-pass transmembrane protein that forms antiparallel homodimers and functions as a broad GPCR accessory protein: it physically interacts (preferentially as 1:1 complexes) with multiple GPCRs including MC2R–MC5R, GHSR1a, PKR1/2, OX1R, MCHR1, and melatonin receptors, using its transmembrane domain to contact receptor transmembrane helices 5 and 6 while its cytoplasmic C-terminal region modulates constitutive activity; MRAP2 enhances G-protein-mediated signaling, disrupts receptor homodimerization, blocks β-arrestin recruitment by preventing GRK2/PKC-mediated receptor phosphorylation, facilitates MC4R targeting to neuronal primary cilia, and is essential in hypothalamic and pancreatic δ-cell circuits for ghrelin-driven hunger signaling and insulinostasis, with loss-of-function variants causing monogenic hyperphagic obesity, hyperglycemia, and hypertension in humans."},"narrative":{"mechanistic_narrative":"MRAP2 is a single-pass transmembrane accessory protein that broadly regulates G-protein-coupled receptor signaling in central and peripheral circuits controlling energy homeostasis [PMID:19329486, PMID:28959025]. It assembles as a symmetric antiparallel homodimer at the plasma membrane, and proper domain orientation within this dimer is essential for its regulatory output [PMID:34759891], while a conserved arginine (R125) governs dimer formation and receptor binding [PMID:36077245]. MRAP2 physically engages a diverse set of GPCRs — the melanocortin receptors MC1R–MC5R [PMID:19329486], the ghrelin receptor GHSR1a [PMID:28959025], prokineticin and orexin receptors [PMID:28939058], MCHR1 [PMID:35311242], and melatonin receptors [PMID:40510472] — preferentially forming 1:1 complexes that disrupt receptor homodimerization and contact receptor transmembrane helices 5 and 6 through the MRAP2 transmembrane domain, with the cytoplasmic C-terminus tuning constitutive activity [PMID:41401256, PMID:41722048]. Mechanistically, MRAP2 biases signaling toward G-protein pathways while suppressing β-arrestin recruitment: at GHSR1a it blocks GRK2/PKC-mediated phosphorylation of Ser252 and Thr261 in the third intracellular loop, preventing β-arrestin recruitment and receptor internalization [PMID:35605660, PMID:31911434], and it exerts the same enhancement of G-protein signaling with reduced internalization at MC4R and MC3R [PMID:40998819, PMID:41401256]. In vivo, MRAP2 is required in hypothalamic AgRP neurons for fasting- and ghrelin-driven hunger signaling [PMID:28959025], facilitates MC4R targeting to neuronal primary cilia for long-term energy balance [PMID:36692018], and acts in pancreatic δ cells where it is required for ghrelin's insulinostatic calcium response [PMID:32535024]; its loss causes severe obesity with disrupted central melanocortin pathway gene expression [PMID:27106110]. Loss-of-function MRAP2 variants in humans cause monogenic hyperphagic obesity, hyperglycemia, and hypertension, impairing MRAP2-regulated GPCR function [PMID:31700171].","teleology":[{"year":2009,"claim":"Established MRAP2 as a direct, bidirectional regulator of the melanocortin receptor family, defining its core role as a GPCR accessory protein rather than a single-receptor chaperone.","evidence":"Co-IP and cAMP signaling assays across MC1R–MC5R in heterologous cells","pmids":["19329486"],"confidence":"High","gaps":["Did not resolve the structural basis of the interaction","In vivo physiological relevance not yet addressed"]},{"year":2013,"claim":"Showed MRAP2 control of MC4R is isoform- and development-specific in vivo, linking receptor modulation to organismal growth and feeding.","evidence":"Zebrafish genetic model with paralog-specific MRAP2a/MRAP2b binding and signaling assays","pmids":["23869017"],"confidence":"High","gaps":["Mammalian equivalence of paralog-specific effects unclear","Molecular determinants of opposite paralog effects not mapped"]},{"year":2016,"claim":"Placed MRAP2 within the central melanocortin pathway by showing its deletion causes obesity and downregulates PVN neuropeptide gene expression.","evidence":"Two independent Mrap2-deficient mouse lines with body composition, PVN transcriptomics, and plasma lipid analysis","pmids":["27106110"],"confidence":"High","gaps":["Did not identify which receptor(s) mediate the phenotype","Causal link between transcriptomic changes and obesity not established"]},{"year":2017,"claim":"Extended MRAP2 function to the ghrelin axis, demonstrating it is required in vivo for fasting and ghrelin to activate orexigenic AgRP neurons.","evidence":"Reciprocal Co-IP, in vitro signaling, and mouse KO with neuronal/behavioral readouts","pmids":["28959025"],"confidence":"High","gaps":["Did not resolve the signaling-bias mechanism","Domain requirements not yet defined"]},{"year":2017,"claim":"Demonstrated that distinct MRAP2 domains regulate distinct GPCRs, showing the protein is not a uniform modulator across its receptor repertoire.","evidence":"Trafficking and signaling assays with MRAP2 deletion/truncation mutants on PKR1 and OX1R","pmids":["28939058"],"confidence":"Medium","gaps":["Single lab","Precise domain-receptor contacts not structurally defined"]},{"year":2018,"claim":"Provided gain-of-function evidence that MRAP2 in PVN MC4R neurons potentiates melanocortin signaling and energy expenditure, complementing loss-of-function data.","evidence":"AAV-mediated MRAP2 overexpression in Mc4r-cre mice with energy homeostasis and c-Fos readouts","pmids":["30352741"],"confidence":"Medium","gaps":["Single lab","Overexpression may not reflect physiological stoichiometry"]},{"year":2019,"claim":"Connected MRAP2 to human disease, establishing loss-of-function variants as causative for monogenic hyperphagic obesity with metabolic comorbidities.","evidence":"Sequencing of 9,418 individuals with functional assessment of each variant including pancreatic islet relevance","pmids":["31700171"],"confidence":"High","gaps":["Tissue-specific contribution of each comorbidity not dissected","Variant effects on specific receptors not individually resolved"]},{"year":2020,"claim":"Defined the signaling-bias mechanism at GHSR1a, showing MRAP2 suppresses constitutive activity and β-arrestin recruitment while enhancing G-protein signaling via independent domains.","evidence":"cAMP, IP1, and β-arrestin recruitment assays with MRAP2 domain deletion constructs","pmids":["31911434"],"confidence":"High","gaps":["Molecular block of β-arrestin recruitment not yet identified at residue level"]},{"year":2020,"claim":"Extended MRAP2's role to the endocrine pancreas, showing it is required in δ cells for ghrelin-evoked calcium responses and insulinostasis.","evidence":"Calcium imaging, global and δ-cell-conditional MRAP2 KO mice, and insulin secretion assays","pmids":["32535024"],"confidence":"High","gaps":["Direct demonstration of δ-cell GHSR1a–MRAP2 complex in vivo not shown"]},{"year":2021,"claim":"Established that MRAP2 forms symmetric antiparallel homodimers and that domain orientation, not merely dimer assembly, dictates regulatory output on MC4R.","evidence":"Domain inversion mutagenesis with dimerization, surface expression, and cAMP assays","pmids":["34759891"],"confidence":"Medium","gaps":["Single lab","No experimental structure of the dimer"]},{"year":2022,"claim":"Identified the molecular events by which MRAP2 blocks β-arrestin recruitment at GHSR1a, pinpointing kinase exclusion and specific phospho-sites.","evidence":"Phosphosite mutagenesis (Ser252/Thr261), GRK2/PKC interaction assays, and β-arrestin recruitment assays","pmids":["35605660"],"confidence":"High","gaps":["Whether the same kinase-exclusion mechanism operates at other GPCRs not tested"]},{"year":2022,"claim":"Tied a conserved residue (R125) to MRAP2 conformation, dimerization, and receptor binding, providing a structural rationale for human R125 obesity variants.","evidence":"Site-directed mutagenesis with dimerization and PKR2 binding assays, cross-species comparison","pmids":["36077245"],"confidence":"Medium","gaps":["Single lab","Structural mechanism of R125 action not resolved"]},{"year":2022,"claim":"Broadened the receptor repertoire to MCHR1, showing C-terminal-dependent inhibition of calcium signaling and trafficking.","evidence":"Co-IP, BiFC, Ca2+ signaling assays, and MRAP2 truncation mutants","pmids":["35311242"],"confidence":"Medium","gaps":["Single lab","No in vivo validation of MCHR1 regulation"]},{"year":2023,"claim":"Revealed a trafficking-based mechanism by which MRAP2 controls MC4R, showing it is required for ciliary targeting of the receptor for long-term energy balance.","evidence":"Ciliary localization microscopy and MRAP2 KO mouse models with energy homeostasis readouts","pmids":["36692018"],"confidence":"High","gaps":["Molecular machinery linking MRAP2 to ciliary transport not defined"]},{"year":2024,"claim":"Added PKR2 to the set of receptors whose β-arrestin signaling MRAP2 inhibits.","evidence":"β-arrestin-2 recruitment assays with PKR2/MRAP2 co-expression","pmids":["38392222"],"confidence":"Low","gaps":["Single method described in abstract with limited mechanistic detail","No reciprocal interaction validation","No in vivo support"]},{"year":2025,"claim":"Unified MRAP2's mechanism across MC4R, MC3R, and GHSR, showing preferential 1:1 complexes, disruption of receptor homodimers, shared transmembrane contacts, and C-terminal modulation of constitutive activity.","evidence":"Single-molecule pull-down, stoichiometry by photobleaching, cAMP/β-arrestin assays, alanine and domain-deletion mutagenesis, and structural homology modeling across receptors","pmids":["40998819","41401256","41722048","39807633"],"confidence":"High","gaps":["Structural models not experimentally validated by high-resolution structure","Some studies single-lab"]},{"year":2025,"claim":"Further extended the repertoire to melatonin receptors with receptor-divergent trafficking outcomes, reinforcing MRAP2 as a broad GPCR accessory protein.","evidence":"Co-IP, BiFC, GloSensor Gi signaling, and surface trafficking ELISA on MTNR1A/MTNR1B","pmids":["40510472"],"confidence":"Medium","gaps":["Single lab","Physiological relevance of melatonin receptor regulation not established"]},{"year":null,"claim":"How MRAP2 mechanistically distinguishes G-protein potentiation from β-arrestin/trafficking suppression across its diverse receptor partners, and whether a single structural mode generalizes, remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No experimentally determined high-resolution structure of any MRAP2–GPCR complex","Tissue-specific receptor selectivity in vivo only partially mapped","Mechanism of ciliary MC4R targeting unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,2,4,11,12]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[4,7,10,11]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[14,15]},{"term_id":"GO:0005929","term_label":"cilium","supporting_discovery_ids":[6]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[0,2,4,11,12]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[5,8,18]}],"complexes":[],"partners":["MC4R","MC3R","MC2R","GHSR1A","PKR2","OX1R","MCHR1","MTNR1A"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q96G30","full_name":"Melanocortin-2 receptor accessory protein 2","aliases":[],"length_aa":205,"mass_kda":23.5,"function":"Modulator of melanocortin receptor 4 (MC4R), a receptor involved in energy homeostasis. Plays a central role in the control of energy homeostasis and body weight regulation by increasing ligand-sensitivity of MC4R and MC4R-mediated generation of cAMP (By similarity). May also act as a negative regulator of MC2R: competes with MRAP for binding to MC2R and impairs the binding of corticotropin (ACTH) to MC2R. May also regulate activity of other melanocortin receptors (MC1R, MC3R and MC5R); however, additional evidence is required in vivo","subcellular_location":"Cell membrane; Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/Q96G30/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/MRAP2","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/MRAP2","total_profiled":1310},"omim":[{"mim_id":"615457","title":"BODY MASS INDEX QUANTITATIVE TRAIT LOCUS 18; BMIQ18","url":"https://www.omim.org/entry/615457"},{"mim_id":"615410","title":"MELANOCORTIN 2 RECEPTOR ACCESSORY PROTEIN 2; MRAP2","url":"https://www.omim.org/entry/615410"},{"mim_id":"609196","title":"MELANOCORTIN 2 RECEPTOR ACCESSORY PROTEIN; MRAP","url":"https://www.omim.org/entry/609196"},{"mim_id":"606641","title":"BODY MASS INDEX QUANTITATIVE TRAIT LOCUS 1; BMIQ1","url":"https://www.omim.org/entry/606641"},{"mim_id":"155540","title":"MELANOCORTIN 3 RECEPTOR; MC3R","url":"https://www.omim.org/entry/155540"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enriched","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"blood vessel","ntpm":97.2}],"url":"https://www.proteinatlas.org/search/MRAP2"},"hgnc":{"alias_symbol":["bA51G5.2"],"prev_symbol":["C6orf117"]},"alphafold":{"accession":"Q96G30","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96G30","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96G30-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96G30-F1-predicted_aligned_error_v6.png","plddt_mean":57.09},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=MRAP2","jax_strain_url":"https://www.jax.org/strain/search?query=MRAP2"},"sequence":{"accession":"Q96G30","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96G30.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96G30/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96G30"}},"corpus_meta":[{"pmid":"19329486","id":"PMC_19329486","title":"MRAP and MRAP2 are bidirectional regulators of the melanocortin receptor family.","date":"2009","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/19329486","citation_count":197,"is_preprint":false},{"pmid":"23869017","id":"PMC_23869017","title":"Developmental control of the melanocortin-4 receptor by MRAP2 proteins in zebrafish.","date":"2013","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/23869017","citation_count":137,"is_preprint":false},{"pmid":"31700171","id":"PMC_31700171","title":"Loss-of-function mutations in MRAP2 are pathogenic in hyperphagic obesity with hyperglycemia and hypertension.","date":"2019","source":"Nature medicine","url":"https://pubmed.ncbi.nlm.nih.gov/31700171","citation_count":82,"is_preprint":false},{"pmid":"28959025","id":"PMC_28959025","title":"MRAP2 regulates ghrelin receptor signaling and hunger sensing.","date":"2017","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/28959025","citation_count":66,"is_preprint":false},{"pmid":"28939058","id":"PMC_28939058","title":"Regions of MRAP2 required for the inhibition of orexin and prokineticin receptor signaling.","date":"2017","source":"Biochimica et biophysica acta. Molecular cell research","url":"https://pubmed.ncbi.nlm.nih.gov/28939058","citation_count":57,"is_preprint":false},{"pmid":"31911434","id":"PMC_31911434","title":"The GPCR accessory protein MRAP2 regulates both biased signaling and constitutive activity of the ghrelin receptor GHSR1a.","date":"2020","source":"Science signaling","url":"https://pubmed.ncbi.nlm.nih.gov/31911434","citation_count":52,"is_preprint":false},{"pmid":"28512117","id":"PMC_28512117","title":"The interaction of MC3R and MC4R with MRAP2, ACTH, α-MSH and AgRP in chickens.","date":"2017","source":"The Journal of endocrinology","url":"https://pubmed.ncbi.nlm.nih.gov/28512117","citation_count":48,"is_preprint":false},{"pmid":"32679290","id":"PMC_32679290","title":"Emerging roles of melanocortin receptor accessory proteins (MRAP and MRAP2) in physiology and 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This interaction enables MC2R surface expression and signaling, while reducing MC1R, MC3R, MC4R, and MC5R responsiveness to NDP-MSH, establishing MRAP2 as a bidirectional regulator of the MCR family.\",\n      \"method\": \"Co-immunoprecipitation, cell surface expression assays, cAMP signaling assays in heterologous cells\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP and functional signaling assays across multiple receptors, replicated in multiple follow-up studies\",\n      \"pmids\": [\"19329486\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"In zebrafish, MRAP2a binds MC4R and reduces its ability to bind α-MSH, blocking MC4R function and stimulating larval growth; a paralog MRAP2b also binds MC4R but increases ligand sensitivity, demonstrating developmental isoform-specific control of MC4R activity.\",\n      \"method\": \"Cell culture binding assays, signaling assays, zebrafish genetic model\",\n      \"journal\": \"Science (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo zebrafish model combined with in vitro binding/signaling assays, published in high-impact journal\",\n      \"pmids\": [\"23869017\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"MRAP2 interacts with the ghrelin receptor GHSR1a and potentiates ghrelin-stimulated G-protein signaling both in vitro and in vivo. In the absence of MRAP2, fasting fails to activate AgRP neurons and the orexigenic effect of ghrelin is lost in mice.\",\n      \"method\": \"Co-immunoprecipitation, in vitro signaling assays, mouse knockout model with neuronal activity readouts\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal Co-IP plus in vivo mouse KO with defined neuronal and behavioral phenotype\",\n      \"pmids\": [\"28959025\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"MRAP2 inhibits the trafficking and signaling of prokineticin receptor 1 (PKR1) and orexin receptor 1 (OX1R). Specific regions of MRAP2 are required for regulation of OX1R and PKR1, establishing that different MRAP2 domains mediate regulation of distinct GPCRs.\",\n      \"method\": \"Cell-based trafficking assays, signaling assays, MRAP2 deletion/truncation mutants\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — domain mapping with multiple deletion mutants, single lab, two orthogonal readouts (trafficking + signaling)\",\n      \"pmids\": [\"28939058\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"MRAP2 inhibits GHSR1a constitutive activity, enhances G-protein-dependent signaling in response to ghrelin, and blocks β-arrestin recruitment and signaling. The effects on Gαq and β-arrestin pathways are independent and involve distinct regions of MRAP2, establishing that MRAP2 biases GHSR1a signaling.\",\n      \"method\": \"In vitro signaling assays (cAMP, IP1, β-arrestin recruitment assays), MRAP2 domain deletion constructs\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal signaling assays plus domain-mapping mutagenesis, mechanistically detailed\",\n      \"pmids\": [\"31911434\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Loss-of-function MRAP2 variants in humans are pathogenic for monogenic hyperphagic obesity, hyperglycemia, and hypertension, with functional assessment demonstrating these variants impair MRAP2-regulated GPCR function in relevant tissues including pancreatic islets.\",\n      \"method\": \"Large-scale sequencing of 9,418 individuals, functional assessment of each variant\",\n      \"journal\": \"Nature medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — large cohort with functional validation of each variant, multiple independent loss-of-function mutations confirmed\",\n      \"pmids\": [\"31700171\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"MRAP2 is critical for the ciliary localization of MC4R in neurons. Loss of MRAP2 abolishes MC4R targeting to neuronal primary cilia, and disruption of ciliary MC4R localization impairs long-term energy homeostasis regulation.\",\n      \"method\": \"Fluorescence microscopy for ciliary localization, mouse genetic models (MRAP2 KO), functional energy homeostasis measurements\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — direct localization experiment tied to functional consequence, in vivo mouse model with defined phenotype\",\n      \"pmids\": [\"36692018\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"MRAP2 inhibits β-arrestin recruitment to GHSR1a by blocking GRK2 and PKC interaction with the receptor, thereby preventing phosphorylation of Ser252 and Thr261 in the third intracellular loop of GHSR1a that are required for β-arrestin recruitment.\",\n      \"method\": \"Phosphorylation site mutagenesis, GRK2/PKC interaction assays, β-arrestin recruitment assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — active-site mutagenesis identifying specific phosphorylation residues and blocking kinase interactions, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"35605660\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"MRAP2 is expressed in pancreatic islet δ cells and is required for ghrelin to elicit a calcium response in those cells. Global and δ-cell-targeted deletion of MRAP2 abrogates the insulinostatic effect of ghrelin, establishing MRAP2 as a regulator of insulin secretion via δ cell GHSR1a signaling.\",\n      \"method\": \"Calcium imaging in δ cells, conditional and global MRAP2 knockout mice, insulin secretion assays\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — cell-type-specific conditional KO with mechanistic readouts (calcium response and insulin secretion), multiple genetic models tested\",\n      \"pmids\": [\"32535024\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Overexpression of MRAP2 specifically in PVN MC4R-expressing neurons reduces food intake, increases energy expenditure and core body temperature, and increases neuronal activation in response to melanocortin agonist MTII, demonstrating a site-specific role for MRAP2 in potentiating MC4R neuronal activation.\",\n      \"method\": \"Stereotaxic AAV-mediated MRAP2 overexpression in Mc4r-cre mice, energy homeostasis measurements, glucose/insulin profiling, c-Fos neuronal activation\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — targeted in vivo overexpression with multiple functional readouts, single lab\",\n      \"pmids\": [\"30352741\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"MRAP2 co-expression with MC4R enhances G-protein-mediated signaling, impairs β-arrestin2 recruitment and internalization, and disrupts MC4R oligomers by increasing the fraction of MC4R monomers. A structural homology model suggests MRAP2 contacts MC4R at transmembrane helices 5 and 6.\",\n      \"method\": \"cAMP and β-arrestin signaling assays, single-molecule imaging for oligomerization state, structural homology modeling\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal functional assays plus oligomerization measurement, single lab, structural model not experimentally validated\",\n      \"pmids\": [\"40998819\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"MRAP2 directly interacts with MC3R with a preferential 1:1 stoichiometry, enhancing MC3R cAMP signaling, impairing β-arrestin recruitment, and reducing internalization. Five MRAP2 and two MC3R transmembrane residues identified by structural homology modeling are required for the interaction, and obesity-associated MRAP2 variants fail to enhance MC3R signaling.\",\n      \"method\": \"Single-molecule pull-down (SiMPull), fluorescence photobleaching stoichiometry, cAMP assays, β-arrestin recruitment assays, alanine mutagenesis\",\n      \"journal\": \"Science signaling\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — single-molecule interaction assay with stoichiometry determination, multiple orthogonal functional assays, and mutagenesis validating predicted contact residues\",\n      \"pmids\": [\"41401256\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"MRAP2 regulates MC4R, MC3R, and GHSR via shared molecular mechanisms: all three GPCRs preferentially form 1:1 complexes with MRAP2, MRAP2 binding disrupts GPCR homodimerization, MRAP2 interacts with the same transmembrane regions across receptors, and deletion of the MRAP2 cytoplasmic region impairs GPCR signaling by modulating constitutive activity. Obesity-associated MRAP2 variants alter constitutive activity of all three GPCRs.\",\n      \"method\": \"Single-molecule pull-down, β-arrestin recruitment assays, constitutive activity measurements, MRAP2 domain deletion constructs, functional variant analysis\",\n      \"journal\": \"Cell reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — convergent evidence across three GPCRs with multiple orthogonal methods including single-molecule interaction assays and domain deletion mutagenesis\",\n      \"pmids\": [\"41722048\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Twelve obesity-associated MRAP2 variants impair MC4R function across multiple signaling pathways: seven impair cAMP signaling and nine impair IP3 signaling when expressed at equal concentrations; four C-terminal mutations affect MC4R internalization. Structural models predict MRAP2 interacts with MC4R transmembrane helices 5 and 6, and mutagenesis of two putative contact residues impaired MRAP2 facilitation of MC4R signaling.\",\n      \"method\": \"cAMP signaling assays, IP3 signaling assays, internalization assays, MRAP2 variant expression at equal concentrations, alanine mutagenesis of predicted contact sites\",\n      \"journal\": \"Human molecular genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple signaling pathway readouts and mutagenesis, single lab\",\n      \"pmids\": [\"39807633\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"MRAP2 exists as a symmetric antiparallel homodimer on the plasma membrane. The arginine at position 125 (R125) is essential for MRAP2 function, affecting protein conformation, dimer formation, and PKR2 binding. Human obesity-associated mutations R125H and R125C impair these properties.\",\n      \"method\": \"MRAP2 mutant expression, dimerization assays, PKR2 binding assays, comparison with mouse MRAP2 (where position 125 is naturally histidine)\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — site-directed mutagenesis with multiple functional readouts, single lab\",\n      \"pmids\": [\"36077245\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"MRAP2 forms symmetric antiparallel homodimers on the plasma membrane; inversion of N-terminal, transmembrane, and C-terminal domains in six MRAP2 conformational variants showed that proper antiparallel homodimer assembly is maintained but regulatory profiles on MC4R surface expression and cAMP signaling are altered, revealing the functional importance of domain orientation.\",\n      \"method\": \"Domain inversion mutagenesis, dimerization assays, MC4R surface expression assays, cAMP signaling assays\",\n      \"journal\": \"Frontiers in endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — systematic domain inversion mutagenesis with multiple functional readouts, single lab\",\n      \"pmids\": [\"34759891\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"MRAP2 inhibits β-arrestin-2 recruitment to prokineticin receptor 2 (PKR2) and modulates PKR2-mediated β-arrestin signaling.\",\n      \"method\": \"β-arrestin-2 recruitment assays, PKR2/MRAP2 co-expression in cells\",\n      \"journal\": \"Current issues in molecular biology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single method described in abstract, limited detail on mechanism\",\n      \"pmids\": [\"38392222\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"MRAP2 interacts with melanin-concentrating hormone receptor 1 (MCHR1) and inhibits MCHR1 signaling in vitro. The C-terminal domain of MRAP2 is required for pharmacological modulation of intracellular Ca2+-coupled cascades and membrane transport of MCHR1.\",\n      \"method\": \"Co-immunoprecipitation, bimolecular fluorescence complementation (BiFC), Ca2+ signaling assays, MRAP2 truncation mutants\",\n      \"journal\": \"Frontiers in endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal Co-IP plus BiFC plus functional domain truncation, single lab\",\n      \"pmids\": [\"35311242\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Brain-specific and global deletion of Mrap2 in mice causes severe obesity with increased fat and lean mass. Transcriptomic analysis of Mrap2-deficient hypothalamic PVN revealed significantly decreased expression of Sim1, Trh, Oxt, and Crh, and circulating HDL and LDL were significantly elevated, indicating defective central melanocortin signaling downstream of MRAP2 loss.\",\n      \"method\": \"Two independently derived Mrap2-deficient mouse lines, body composition analysis, transcriptomic analysis of PVN, plasma lipid measurements\",\n      \"journal\": \"The Journal of endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — two independent KO lines with molecular and metabolic phenotyping, pathway placement via downstream gene expression\",\n      \"pmids\": [\"27106110\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"MRAP2 interacts with melatonin receptors MTNR1A and MTNR1B (shown by Co-IP and BiFC), inhibits their constitutive activity, enhances maximal agonist potency, suppresses membrane trafficking of MTNR1A, and promotes surface trafficking of MTNR1B.\",\n      \"method\": \"Co-immunoprecipitation, BiFC, GloSensor luminescence assay (Gi signaling), fixed-cell ELISA for surface trafficking\",\n      \"journal\": \"Frontiers in endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods (Co-IP, BiFC, functional assay, trafficking assay), single lab\",\n      \"pmids\": [\"40510472\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"MRAP2 is a single-pass transmembrane protein that forms antiparallel homodimers and functions as a broad GPCR accessory protein: it physically interacts (preferentially as 1:1 complexes) with multiple GPCRs including MC2R–MC5R, GHSR1a, PKR1/2, OX1R, MCHR1, and melatonin receptors, using its transmembrane domain to contact receptor transmembrane helices 5 and 6 while its cytoplasmic C-terminal region modulates constitutive activity; MRAP2 enhances G-protein-mediated signaling, disrupts receptor homodimerization, blocks β-arrestin recruitment by preventing GRK2/PKC-mediated receptor phosphorylation, facilitates MC4R targeting to neuronal primary cilia, and is essential in hypothalamic and pancreatic δ-cell circuits for ghrelin-driven hunger signaling and insulinostasis, with loss-of-function variants causing monogenic hyperphagic obesity, hyperglycemia, and hypertension in humans.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"MRAP2 is a single-pass transmembrane accessory protein that broadly regulates G-protein-coupled receptor signaling in central and peripheral circuits controlling energy homeostasis [#0, #2]. It assembles as a symmetric antiparallel homodimer at the plasma membrane, and proper domain orientation within this dimer is essential for its regulatory output [#15], while a conserved arginine (R125) governs dimer formation and receptor binding [#14]. MRAP2 physically engages a diverse set of GPCRs — the melanocortin receptors MC1R–MC5R [#0], the ghrelin receptor GHSR1a [#2], prokineticin and orexin receptors [#3], MCHR1 [#17], and melatonin receptors [#19] — preferentially forming 1:1 complexes that disrupt receptor homodimerization and contact receptor transmembrane helices 5 and 6 through the MRAP2 transmembrane domain, with the cytoplasmic C-terminus tuning constitutive activity [#11, #12]. Mechanistically, MRAP2 biases signaling toward G-protein pathways while suppressing β-arrestin recruitment: at GHSR1a it blocks GRK2/PKC-mediated phosphorylation of Ser252 and Thr261 in the third intracellular loop, preventing β-arrestin recruitment and receptor internalization [#7, #4], and it exerts the same enhancement of G-protein signaling with reduced internalization at MC4R and MC3R [#10, #11]. In vivo, MRAP2 is required in hypothalamic AgRP neurons for fasting- and ghrelin-driven hunger signaling [#2], facilitates MC4R targeting to neuronal primary cilia for long-term energy balance [#6], and acts in pancreatic δ cells where it is required for ghrelin's insulinostatic calcium response [#8]; its loss causes severe obesity with disrupted central melanocortin pathway gene expression [#18]. Loss-of-function MRAP2 variants in humans cause monogenic hyperphagic obesity, hyperglycemia, and hypertension, impairing MRAP2-regulated GPCR function [#5].\",\n  \"teleology\": [\n    {\n      \"year\": 2009,\n      \"claim\": \"Established MRAP2 as a direct, bidirectional regulator of the melanocortin receptor family, defining its core role as a GPCR accessory protein rather than a single-receptor chaperone.\",\n      \"evidence\": \"Co-IP and cAMP signaling assays across MC1R–MC5R in heterologous cells\",\n      \"pmids\": [\"19329486\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the structural basis of the interaction\", \"In vivo physiological relevance not yet addressed\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Showed MRAP2 control of MC4R is isoform- and development-specific in vivo, linking receptor modulation to organismal growth and feeding.\",\n      \"evidence\": \"Zebrafish genetic model with paralog-specific MRAP2a/MRAP2b binding and signaling assays\",\n      \"pmids\": [\"23869017\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mammalian equivalence of paralog-specific effects unclear\", \"Molecular determinants of opposite paralog effects not mapped\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Placed MRAP2 within the central melanocortin pathway by showing its deletion causes obesity and downregulates PVN neuropeptide gene expression.\",\n      \"evidence\": \"Two independent Mrap2-deficient mouse lines with body composition, PVN transcriptomics, and plasma lipid analysis\",\n      \"pmids\": [\"27106110\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify which receptor(s) mediate the phenotype\", \"Causal link between transcriptomic changes and obesity not established\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Extended MRAP2 function to the ghrelin axis, demonstrating it is required in vivo for fasting and ghrelin to activate orexigenic AgRP neurons.\",\n      \"evidence\": \"Reciprocal Co-IP, in vitro signaling, and mouse KO with neuronal/behavioral readouts\",\n      \"pmids\": [\"28959025\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the signaling-bias mechanism\", \"Domain requirements not yet defined\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Demonstrated that distinct MRAP2 domains regulate distinct GPCRs, showing the protein is not a uniform modulator across its receptor repertoire.\",\n      \"evidence\": \"Trafficking and signaling assays with MRAP2 deletion/truncation mutants on PKR1 and OX1R\",\n      \"pmids\": [\"28939058\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Precise domain-receptor contacts not structurally defined\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Provided gain-of-function evidence that MRAP2 in PVN MC4R neurons potentiates melanocortin signaling and energy expenditure, complementing loss-of-function data.\",\n      \"evidence\": \"AAV-mediated MRAP2 overexpression in Mc4r-cre mice with energy homeostasis and c-Fos readouts\",\n      \"pmids\": [\"30352741\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Overexpression may not reflect physiological stoichiometry\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Connected MRAP2 to human disease, establishing loss-of-function variants as causative for monogenic hyperphagic obesity with metabolic comorbidities.\",\n      \"evidence\": \"Sequencing of 9,418 individuals with functional assessment of each variant including pancreatic islet relevance\",\n      \"pmids\": [\"31700171\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific contribution of each comorbidity not dissected\", \"Variant effects on specific receptors not individually resolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Defined the signaling-bias mechanism at GHSR1a, showing MRAP2 suppresses constitutive activity and β-arrestin recruitment while enhancing G-protein signaling via independent domains.\",\n      \"evidence\": \"cAMP, IP1, and β-arrestin recruitment assays with MRAP2 domain deletion constructs\",\n      \"pmids\": [\"31911434\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular block of β-arrestin recruitment not yet identified at residue level\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Extended MRAP2's role to the endocrine pancreas, showing it is required in δ cells for ghrelin-evoked calcium responses and insulinostasis.\",\n      \"evidence\": \"Calcium imaging, global and δ-cell-conditional MRAP2 KO mice, and insulin secretion assays\",\n      \"pmids\": [\"32535024\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct demonstration of δ-cell GHSR1a–MRAP2 complex in vivo not shown\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Established that MRAP2 forms symmetric antiparallel homodimers and that domain orientation, not merely dimer assembly, dictates regulatory output on MC4R.\",\n      \"evidence\": \"Domain inversion mutagenesis with dimerization, surface expression, and cAMP assays\",\n      \"pmids\": [\"34759891\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"No experimental structure of the dimer\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Identified the molecular events by which MRAP2 blocks β-arrestin recruitment at GHSR1a, pinpointing kinase exclusion and specific phospho-sites.\",\n      \"evidence\": \"Phosphosite mutagenesis (Ser252/Thr261), GRK2/PKC interaction assays, and β-arrestin recruitment assays\",\n      \"pmids\": [\"35605660\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the same kinase-exclusion mechanism operates at other GPCRs not tested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Tied a conserved residue (R125) to MRAP2 conformation, dimerization, and receptor binding, providing a structural rationale for human R125 obesity variants.\",\n      \"evidence\": \"Site-directed mutagenesis with dimerization and PKR2 binding assays, cross-species comparison\",\n      \"pmids\": [\"36077245\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Structural mechanism of R125 action not resolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Broadened the receptor repertoire to MCHR1, showing C-terminal-dependent inhibition of calcium signaling and trafficking.\",\n      \"evidence\": \"Co-IP, BiFC, Ca2+ signaling assays, and MRAP2 truncation mutants\",\n      \"pmids\": [\"35311242\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"No in vivo validation of MCHR1 regulation\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Revealed a trafficking-based mechanism by which MRAP2 controls MC4R, showing it is required for ciliary targeting of the receptor for long-term energy balance.\",\n      \"evidence\": \"Ciliary localization microscopy and MRAP2 KO mouse models with energy homeostasis readouts\",\n      \"pmids\": [\"36692018\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular machinery linking MRAP2 to ciliary transport not defined\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Added PKR2 to the set of receptors whose β-arrestin signaling MRAP2 inhibits.\",\n      \"evidence\": \"β-arrestin-2 recruitment assays with PKR2/MRAP2 co-expression\",\n      \"pmids\": [\"38392222\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Single method described in abstract with limited mechanistic detail\", \"No reciprocal interaction validation\", \"No in vivo support\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Unified MRAP2's mechanism across MC4R, MC3R, and GHSR, showing preferential 1:1 complexes, disruption of receptor homodimers, shared transmembrane contacts, and C-terminal modulation of constitutive activity.\",\n      \"evidence\": \"Single-molecule pull-down, stoichiometry by photobleaching, cAMP/β-arrestin assays, alanine and domain-deletion mutagenesis, and structural homology modeling across receptors\",\n      \"pmids\": [\"40998819\", \"41401256\", \"41722048\", \"39807633\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural models not experimentally validated by high-resolution structure\", \"Some studies single-lab\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Further extended the repertoire to melatonin receptors with receptor-divergent trafficking outcomes, reinforcing MRAP2 as a broad GPCR accessory protein.\",\n      \"evidence\": \"Co-IP, BiFC, GloSensor Gi signaling, and surface trafficking ELISA on MTNR1A/MTNR1B\",\n      \"pmids\": [\"40510472\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single lab\", \"Physiological relevance of melatonin receptor regulation not established\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How MRAP2 mechanistically distinguishes G-protein potentiation from β-arrestin/trafficking suppression across its diverse receptor partners, and whether a single structural mode generalizes, remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No experimentally determined high-resolution structure of any MRAP2–GPCR complex\", \"Tissue-specific receptor selectivity in vivo only partially mapped\", \"Mechanism of ciliary MC4R targeting unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 2, 4, 11, 12]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [4, 7, 10, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [14, 15]},\n      {\"term_id\": \"GO:0005929\", \"supporting_discovery_ids\": [6]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [0, 2, 4, 11, 12]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [5, 8, 18]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"MC4R\", \"MC3R\", \"MC2R\", \"GHSR1a\", \"PKR2\", \"OX1R\", \"MCHR1\", \"MTNR1A\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}