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. 2010 Jul 29;2010:baq018. doi: 10.1093/database/baq018

The systematic annotation of the three main GPCR families in Reactome

Bijay Jassal 1, Steven Jupe 1, Michael Caudy 2, Ewan Birney 1, Lincoln Stein 2, Henning Hermjakob 1, Peter D’Eustachio 3,*
PMCID: PMC2945921  PMID: 20671204

Abstract

Reactome is an open-source, freely available database of human biological pathways and processes. A major goal of our work is to provide an integrated view of cellular signalling processes that spans from ligand–receptor interactions to molecular readouts at the level of metabolic and transcriptional events. To this end, we have built the first catalogue of all human G protein-coupled receptors (GPCRs) known to bind endogenous or natural ligands. The UniProt database has records for 797 proteins classified as GPCRs and sorted into families A/1, B/2 and C/3 on the basis of amino accid sequence. To these records we have added details from the IUPHAR database and our own manual curation of relevant literature to create reactions in which 563 GPCRs bind ligands and also interact with specific G-proteins to initiate signalling cascades. We believe the remaining 234 GPCRs are true orphans. The Reactome GPCR pathway can be viewed as a detailed interactive diagram and can be exported in many forms. It provides a template for the orthology-based inference of GPCR reactions for diverse model organism species, and can be overlaid with protein–protein interaction and gene expression datasets to facilitate overrepresentation studies and other forms of pathway analysis.

Database URL: http://www.reactome.org

Introduction

G protein-coupled receptors (GPCRs), also known as 7-transmembrane (7TM) domain receptors, comprise the largest and most diverse gene super-family in humans—>1% of the total protein-coding human genome. Estimates of the exact number of GPCR genes vary but a recent phylogenetic analysis identified over 800 (1). Of these, 701 were classified within the rhodopsin family (type A) including 241 non-olfactory receptors. Many protein coding genes are alternatively spliced giving rise to isoforms so the true number of functionally unique receptors may be much higher than estimates based on gene numbers.

These GPCRs sense extracellular molecules and, through their interaction with G proteins, activate downstream signal transduction pathways. GPCRs respond to a huge range of stimuli, including light, odours, hormones, neurotransmitters and peptides (2). GPCRs represent around half of cell surface drug targets (3) and are a very successful therapeutic target family for the pharmaceutical industry accounting for the majority of best-selling drugs, ∼30% of all prescription pharmaceuticals on the market (4). The potential for further exploitation remains high, as only 10% of GPCRs are targeted by these marketed drugs (5).

Reactome is a free, open-source pathways database. Information in Reactome is captured by expert curators and peer-reviewed by experts in their fields of biology. The data is extensively cross-referenced to databases such as Ensembl [http://www.ensembl.org/index.html (6)], GO [http://www.ebi.ac.uk/QuickGO/ (7)], PubMed (http://www.ncbi.nlm.nih.gov/pubmed), ChEBI [http://www.ebi.ac.uk/chebi/index.jsp (8)], UniProt [http://www.uniprot.org/ (9)] and OMIM [http://www.ncbi.nlm.nih.gov/omim (10)]. Reactions for other species are inferred by orthology from curated human ones. Reactions can be viewed in the context of their pathways and interaction data can be overlaid to further expand the data richness. Tools are available in Reactome to help users with analyses such as pathway over-representation (enrichment) and pathway differential expression, and data including tables of pairwise protein–protein interactions computed from manually curated reactions and complexes can be downloaded in a range of formats.

Several resources hold rich data for GPCRs. UniProt is a comprehensive protein knowledgebase of protein sequence and functional information. IUPHAR-db (International Union of basic and clinical PHARmacology, http://www.iuphar-db.org/) is a database of receptor nomenclature and drug classification. Its GPCR section is arranged according to the sequence homology and functional similarity of these receptors. It also contains orphan GPCR lists. These resources were used as a starting point to catalogue the GPCR project in Reactome.

Materials and methods

In UniProt, a query was constructed to search for all manually annotated and reviewed human GPCRs.

Information in Reactome is annotated by database curators. These in-house experts systematically reviewed the literature for the three GPCR families. GPCRs whose ligands were identified from published experimental data were captured via the Curator Tool, an interface which allows the curator to annotate and structure data in accord with Reactome’s frame-based data model, and commit the results to a central repository (11). Data was organized into the three main GPCR families, A/1, B/2 and C/3. Within each family, details were further structured based on the type of ligand. Attributes of a reaction captured by Reactome are:

  • input molecule(s)

  • output molecule(s)

  • catalyst (where appropriate)

  • compartment where reaction takes place

  • literature reference

  • species

  • links to preceding reactions (e.g. the causal connection between ligand binding and G-protein interaction)

  • membership in a pathway

  • text summary of reaction

  • curator

  • expert reviewer

Input and output entities can be composed of proteins, simple chemicals or combinations of these entities (complexes).

Useful information captured from IUPHAR-db by the curation team included:

  • the type of ligand the receptor bound, which helped in the organization of the project in Reactome. Ligands were represented if they are generally accepted in the literature to be the endogenous or natural ones

  • literature references associated with human receptors

  • where appropriate, orthologous proteins in other species

  • the primary transduction mechanism, i.e. the G protein(s) that coupled with the receptor: ligand complex

  • receptor nomenclature

As of October 2009, there were 356 GPCRs captured by the IUPHAR database. The database also contains lists of orphans. Orphans are proteins classified as GPCR protein family members based on sequence similarity but whose endogenous ligands are unknown. These were investigated by Reactome curators to determine if recent advances assigned ligands to some of them.

Results

From UniProt, we retrieved records for the three main families of human GPCRs with the query:

family: ‘G-protein coupled receptor’ and organism:human and reviewed: yes

The query resulted in 836 protein matches. Of these, 797 proteins matched the three main families (A/1, B/2 and C/3). We then queried IUPHAR-db and searched published literature to identify ligands for these proteins, with the results

Class A/1—726 UniProt records; ligands found for 519

Class B/2—49 UniProt records; ligands found for 29

Class C/3—22 UniProt records; ligands found for 15

Tables 1. (Class A/1) and 2 (classes B/2 and C/3) show the GPCRs, their gene names and accession numbers, and the G protein with which each one interacts.

Table 1.

Family A GPCR names, accession numbers and G protein coupled (Receptor names link out to the corresponding reaction in Reactome)

GPCR family A Receptors Gene Name Acc. No. Gα subunit(s) coupled GPCR family A Receptors Gene Name Acc. No. Gα subunit(s) oupled
5-hydroxytryptamine receptor 1A HTR1A P08908 i Type-2 angiotensin II receptor (AT2) AGTR2 P50052 i, q/11
5-hydroxytryptamine receptor 1B HTR1B P28222 i Apelin receptor APLNR P35414 i
5-hydroxytryptamine receptor 1D HTR1D P28221 i B1 bradykinin receptor BDKRB1 P46663 i, q/11
5-hydroxytryptamine receptor 1E HTR1E P28566 i B2 bradykinin receptor BDKRB2 P30411 i, q/11
5-hydroxytryptamine receptor 1F HTR1F P30939 i C5a anaphylatoxin chemotactic receptor C5AR1 P21730 i
5-hydroxytryptamine receptor 2A HTR2A P28223 q/11 Cholecystokinin receptor type A CCKAR P32238 q/11
5-hydroxytryptamine receptor 2B HTR2B P41595 q/11 C-C chemokine receptor type 10 CCR10 P46092 i
5-hydroxytryptamine receptor 2C HTR2C P28335 q/11 C-C chemokine receptor type 1 CCR1 P32246 i
5-hydroxytryptamine receptor 4 HTR4 Q13639 s C-C chemokine receptor type 2 CCR2 P41597 i
5-hydroxytryptamine receptor 5A HTR5A P47898 i C-C chemokine receptor type 3 CCR3 P51677 i
5-hydroxytryptamine receptor 6 HTR6 P50406 s C-C chemokine receptor type 4 CCR4 P51679 i
5-hydroxytryptamine receptor HTR7 P34969 s C-C chemokine receptor type 5 CCR5 P51681 i
Adenosine receptor A1 ADORA1 P30542 i C-C chemokine receptor type 6 CCR6 P51684 i
Adenosine receptor A2a ADORA2A P29274 s C-C chemokine receptor type 7 CCR7 P32248 i
Adenosine receptor A2b ADORA2B P29275 s C-C chemokine receptor type 8 CCR8 P51685 i
Adenosine A3 receptor ADORA3 P33765 i C-C chemokine receptor type 9 CCR9 P51686 i
Muscarinic acetylcholine receptor M1 CHRM1 P11229 q/11 Cysteinyl leukotriene receptor 1 CYSLTR1 Q9Y271 q/11
Muscarinic acetylcholine receptor M2 CHRM2 P08172 i Cysteinyl leukotriene receptor 2 CYSLTR2 Q9NS75 q/11
Muscarinic acetylcholine receptor M3 CHRM3 P20309 q/11 Cannabinoid receptor 1 CNR1 P21554 i
Muscarinic acetylcholine receptor M4 CHRM4 P08173 i Cannabinoid receptor 2 CNR2 P34972 i
Muscarinic acetylcholine receptor M5 CHRM5 P08912 q/11 CX3C chemokine receptor 1 CX3CR1 P49238 unknown
Adrenocorticotropic hormone receptor MC2R Q01718 s High affinity interleukin-8 receptor A IL8RA P25024 i
α-1A adrenergic receptor ADRA1A P35348 q/11 High affinity interleukin-8 receptor B IL8RB P25025 i
α-1B adrenergic receptor ADRA1B P35368 q/11 C-X-C chemokine receptor type 3 CXCR3 P49682 i
α-1D adrenergic receptor ADRA1D P25100 q/11 C-X-C chemokine receptor type 4 CXCR4 P61073 i
α-2A adrenergic receptor ADRA2A P08913 i C-X-C chemokine receptor type 6 CXCR6 O00574 i
α-2B adrenergic receptor ADRA2B P18089 i C-X-C chemokine receptor type 7 CXCR7 P25106 i
α-2C adrenergic receptor ADRA2C P18825 i D(1A) dopamine receptor DRD1 P21728 s
β-1 adrenergic receptor ADRB1 P08588 s D(2) dopamine receptor DRD2 P14416 i
β-2 adrenergic receptor ADRB2 P07550 s D(3) dopamine receptor DRD3 P35462 i
β-3 adrenergic receptor ADRB3 P13945 s D(4) dopamine receptor DRD4 P21917 i
Type-1 angiotensin II receptor AGTR1 P30556 q/11 D(1B) dopamine receptor DRD5 P21918 s
Duffy antigen/chemokine receptor DARC Q16570 unknown Melanocyte-stimulating hormone receptor MC1R Q01726 s
Endothelin-1 receptor EDNRA P25101 q/11 Melatonin receptor type 1A MTNR1A P48039 i
Endothelin B receptor EDNRB P24530 q/11 Melatonin receptor type 1B MTNR1B P49286 i
N-formyl peptide receptor 2 FPR2 P25090 q/11 Substance-P receptor TACR1 P25103 q/11
Follicle-stimulating hormone receptor FSHR P23945 s Substance-K receptor TACR2 P21452 q/11
Galanin receptor type 1 GALR1 P47211 i Neuromedin-K receptor TACR3 P29371 q/11
Galanin receptor type 2 GALR2 O43603 i Neuromedin-B receptor NMBR P28336 q/11
Galanin receptor type 3 GALR3 O60755 i Neuropeptides B/W receptor type 1 NPBWR1 P48145 i
Gastrin/cholecystokinin type B receptor CCKBR P32239 q/11 Neuropeptides B/W receptor type 2 NPBWR2 P48146 i
Gonadotropin-releasing hormone receptor GNRHR P30968 q/11 Neuropeptide FF receptor 1 NPFFR1 Q9GZQ6 q/11
Putative gonadotropin-releasing hormone II receptor GNRHR2 Q96P88 q/11 Neuropeptide FF receptor 2 NPFFR2 Q9Y5X5 q/11
G-protein coupled oestrogen receptor 1 GPER Q99527 i Neuropeptide Y receptor type 1 NPY1R P25929 i
Uracil nucleotide/cysteinyl leukotriene receptor GPR17 Q13304 i, q/11 Neuropeptide Y receptor type 2 NPY2R P49146 i
Putative G-protein coupled receptor 44 GPR44 Q9Y5Y4 i Neuropeptide Y receptor type 4 PPYR1 P50391 i
G-protein coupled receptor 55 GPR55 Q9Y2T6 i Neuropeptide Y receptor type 5 NPY5R Q15761 i
Gastrin-releasing peptide receptor GRPR P30550 q/11 Neurotensin receptor type 1 NTSR1 P30989 q/11
Histamine H1 receptor HRH1 P35367 q/11 Neurotensin receptor type 2 NTSR2 O95665 q/11
Histamine H2 receptor HRH2 P25021 s Olfactory receptor 10A2 OR10A2 Q9H208 olf
Histamine H3 receptor HRH3 Q9Y5N1 i Olfactory receptor 10A3 OR10A3 P58181 olf
Histamine H4 receptor HRH4 Q9H3N8 i Olfactory receptor 10A4 OR10A4 Q9H209 olf
KiSS-1 receptor KISS1R Q969F8 q/11 Olfactory receptor 10A5 OR10A5 Q9H207 olf
Lysophosphatidic acid receptor 1 LPAR1 Q92633 i, q/11 Olfactory receptor 10A6 OR10A6 Q8NH74 olf
Lysophosphatidic acid receptor 2 LPAR2 Q9HBW0 i, q/11 Olfactory receptor 10A7 OR10A7 Q8NGE5 olf
Lysophosphatidic acid receptor 3 LPAR3 Q9UBY5 i, q/11 Olfactory receptor 10AD1 OR10AD1 Q8NGE0 olf
Lysophosphatidic acid receptor 4 LPAR4 Q99677 q/11 Olfactory receptor 10AG1 OR10AG1 Q8NH19 olf
Lysophosphatidic acid receptor 6 LPAR6 P43657 q/11 Olfactory receptor 10C1 OR10C1 Q96KK4 olf
Lutropin-choriogonadotropic hormone receptor LHCGR P22888 s Olfactory receptor 10G2 OR10G2 Q8NGC3 olf
Leukotriene B4 receptor 1 LTB4R Q15722 q/11 Olfactory receptor 10G3 OR10G3 Q8NGC4 olf
Leukotriene B4 receptor 2 LTB4R2 Q9NPC1 q/11 Olfactory receptor 10G4 OR10G4 Q8NGN3 olf
Melanocortin receptor 3 MC3R P41968 s Olfactory receptor 10G6 OR10G6 Q8NH81 olf
Melanocortin receptor 4 MC4R P32245 s Olfactory receptor 10G7 OR10G7 Q8NGN6 olf
Melanocortin receptor 5 MC5R P33032 s Olfactory receptor 10G8 OR10G8 Q8NGN5 olf
Olfactory receptor 10G9 OR10G9 Q8NGN4 olf Olfactory receptor 2T10 OR2T10 Q8NGZ9 olf
Olfactory receptor 10H1 OR10H1 Q9Y4A9 olf Olfactory receptor 2T11 OR2T11 Q8NH01 olf
Olfactory receptor 10H2 OR10H2 O60403 olf Olfactory receptor 2T12 OR2T12 Q8NG77 olf
Olfactory receptor 10H3 OR10H3 O60404 olf Olfactory receptor 2T27 OR2T27 Q8NH04 olf
Olfactory receptor 10H4 OR10H4 Q8NGA5 olf Olfactory receptor 2T29 OR2T29 Q8NH02 olf
Olfactory receptor 10H5 OR10H5 Q8NGA6 olf Olfactory receptor 2T33 OR2T33 Q8NG76 olf
Olfactory receptor 10J1 OR10J1 P30954 olf Olfactory receptor 2T34 OR2T34 Q8NGX1 olf
Olfactory receptor 10J3 OR10J3 Q5JRS4 olf Olfactory receptor 2T35 OR2T35 Q8NGX2 olf
Olfactory receptor 10J5 OR10J5 Q8NHC4 olf Olfactory receptor 4A15 OR4A15 Q8NGL6 olf
Olfactory receptor 10K1 OR10K1 Q8NGX5 olf Olfactory receptor 4A16 OR4A16 Q8NH70 olf
Olfactory receptor 10K2 OR10K2 Q6IF99 olf Olfactory receptor 4A47 OR4A47 Q6IF82 olf
Olfactory receptor 10P1 OR10P1 Q8NGE3 olf Olfactory receptor 4C45 OR4C45 A6NMZ5 olf
Olfactory receptor 10Q1 OR10Q1 Q8NGQ4 olf Olfactory receptor 4C46 OR4C46 A6NHA9 olf
Olfactory receptor 10R2 OR10R2 Q8NGX6 olf Olfactory receptor 4F15 OR4F15 Q8NGB8 olf
Olfactory receptor 10S1 OR10S1 Q8NGN2 olf Olfactory receptor 4F17 OR4F17 Q8NGA8 olf
Olfactory receptor 10T2 OR10T2 Q8NGX3 olf Olfactory receptor 4F21 OR4F21 O95013 olf
Olfactory receptor 10V1 OR10V1 Q8NGI7 olf Olfactory receptor 51A2 OR51A2 Q8NGJ7 olf
Olfactory receptor 10W1 OR10W1 Q8NGF6 olf Olfactory receptor 51A4 OR51A4 Q8NGJ6 olf
Olfactory receptor 14A2 OR14A2 Q96R54 olf Olfactory receptor 51A7 OR51A7 Q8NH64 olf
Olfactory receptor 14C36 OR14C36 Q8NHC7 olf Olfactory receptor 51B2 OR51B2 Q9Y5P1 olf
Olfactory receptor 14I1 OR14I1 A6ND48 olf Olfactory receptor 51B4 OR51B4 Q9Y5P0 olf
Olfactory receptor 14J1 OR14J1 Q9UGF5 olf Olfactory receptor 51B5 OR51B5 Q9H339 olf
Olfactory receptor 14K1 OR14K1 Q8NGZ2 olf Olfactory receptor 51B5 OR51B6 Q9H340 olf
Olfactory receptor 2A12 OR2A12 Q8NGT7 olf Olfactory receptor 51D1 OR51D1 Q8NGF3 olf
Olfactory receptor 2A14 OR2A14 Q96R47 olf Olfactory receptor 51E1 OR51E1 Q8TCB6 olf
Olfactory receptor 2A25 OR2A25 A4D2G3 olf Olfactory receptor 51E2 OR51E2 Q9H255 olf
Olfactory receptor 2AG1 OR2AG1 Q9H205 olf Olfactory receptor 51F1 OR51F1 A6NGY5 olf
Olfactory receptor 2AG2 OR2AG2 A6NM03 olf Olfactory receptor 51F2 OR51F2 Q8NH61 olf
Olfactory receptor 2AJ1 OR2AJ1 Q8NGZ0 olf Olfactory receptor 51G1 OR51G1 Q8NGK1 olf
Olfactory receptor 2AK2 OR2AK2 Q8NG84 olf Olfactory receptor 51G2 OR51G2 Q8NGK0 olf
Olfactory receptor 2AP1 OR2AP1 Q8NGE2 olf Putative olfactory receptor 51H1 OR51H1P Q8NH63 olf
Olfactory receptor 2AT4 OR2AT4 A6NND4 olf Olfactory receptor 51I1 OR51I1 Q9H343 olf
Olfactory receptor 51I2 OR51I2 Q9H344 olf Olfactory receptor 56A1 OR56A1 Q8NGH5 olf
Olfactory receptor 51J1 OR51J1 Q9H342 olf Olfactory receptor 56A3 OR56A3 Q8NH54 olf
Olfactory receptor 51L1 OR51L1 Q8NGJ5 olf Olfactory receptor 56A4 OR56A4 Q8NGH8 olf
Olfactory receptor 51M1 OR51M1 Q9H341 olf Olfactory receptor 56A5 OR56A5 P0C7T3 olf
Olfactory receptor 51Q1 OR51Q1 Q8NH59 olf Olfactory receptor 56B1 OR56B1 Q8NGI3 olf
Olfactory receptor 51S1 OR51S1 Q8NGJ8 olf Olfactory receptor 56B4 OR56B4 Q8NH76 olf
Olfactory receptor 51T1 OR51T1 Q8NGJ9 olf Olfactory receptor 5AC2 OR5AC2 Q9NZP5 olf
Olfactory receptor 51V1 OR51V1 Q9H2C8 olf Olfactory receptor 5AK2 OR5AK2 Q8NH90 olf
Olfactory receptor 52A1 OR52A1 Q9UKL2 olf Olfactory receptor 5AN1 OR5AN1 Q8NGI8 olf
Olfactory receptor 52A5 OR52A5 Q9H2C5 olf Olfactory receptor 5AP2 OR5AP2 Q8NGF4 olf
Olfactory receptor 52B2 OR52B2 Q96RD2 olf Olfactory receptor 5AR1 OR5AR1 Q8NGP9 olf
Olfactory receptor 52B4 OR52B4 Q8NGK2 olf Olfactory receptor 5AS1 OR5AS1 Q8N127 olf
Olfactory receptor 52B6 OR52B6 Q8NGF0 olf Olfactory receptor 5AU1 OR5AU1 Q8NGC0 olf
Olfactory receptor 52D1 OR52D1 Q9H346 olf Olfactory receptor 5H14 OR5H14 A6NHG9 olf
Olfactory receptor 52E2 OR52E2 Q8NGJ4 olf Olfactory receptor 5H15 OR5H15 A6NDH6 olf
Olfactory receptor 52E4 OR52E4 Q8NGH9 olf Olfactory receptor 6C65 OR6C65 A6NJZ3 olf
Olfactory receptor 52E5 OR52E5 Q8NH55 olf Olfactory receptor 6C68 OR6C68 A6NDL8 olf
Olfactory receptor 52E6 OR52E6 Q96RD3 olf Olfactory receptor 6C70 OR6C70 A6NIJ9 olf
Olfactory receptor 52E8 OR52E8 Q6IFG1 olf Olfactory receptor 6C74 OR6C74 A6NCV1 olf
Olfactory receptor 52H1 OR52H1 Q8NGJ2 olf Olfactory receptor 6C75 OR6C75 A6NL08 olf
Olfactory receptor 52I1 OR52I1 Q8NGK6 olf Olfactory receptor 6C76 OR6C76 A6NM76 olf
Olfactory receptor 52I2 OR52I2 Q8NH67 olf Olfactory receptor 7E24 OR7E24 Q6IFN5 olf
Olfactory receptor 52K1 OR52K1 Q8NGK4 olf Opsin-3 OPN3 Q9H1Y3 t
Olfactory receptor 52K2 OR52K2 Q8NGK3 olf Melanopsin OPN4 Q9UHM6 q/11
Olfactory receptor 52L1 OR52L1 Q8NGH7 olf Opsin-5 OPN5 Q6U736 t
Olfactory receptor 52M1 OR52M1 Q8NGK5 olf δ-type opioid receptor OPRD1 P41143 I
Olfactory receptor 52N1 OR52N1 Q8NH53 olf κ-type opioid receptor OPRK1 P41145 I
Olfactory receptor 52N2 OR52N2 Q8NGI0 olf µ-type opioid receptor OPRM1 P35372 i
Olfactory receptor 52N4 OR52N4 Q8NGI2 olf Nociceptin receptor OPRL1 P41146 i
Olfactory receptor 52N5 OR52N5 Q8NH56 olf Blue-sensitive opsin OPN1SW P03999 t
Olfactory receptor 52R1 OR52R1 Q8NGF1 olf Rhodopsin RHO P08100 t
Olfactory receptor 52W1 OR52W1 Q6IF63 olf Green-sensitive opsin OPN1MW P04001 t
Red-sensitive opsin OPN1LW P04000 t Olfactory receptor 2B11 OR2B11 Q5JQS5 olf
Visual pigment-like receptor peropsin RRH O14718 t Olfactory receptor 2C1 OR2C1 O95371 olf
Olfactory receptor 1A1 OR1A1 Q9P1Q5 olf Olfactory receptor 2C3 OR2C3 Q8N628 olf
Olfactory receptor 1A2 OR1A2 Q9Y585 olf Olfactory receptor 2D2 OR2D2 Q9H210 olf
Olfactory receptor 1B1 OR1B1 Q8NGR6 olf Olfactory receptor 2D3 OR2D3 Q8NGH3 olf
Olfactory receptor 1C1 OR1C1 Q15619 olf Olfactory receptor 2F1 OR2F1 Q13607 olf
Olfactory receptor 1D2 OR1D2 P34982 olf Olfactory receptor 2F2 OR2F2 O95006 olf
Olfactory receptor 1F1 OR1F1 O43749 olf Olfactory receptor 2G2 OR2G2 Q8NGZ5 olf
Olfactory receptor 1F12 OR1F12 Q8NHA8 olf Olfactory receptor 2G3 OR2G3 Q8NGZ4 olf
Olfactory receptor 1G1 OR1G1 P47890 olf Olfactory receptor 2G6 OR2G6 Q5TZ20 olf
Olfactory receptor 1I1 OR1I1 O60431 olf Olfactory receptor 2H1 OR2H1 Q9GZK4 olf
Olfactory receptor 1J1 OR1J1 Q8NGS3 olf Olfactory receptor 2H2 OR2H2 O95918 olf
Olfactory receptor 1J2 OR1J2 Q8NGS2 olf Putative olfactory receptor 2I1 OR2I1P Q8NGU4 olf
Olfactory receptor 1J4 OR1J4 Q8NGS1 olf Olfactory receptor 2J1 OR2J1 Q9GZK6 olf
Olfactory receptor 1K1 OR1K1 Q8NGR3 olf Olfactory receptor 2J2 OR2J2 O76002 olf
Olfactory receptor 1L1 OR1L1 Q8NH94 olf Olfactory receptor 2J3 OR2J3 O76001 olf
Olfactory receptor 1L3 OR1L3 Q8NH93 olf Olfactory receptor 2K2 OR2K2 Q8NGT1 olf
Olfactory receptor 1L4 OR1L4 Q8NGR5 olf Olfactory receptor 2L2 OR2L2 Q8NH16 olf
Olfactory receptor 1L6 OR1L6 Q8NGR2 olf Olfactory receptor 2L3 OR2L3 Q8NG85 olf
Olfactory receptor 1L8 OR1L8 Q8NGR8 olf Olfactory receptor 2L5 OR2L5 Q8NG80 olf
Olfactory receptor 1M1 OR1M1 Q8NGA1 olf Olfactory receptor 2L8 OR2L8 Q8NGY9 olf
Olfactory receptor 1N1 OR1N1 Q8NGS0 olf Olfactory receptor 2L13 OR2L13 Q8N349 olf
Olfactory receptor 1N2 OR1N2 Q8NGR9 olf Olfactory receptor 2M2 OR2M2 Q96R28 olf
Olfactory receptor 1Q1 OR1Q1 Q15612 olf Olfactory receptor 2M3 OR2M3 Q8NG83 olf
Olfactory receptor 1S1 OR1S1 Q8NH92 olf Olfactory receptor 2M4 OR2M4 Q96R27 olf
Olfactory receptor 1S2 OR1S2 Q8NGQ3 olf Olfactory receptor 2M5 OR2M5 A3KFT3 olf
Olfactory receptor 2A2 OR2A2 Q6IF42 olf Olfactory receptor 2M7 OR2M7 Q8NG81 olf
Olfactory receptor 2A4 OR2A4 O95047 olf Olfactory receptor 2S2 OR2S2 Q9NQN1 olf
Olfactory receptor 2B2 OR2B2 Q9GZK3 olf Olfactory receptor 2T1 OR2T1 O43869 olf
Putative olfactory receptor 2B3 OR2B3 O76000 olf Olfactory receptor 2T2 OR2T2 Q6IF00 olf
Olfactory receptor 2B6 OR2B6 P58173 olf Olfactory receptor 2T3 OR2T3 Q8NH03 olf
Putative olfactory receptor 2B8 OR2B8P P59922 olf Olfactory receptor 2T4 OR2T4 Q8NH00 olf
Olfactory receptor 2T5 OR2T5 Q6IEZ7 olf Olfactory receptor 4E1 OR4E1 P0C645 olf
Olfactory receptor 2T6 OR2T6 Q8NHC8 olf Olfactory receptor 4E2 OR4E2 Q8NGC2 olf
Olfactory receptor 2T8 OR2T8 A6NH00 olf Olfactory receptor 4F3/4F16/4F29 OR4F3 Q6IEY1 olf
Olfactory receptor 2V1 OR2V1 Q8NHB1 olf Olfactory receptor 4F4 OR4F4 Q96R69 olf
Olfactory receptor 2V2 OR2V2 Q96R30 olf Olfactory receptor 4F5 OR4F5 Q8NH21 olf
Olfactory receptor 2W1 OR2W1 Q9Y3N9 olf Olfactory receptor 4F6 OR4F6 Q8NGB9 olf
Olfactory receptor 2W3 OR2W3 Q7Z3T1 olf Olfactory receptor 4K1 OR4K1 Q8NGD4 olf
Olfactory receptor 2Y1 OR2Y1 Q8NGV0 olf Olfactory receptor 4K2 OR4K2 Q8NGD2 olf
Olfactory receptor 2Z1 OR2Z1 Q8NG97 olf Olfactory receptor 4K3 OR4K3 Q96R72 olf
Olfactory receptor 3A1 OR3A1 P47881 olf Olfactory receptor 4K5 OR4K5 Q8NGD3 olf
Olfactory receptor 3A2 OR3A2 P47893 olf Olfactory receptor 4K13 OR4K13 Q8NH42 olf
Olfactory receptor 3A3 OR3A3 P47888 olf Olfactory receptor 4K14 OR4K14 Q8NGD5 olf
Olfactory receptor 3A4 OR3A4 P47883 olf Olfactory receptor 4K15 OR4K15 Q8NH41 olf
Putative olfactory receptor 4A4 OR4A4P Q8NGN8 olf Olfactory receptor 4K17 OR4K17 Q8NGC6 olf
Olfactory receptor 4A5 OR4A5 Q8NH83 olf Olfactory receptor 4L1 OR4L1 Q8NH43 olf
Olfactory receptor 4A8 OR4A8P P0C604 olf Olfactory receptor 4M1 OR4M1 Q8NGD0 olf
Olfactory receptor 4B1 OR4B1 Q8NGF8 olf Olfactory receptor 4M2 OR4M2 Q8NGB6 olf
Olfactory receptor 4C3 OR4C3 Q8NH37 olf Olfactory receptor 4N2 OR4N2 Q8NGD1 olf
Olfactory receptor 4C5 OR4C5 Q8NGB2 olf Olfactory receptor 4N4 OR4N4 Q8N0Y3 olf
Olfactory receptor 4C6 OR4C6 Q8NH72 olf Olfactory receptor 4N5 OR4N5 Q8IXE1 olf
Olfactory receptor 4C11 OR4C11 Q6IEV9 olf Olfactory receptor 4P4 OR4P4 Q8NGL7 olf
Olfactory receptor 4C12 OR4C12 Q96R67 olf Olfactory receptor 4Q2 OR4Q2 P0C623 olf
Olfactory receptor 4C13 OR4C13 Q8NGP0 olf Olfactory receptor 4Q3 OR4Q3 Q8NH05 olf
Olfactory receptor 4C15 OR4C15 Q8NGM1 olf Olfactory receptor 4S1 OR4S1 Q8NGB4 olf
Olfactory receptor 4C16 OR4C16 Q8NGL9 olf Olfactory receptor 4S2 OR4S2 Q8NH73 olf
Olfactory receptor 4D1 OR4D1 Q15615 olf Olfactory receptor 4X1 OR4X1 Q8NH49 olf
Olfactory receptor 4D2 OR4D2 P58180 olf Olfactory receptor 4X2 OR4X2 Q8NGF9 olf
Olfactory receptor 4D5 OR4D5 Q8NGN0 olf Olfactory receptor 5A1 OR5A1 Q8NGJ0 olf
Olfactory receptor 4D6 OR4D6 Q8NGJ1 olf Olfactory receptor 5A2 OR5A2 Q8NGI9 olf
Olfactory receptor 4D9 OR4D9 Q8NGE8 olf Olfactory receptor 5B2 OR5B2 Q96R09 olf
Olfactory receptor 4D10 OR4D10 Q8NGI6 olf Olfactory receptor 5B3 OR5B3 Q8NH48 olf
Olfactory receptor 4D11 OR4D11 Q8NGI4 olf Olfactory receptor 5B12 OR5B12 Q96R08 olf
Olfactory receptor 5B17 OR5B17 Q8NGF7 olf Olfactory receptor 6A2 OR6A2 O95222 olf
Olfactory receptor 5B21 OR5B21 A6NL26 olf Olfactory receptor 6B1 OR6B1 O95007 olf
Olfactory receptor 5C1 OR5C1 Q8NGR4 olf Olfactory receptor 6B2 OR6B2 Q6IFH4 olf
Olfactory receptor 5D13 OR5D13 Q8NGL4 olf Olfactory receptor 6B3 OR6B3 Q8NGW1 olf
Olfactory receptor 5D14 OR5D14 Q8NGL3 olf Olfactory receptor 6C1 OR6C1 Q96RD1 olf
Olfactory receptor 5D16 OR5D16 Q8NGK9 olf Olfactory receptor 6C2 OR6C2 Q9NZP2 olf
Olfactory receptor 5D18 OR5D18 Q8NGL1 olf Olfactory receptor 6C3 OR6C3 Q9NZP0 olf
Olfactory receptor 5F1 OR5F1 O95221 olf Olfactory receptor 6C4 OR6C4 Q8NGE1 olf
Olfactory receptor 5H1 OR5H1 A6NKK0 olf Olfactory receptor 6C6 OR6C6 A6NF89 olf
Olfactory receptor 5H2 OR5H2 Q8NGV7 olf Olfactory receptor 6F1 OR6F1 Q8NGZ6 olf
Olfactory receptor 5H6 OR5H6 Q8NGV6 olf Olfactory receptor 6J1 OR6J1 Q8NGC5 olf
Olfactory receptor 5I1 OR5I1 Q13606 olf Olfactory receptor 6K2 OR6K2 Q8NGY2 olf
Olfactory receptor 5J2 OR5J2 Q8NH18 olf Olfactory receptor 6K3 OR6K3 Q8NGY3 olf
Olfactory receptor 5K1 OR5K1 Q8NHB7 olf Olfactory receptor 6K6 OR6K6 Q8NGW6 olf
Olfactory receptor 5K2 OR5K2 Q8NHB8 olf Olfactory receptor 6M1 OR6M1 Q8NGM8 olf
Olfactory receptor 5K3 OR5K3 A6NET4 olf Olfactory receptor 6N1 OR6N1 Q8NGY5 olf
Olfactory receptor 5K4 OR5K4 A6NMS3 olf Olfactory receptor 6N2 OR6N2 Q8NGY6 olf
Olfactory receptor 5L1 OR5L1 Q8NGL2 olf Olfactory receptor 6P1 OR6P1 Q8NGX9 olf
Olfactory receptor 5L2 OR5L2 Q8NGL0 olf Olfactory receptor 6Q1 OR6Q1 Q8NGQ2 olf
Olfactory receptor 5M1 OR5M1 Q8NGP8 olf Olfactory receptor 6S1 OR6S1 Q8NH40 olf
Olfactory receptor 5M3 OR5M3 Q8NGP4 olf Olfactory receptor 6T1 OR6T1 Q8NGN1 olf
Olfactory receptor 5M8 OR5M8 Q8NGP6 olf Olfactory receptor 6V1 OR6V1 Q8N148 olf
Olfactory receptor 5M9 OR5M9 Q8NGP3 olf Olfactory receptor 6X1 OR6X1 Q8NH79 olf
Olfactory receptor 5M10 OR5M10 Q6IEU7 olf Olfactory receptor 6Y1 OR6Y1 Q8NGX8 olf
Olfactory receptor 5M11 OR5M11 Q96RB7 olf Olfactory receptor 7A5 OR7A5 Q15622 olf
Olfactory receptor 5P2 OR5P2 Q8WZ92 olf Olfactory receptor 7A10 OR7A10 O76100 olf
Olfactory receptor 5P3 OR5P3 Q8WZ94 olf Olfactory receptor 7A17 OR7A17 O14581 olf
Olfactory receptor 5R1 OR5R1 Q8NH85 olf Olfactory receptor 7C1 OR7C1 O76099 olf
Olfactory receptor 5T1 OR5T1 Q8NG75 olf Olfactory receptor 7C2 OR7C2 O60412 olf
Olfactory receptor 5T2 OR5T2 Q8NGG2 olf Olfactory receptor 7D4 OR7D4 Q8NG98 olf
Olfactory receptor 5T3 OR5T3 Q8NGG3 olf Olfactory receptor 7G1 OR7G1 Q8NGA0 olf
Olfactory receptor 5W2 OR5W2 Q8NH69 olf Olfactory receptor 7G2 OR7G2 Q8NG99 olf
Olfactory receptor 7G3 OR7G3 Q8NG95 olf Prostaglandin F2-α receptor PTGFR P43088 q/11
Olfactory receptor 8A1 OR8A1 Q8NGG7 olf Prostacyclin receptor PTGIR P43119 s
Olfactory receptor 8B3 OR8B3 Q8NGG8 olf Prolactin-releasing peptide receptor PRLHR P49683 unknown
Olfactory receptor 8B4 OR8B4 Q96RC9 olf Platelet-activating factor receptor PTAFR P25105 q/11
Olfactory receptor 8B8 OR8B8 Q15620 olf Pyroglutamylated RFamide peptide receptor QRFPR Q96P65 q/11
Olfactory receptor 8B12 OR8B12 Q8NGG6 olf RPE-retinal G protein-coupled receptor RGR P47804 t
Olfactory receptor 8D1 OR8D1 Q8WZ84 olf Sphingosine 1-phosphate receptor 1 S1PR1 P21453 i
Olfactory receptor 8D2 OR8D2 Q9GZM6 olf Sphingosine 1-phosphate receptor 2 S1PR2 O95136 i, q/11
Olfactory receptor 8D4 OR8D4 Q8NGM9 olf Sphingosine 1-phosphate receptor 3 S1PR3 Q99500 i, q/11, 12/13
Orexin receptor type 1 HCRTR1 O43613 q/11 Sphingosine 1-phosphate receptor 4 S1PR4 O95977 i
Orexin receptor type 2 HCRTR2 O43614 q/11 Sphingosine 1-phosphate receptor 5 S1PR5 Q9H228 i
Oxoeicosanoid receptor 1 OXER1 Q8TDS5 i Somatostatin receptor type 1 SSTR1 P30872 i
Oxytocin receptor OXTR P30559 q/11 Somatostatin receptor type 2 SSTR2 P30874 i
P2Y purinoceptor 1 P2RY1 P47900 q/11 Somatostatin receptor type 3 SSTR3 P32745 i
P2Y purinoceptor 2 P2RY2 P41231 q/11 Somatostatin receptor type 4 SSTR4 P31391 i
P2Y purinoceptor 4 P2RY4 P51582 i Somatostatin receptor type 5 SSTR5 P35346 i
P2Y purinoceptor 6 P2RY6 Q15077 q/11 Thromboxane A2 receptor TBXA2R P21731 q/11, 13
P2Y purinoceptor 8 P2RY8 Q86VZ1 unknown Trace amine-associated receptor 1 TAAR1 Q96RJ0 s
Putative P2Y purinoceptor 10 P2RY10 O00398 q/11 Trace amine-associated receptor 2 TAAR2 Q9P1P5 s
P2Y purinoceptor 11 P2RY11 Q96G91 s, q/11 Putative trace amine-associated receptor 3 TAAR3 Q9P1P4 s
P2Y purinoceptor 12 P2RY12 Q9H244 i Trace amine-associated receptor 5 TAAR5 O14804 s
P2Y purinoceptor 13 P2RY13 Q9BPV8 i Trace amine-associated receptor 6 TAAR6 Q96RI8 s
P2Y purinoceptor 14 P2RY14 Q15391 i Trace amine-associated receptor 8 TAAR8 Q969N4 s
Proteinase-activated receptor 1 F2R P25116 q/11 Trace amine-associated receptor 9 TAAR9 Q96RI9 s
Proteinase-activated receptor 2 F2RL1 P55085 q/11 Thyrotropin receptor TSHR P16473 s
Proteinase-activated receptor 3 F2RL2 O00254 q/11 Vasopressin V1a receptor AVPR1A P37288 q/11
Proteinase-activated receptor 4 F2RL3 Q96RI0 q/11 Vasopressin V1b receptor AVPR1B P47901 q/11
Prostaglandin D2 receptor PTGDR Q13258 s Vasopressin V2 receptor AVPR2 P30518 s
Prostaglandin E2 receptor EP1 subtype PTGER1 P34995 q/11 Chemokine XC receptor 1 XCR1 P46094 q/11
Prostaglandin E2 receptor EP2 subtype PTGER2 P43116 s
Prostaglandin E2 receptor EP3 subtype PTGER3 P43115 i
Prostaglandin E2 receptor EP4 subtype PTGER4 P35408 s
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Table 2.

Family B and C GPCR names, accession numbers and G protein coupled (GPCR names link out to the corresponding Reactome reaction)

GPCR family 2/B Receptors Gene Name Acc. No. Gα subunit(s) coupled GPCR family 3/C Receptors Gene Name Acc. No. Gα subunit(s) coupled
Brain-specific angiogenesis inhibitor 1 BAI1 O14514 unknown Extracellular calcium-sensing receptor CASR P41180 i, q/11
Brain-specific angiogenesis inhibitor 2 BAI2 O60241 unknown GABA type B receptor 1subunit 1 GABBR1 Q9UBS5 i
Brain-specific angiogenesis inhibitor 3 BAI3 O60242 unknown GABA type B receptor subunit 2 GABBR2 O75899 i
Calcitonin receptor CALCR P30988 s GPCR family C group 6 member A GPRC6A Q5T6X5 q/11
Calcitonin gene-related peptide type 1 receptor CALCRL Q16602 s Metabotropic glutamate receptor 1 GRM1 Q13255 q/11
Corticotropin-releasing factor receptor 1 CRHR1 P34998 s Metabotropic glutamate receptor 2 GRM2 Q14416 i
Corticotropin-releasing factor receptor 2 CRHR2 Q13324 s Metabotropic glutamate receptor 3 GRM3 Q14832 i
Growth hormone-releasing hormone receptor GHRHR Q02643 s Metabotropic glutamate receptor 4 GRM4 Q14833 i
Gastric inhibitory polypeptide receptor GIPR P48546 s Metabotropic glutamate receptor 5 GRM5 P41594 q/11
Glucagon-like peptide 1 receptor GLP1R P43220 s Metabotropic glutamate receptor 6 GRM6 O15303 i
Glucagon-like peptide 2 receptor GLP2R O95838 s Metabotropic glutamate receptor 7 GRM7 Q14831 i
Glucagon receptor GCGR P47871 q/11, s Metabotropic glutamate receptor 8 GRM8 O00222 i
Pituitary adenylate cyclase-activating polypeptide type I receptor ADCYAP1R1 P41586 s Taste receptor 1 member 1 TAS1R1 Q7RTX1 unknown
Taste receptor 1 member 2 TAS1R2 Q8TE23 unknown
Parathyroid hormone receptor 1 PTH1R Q03431 s Taste receptor 1 member 3 TAS1R3 Q7RTX0 unknown
Parathyroid hormone 2 receptor PTH2R P49190 s
Secretin receptor SCTR P47872 s
Vasoactive intestinal polypeptide receptor 1 VIPR1 P32241 s
Vasoactive intestinal polypeptide receptor 2 VIPR2 P41587 s
Frizzled-10 FZD10 Q9ULW2 i
Frizzled-1 FZD1 Q9UP38 i
Frizzled-2 FZD2 Q14332 i
Frizzled-3 FZD3 Q9NPG1 i
Frizzled-4 FZD4 Q9ULV1 i
Frizzled-5 FZD5 Q13467 i
Frizzled-6 FZD6 O60353 i
Frizzled-7 FZD7 O75084 i
Frizzled-8 FZD8 Q9H461 i
Frizzled-9 FZD9 (FZD3) O00144 i
Smoothened homologue SMO (SMOH) Q99835 i
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Of 797 GPCRs in families A/1–C/3 screened from UniProt, we were able to catalogue 563 GPCRs that have ligands (70%), supported by information from IUPHAR and appropriate literature references.

We believe the remainder (234) to be true orphans i.e. no credible endogenous ligands have been determined for these receptors.

The project can be viewed here:

http://www.reactome.org/cgi-bin/eventbrowser?DB=gk_current&ID=372790

An on-line description of the features of these web pages and of additional pathway analysis tools that can be applied to the data is available here:

http://www.reactome.org/userguide/Usersguide.html

For each class, there are further subdivisions of the hierarchy, organized into ligand types that bind particular GPCRs.

Discussion

First and foremost, this addition to Reactome provides a computationally-accessible resource for information about ligand-binding GPCRs. The three main families in human are annotated, together with downstream signalling events mediated by coupling to the appropriate G protein. Each receptor protein record has multiple link-outs to key databases related to sequence, genetic disorders, ontology and literature, further enriching the information a user can view. These annotations of GPCRs by protein family complement the extensive annotation by ligand specificity previously compiled by Alexander and colleagues (12).

A total of 563 ligand-binding GPCRs were identified and included in Reactome; an additional 234 with no identifiable ligand were not. Notably, we included a set of GPCRs thought to function as olfactory receptors. In many cases, these GPCRs have been identified and classified based on their interaction when expressed in a model cultured cell with members of a small set of standard test odorant molecules. These studies are generally accepted as establishing the olfactory receptor function of these GPCRs, albeit without identifying the odorant molecule(s) with which they interact under physiological conditions (13).

Though the absence of any identified ligand presents problems for the pharmaceutical industry and for researchers wishing to study a receptor using a tool agonist, orphan receptors can be of interest when linked to a particular subcellular location and/or physiological process. For instance, the predominantly dorsal root ganglion expressed MRGX family of receptors have been extensively studied because of their narrow and therapeutically interesting expression profile (14). The pathway contexts provided by Reactome annotation provide an additional functional grouping that may be useful in generating testable hypotheses about roles of orphan GPCRs.

Orphan GPCRs have been the subject of intensive research including ligand screening by pharmaceutical companies for many years (15,16), so why do so many GPCRs have no identified ligand? There are several reasons why the endogenous ligands are still undetermined for some orphan GPCRs:

  • They may be accessory proteins that enable another GPCR to function. The classic example of this is the GABAB receptor, which functions as an obligate dimer (17). Dimerization may be the minimum functional state of GPCRs (18) and heterodimers may be common. Some orphan receptors may only be functional when they form a heterodimer. This adds a level of complexity that has not been systematically addressed by ligand screening because of the difficulties inherent in simultaneously expressing two receptors.

  • They themselves may need an as yet unknown accessory protein to respond to their ligand. The prime example of this is the amylin (AMY) receptor which is a dimer composed of a calcitonin receptor and one of three RAMPs to create AMY1-3 receptors (19,20). Other examples of accessory proteins that modify receptor surface expression include the REEP and RTPs (21).

  • They may not signal via G proteins i.e. they may have some other downstream effector. There are numerous examples of GPCR signalling that is independent of G proteins (22); a receptor that only signals via these mechanisms is unlikely to be identified by a ligand screening exercise using any of the established technologies.

  • They may require the presence of an accessory protein for successful signalling (23) that is not present in the assay systems routinely used for ligand identification.

  • Ligand screening platforms used to date are unable to detect G12/13 coupled receptors, indeed G12/13 signalling is rarely measured directly, but inferred from remote downstream events (24). The G12/13 signalling route has only recently been studied in any detail (25) so perhaps a portion of orphans fall into this category. This point could be extended to suggest that none of the ligand screening technologies are optimal for every receptor subtype. It could be that some ligand–receptor interactions have been missed because the signal was not detectable using the technology available. Newly emerging technologies such as Surface Plasmon Resonance may help to address this (26).

Overlaying protein–protein interaction data e.g. from IntAct on the curated Reactome GPCR dataset may provide a powerful approach for identifying candidate heterodimer partners and their potential functions and thus provides a novel tool for the study of orphan receptors (27). Overlaying protein-small molecule data from resources such as PubChem, ChEMBL or proprietary sources may enable identification of cofactors or modulators and could identify novel lead compounds.

Reactome contains several tools for the analysis of large-scale data sets that the user can submit to the resource. Results of such analyses are exportable in many formats from PNG images to systems biology data standards such as BioPAX and SBML. Some key features of the data in Reactome are:

  • data in Reactome is free and open-source

  • data can be analysed by the ‘Skypainter’ tool in Reactome to perform pathway enrichment analysis or expression analysis

  • Reactome contains orthologue predictions to many model organisms, providing a template for analysing structure-function conservation in GPCR gene families across species

  • bulk querying of Reactome data can be easily performed using the ‘BioMart’ tool.

Features accessed from the pathway diagrams page (Entity Level Views or ELVs)

  • Protein: Protein interaction overlays could potentially provide researchers with a starting point for novel drug targets

  • Protein: drug interaction overlays could help to distinguish potentially useful drugs that act on a narrow range of similar processes from ones likely to have adverse effects because they interact with many proteins or with proteins involved in a diverse range of processes.

Funding

National Human Genome Research Institute at the National Institutes of Health (P41 HG003751); European Union 6th Framework Programme ‘ENFIN’ (LSHG-CT-2005-518254). Funding for open access charge: National Institutes of Health (P41 HG003751).

Conflict of interest. None declared.

Acknowledgements

Development of the Reactome data model and database is a collaborative project and this work benefited greatly from our interactions with David Croft, Phani Garapati, Marc Gillespie, Gopal Gopinath, Robin Haw, Lisa Matthews, Bruce May, Gavin O’Kelly, Esther Schmidt, and Guanming Wu, and with our colleagues at GO, ChEBI, UniProt and IntAct. The authors thank Joël Bockaert and Leslie Vosshall for their expert reviews of our GPCR annotations. We also thank three anonymous reviewers for their useful comments on an earlier version of this article.

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