seven-transmembrane G protein-coupled receptor superfamily; This hierarchical evolutionary model represents the seven-transmembrane (7TM) receptors, often referred to as G protein-coupled receptors (GPCRs), which transmit physiological signals from the outside of the cell to the inside via G proteins. GPCRs constitute the largest known superfamily of transmembrane receptors across the three kingdoms of life that respond to a wide variety of extracellular stimuli including peptides, lipids, neurotransmitters, amino acids, hormones, and sensory stimuli such as light, smell and taste. All GPCRs share a common structural architecture comprising of seven-transmembrane (TM) alpha-helices interconnected by three extracellular and three intracellular loops. A general feature of GPCR signaling is agonist-induced conformational changes in the receptors, leading to activation of the heterotrimeric G proteins, which consist of the guanine nucleotide-binding G-alpha subunit and the dimeric G-beta-gamma subunits. The activated G proteins then bind to and activate numerous downstream effector proteins, which generate second messengers that mediate a broad range of cellular and physiological processes. However, some 7TM receptors, such as the type 1 microbial rhodopsins, do not activate G proteins. Based on sequence similarity, GPCRs can be divided into six major classes: class A (the rhodopsin-like family), class B (the Methuselah-like, adhesion and secretin-like receptor family), class C (the metabotropic glutamate receptor family), class D (the fungal mating pheromone receptors), class E (the cAMP receptor family), and class F (the frizzled/smoothened receptor family). Nearly 800 human GPCR genes have been identified and are involved essentially in all major physiological processes. Approximately 40% of clinically marketed drugs mediate their effects through modulation of GPCR function for the treatment of a variety of human diseases including bacterial infections.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 93204873  76 SWDCVLQGALSTFANTSSFFWTVAIALYLYLSIVRaaRGPRTDRLLWAFHVVSWGVPLVITVAAVALKKIGYdasdvSVG 155
Cdd:cd14940  65 GFLCYLYAIVITYGSLSCWLWTLCLAISIYLLIVK--REPEPEKFEKYYHFVCWGLPLISTIIMLIKHHYGP-----VGN 137

                ....
gi 93204873 156 WCWI 159
Cdd:cd14940 138 WCWI 141

slime mold cyclic AMP receptor, member of the class E family of seven-transmembrane G protein-coupled receptors; This family represents the class E of seven-transmembrane G-protein coupled receptors found in soil-living amoebas, commonly referred to as slime molds. The class E family includes cAMP receptors (cAR1-4) and cAMP receptors-like proteins (CrlA-C) from Dictyostelium discoideum, and their highly homologous cAMP receptors (TasA and TasB) from Polysphondylium pallidum. So far, four subtypes of cAMP receptors (cAR1-4) have been identified that play an essential role in the detection and transmit of the periodic extracellular cAMP waves that regulate chemotactic cell movement during Dictyostelium development, from the unicellular amoeba aggregate into many multicellular slugs and then differentiate into a sporocarp, a fruiting body with cells specialized for different functions. These four subtypes differ in their expression levels and patterns during development. cAR1 is high-affinity receptor that is the first one to be expressed highly during early aggregation and continues to be expressed at low levels during later developmental stages. cAR1 detects extracellular cAMP and is coupled to G-alpha2 protein. Cells lacking cAR1 fail to aggregate, demonstrating that cAR1 is responsible for aggregation. During later aggregation the high-affinity cAR3 receptor is expressed at low levels. Nonetheless, cells lacking cAR3 do not show an obviously altered pattern of development and are still able to aggregate into fruiting bodies. In contrast, cAR2 and cAR4 are low affinity receptors expressed predominantly after aggregation in pre-stalk cells. cAR2 is essential for normal tip formation and deletion of the receptor arrests development at the mound stage. On the other hand, CAR4 regulates axial patterning and cellular differentiation, and deletion of the receptor results in defects during culmination. Furthermore, three cAMP receptor-like proteins (CrlA-C) were identified in Dictyostelium that show limited sequence similarity to the cAMP receptors. Of these CrlA is thought to be required for normal cell growth and tip formation in developing aggregates.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 93204873  76 SWDCVLQGALSTFANTSSFFWTVAIALYLYLSIVRaaRGPRTDRLLWAFHVVSWGVPLVITVAAVALKKIGYdasdvSVG 155
Cdd:cd14940  65 GFLCYLYAIVITYGSLSCWLWTLCLAISIYLLIVK--REPEPEKFEKYYHFVCWGLPLISTIIMLIKHHYGP-----VGN 137

                ....
gi 93204873 156 WCWI 159
Cdd:cd14940 138 WCWI 141

slime mold cyclic AMP receptor, member of the class E family of seven-transmembrane G protein-coupled receptors; This family represents the class E of seven-transmembrane G-protein coupled receptors found in soil-living amoebas, commonly referred to as slime molds. The class E family includes cAMP receptors (cAR1-4) and cAMP receptors-like proteins (CrlA-C) from Dictyostelium discoideum, and their highly homologous cAMP receptors (TasA and TasB) from Polysphondylium pallidum. So far, four subtypes of cAMP receptors (cAR1-4) have been identified that play an essential role in the detection and transmit of the periodic extracellular cAMP waves that regulate chemotactic cell movement during Dictyostelium development, from the unicellular amoeba aggregate into many multicellular slugs and then differentiate into a sporocarp, a fruiting body with cells specialized for different functions. These four subtypes differ in their expression levels and patterns during development. cAR1 is high-affinity receptor that is the first one to be expressed highly during early aggregation and continues to be expressed at low levels during later developmental stages. cAR1 detects extracellular cAMP and is coupled to G-alpha2 protein. Cells lacking cAR1 fail to aggregate, demonstrating that cAR1 is responsible for aggregation. During later aggregation the high-affinity cAR3 receptor is expressed at low levels. Nonetheless, cells lacking cAR3 do not show an obviously altered pattern of development and are still able to aggregate into fruiting bodies. In contrast, cAR2 and cAR4 are low affinity receptors expressed predominantly after aggregation in pre-stalk cells. cAR2 is essential for normal tip formation and deletion of the receptor arrests development at the mound stage. On the other hand, CAR4 regulates axial patterning and cellular differentiation, and deletion of the receptor results in defects during culmination. Furthermore, three cAMP receptor-like proteins (CrlA-C) were identified in Dictyostelium that show limited sequence similarity to the cAMP receptors. Of these CrlA is thought to be required for normal cell growth and tip formation in developing aggregates.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 93204873  76 SWDCVLQGALSTFANTSSFFWTVAIALYLYLSIVRaaRGPRTDRLLWAFHVVSWGVPLVITVAAVALKKIGYdasdvSVG 155
Cdd:cd14940  65 GFLCYLYAIVITYGSLSCWLWTLCLAISIYLLIVK--REPEPEKFEKYYHFVCWGLPLISTIIMLIKHHYGP-----VGN 137

                ....
gi 93204873 156 WCWI 159
Cdd:cd14940 138 WCWI 141

adhesion receptors, subfamily B2 of the class B family of seven-transmembrane G protein-coupled receptors; The B2 subfamily of class B GPCRs consists of cell-adhesion receptors with 33 members in humans and vertebrates. The adhesion receptors are characterized by the presence of large N-terminal extracellular domains containing a variety of structural motifs, which play critical roles in cell-cell adhesion and cell-matrix interactions, linked to a class B seven-transmembrane domain. These include, for example, EGF (epidermal growth factor)-like domains in CD97, Celsr1 (cadherin family member), Celsr2, Celsr3, EMR1 (EGF-module-containing mucin-like hormone receptor-like 1), EMR2, EMR3, and Flamingo; two laminin A G-type repeats and nine cadherin domains in Flamingo and its human orthologs Celsr1, Celsr2 and Celsr3; olfactomedin-like domains in the latrotoxin receptors; and five or four thrombospondin type 1 repeats in BAI1 (brain-specific angiogenesis inhibitor 1), BAI2 and BAI3. Furthermore, almost all adhesion receptors, except GPR123, contain an evolutionarily conserved GPCR- autoproteolysis inducing (GAIN) domain that undergoes autoproteolytic processing at the GPCR proteolysis site (GPS) motif located immediately N-terminal to the first transmembrane region, to generate N- and C-terminal fragments (NTF and CTF), which may serve important biological functions.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 93204873  65 FYGVLQNFAGPSWdCVLQGALSTFANTSSFFWTVAIALYLYLSIVRAaRGPRTDRLLWAFHVVSWGVPLVITVAAVALKK 144
Cdd:cd15040  55 FLFGINSTDNPVL-CTAVAALLHYFLLASFMWMLVEALLLYLRLVKV-FGTYPRHFILKYALIGWGLPLIIVIITLAVDP 132

                        90
                ....*....|....*.
gi 93204873 145 IGYDASDvsvGWCWID 160
Cdd:cd15040 133 DSYGNSS---GYCWLS 145

7 transmembrane receptor (Secretin family); This family is known as Family B, the secretin-receptor family or family 2 of the G-protein-coupled receptors (GCPRs). They have been described in many animal species, but not in plants, fungi or prokaryotes. Three distinct sub-families are recognized. Subfamily B1 contains classical hormone receptors, such as receptors for secretin and glucagon, that are all involved in cAMP-mediated signalling pathways. Subfamily B2 contains receptors with long extracellular N-termini, such as the leukocyte cell-surface antigen CD97; calcium-independent receptors for latrotoxin, and brain-specific angiogenesis inhibitors amongst others. Subfamily B3 includes Methuselah and other Drosophila proteins. Other than the typical seven-transmembrane region, characteriztic structural features include an amino-terminal extracellular domain involved in ligand binding, and an intracellular loop (IC3) required for specific G-protein coupling.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 93204873    79 CVLQGALSTFANTSSFFWTVAIALYLYLSIVRAarGPRTDRLLWAFHVVSWGVPLVITVAAVALKKIGYDASDVsvgwCW 158
Cdd:pfam00002  75 CKVVAVFLHYFFLANFFWMLVEGLYLYTLLVEV--FFSERKYFWWYLLIGWGVPALVVGIWAGVDPKGYGEDDG----CW 148

                          90
                  ....*....|.
gi 93204873   159 IDLEakDHVLW 169
Cdd:pfam00002 149 LSNE--NGLWW 157

The subfamily B1 of hormone receptors (secretin-like), member of the class B family seven-transmembrane G protein-coupled receptors; The B1 subfamily of class B GPCRs, also referred to as secretin-like receptor family, includes receptors for polypeptide hormones of 27-141 amino-acid residues such as secretin, glucagon, glucagon-like peptide (GLP), calcitonin gene-related peptide, parathyroid hormone (PTH), and corticotropin-releasing factor. These receptors contain the large N-terminal extracellular domain (ECD), which plays a critical role in hormone recognition by binding to the C-terminal portion of the peptide. On the other hand, the N-terminal segment of the hormone induces receptor activation by interacting with the receptor transmembrane domains and connecting extracellular loops, triggering intracellular signaling pathways. All members of this subfamily preferentially couple to G proteins of G(s) family, which positively stimulate adenylate cyclase, leading to increased intracellular cAMP formation and calcium influx. Moreover, the B1 subfamily receptors play key roles in hormone homeostasis and are promising drug targets in various human diseases including diabetes, osteoporosis, obesity, neurodegenerative conditions (Alzheimer###s and Parkinson's), cardiovascular disease, migraine, and psychiatric disorders (anxiety, depression). Furthermore, the subfamilies B2 and B3 consist of receptors that are capable of interacting with epidermal growth factors (EGF) and the Drosophila melanogaster Methuselah gene product (Mth), respectively. The class B GPCRs have been identified in all the vertebrates, from fishes to mammals, as well as invertebrates including Caenorhabditis elegans and Drosophila melanogaster, but are not present in plants, fungi, or prokaryotes.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 93204873  77 WDCVLQGALSTFANTSSFFWTVAIALYLYLSIVRAARGPrtDRLLWAFHVVSWGVPLVITVAAVALKKIGYDASdvsvgw 156
Cdd:cd15041  80 VGCKLLSVLKRYFKSANYFWMLCEGLYLHRLIVVAFFSE--PSSLKLYYAIGWGLPLVIVVIWAIVRALLSNES------ 151

                        90
                ....*....|....*
gi 93204873 157 CWIDlEAKDHVLWML 171
Cdd:cd15041 152 CWIS-YNNGHYEWIL 165

Frizzled/Smoothened family membrane region; This family contains the membrane spanning region of frizzled and smoothened receptors. This membrane region is predicted to contain seven transmembrane alpha helices. Proteins related to Drosophila frizzled are receptors for Wnt (mediating the beta-catenin signalling pathway), but also the planar cell polarity (PCP) pathway and the Wnt/calcium pathway. The predominantly alpha-helical Cys-rich ligand-binding region (CRD) of Frizzled is both necessary and sufficient for Wnt binding. The smoothened receptor mediates hedgehog signalling.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 93204873    76 SWDCVLQGALSTFANTSSFFWTVAIALYLYLSIVRAARGPRTDRLLWAFHVVSWGVPLVITVAAVALKKIgyDASDVSvG 155
Cdd:pfam01534  91 NLSCTVVFLLLYYFGMAASIWWVILTLTWFLAAGLKWGSEAIEKKSSYFHLAAWGIPAVLTITVLALGKV--DGDELT-G 167

                          90
                  ....*....|....*..
gi 93204873   156 WCWIDLEAKDHVLWMLL 172
Cdd:pfam01534 168 ICFVGNQNSDALRGFVL 184

growth-hormone-releasing hormone receptor, member of the class B family of seven-transmembrane G protein-coupled receptors; Growth hormone-releasing hormone receptor (GHRHR) is a member of the group of G protein-coupled receptors for structurally similar peptide hormones that also include secretin, pituitary adenylate cyclase activating polypeptide (PACAP), and vasoactive intestinal peptide. These receptors are classified into the subfamily B1 of class B GRCRs that consists of the classical hormone receptors and have been identified in all the vertebrates, from fishes to mammals, but are not present in plants, fungi, or prokaryotes. For all class B receptors, the large N-terminal extracellular domain plays a critical role in peptide hormone recognition. GHRHR is a specific receptor for the growth hormone-releasing hormone (GHRH) that controls the synthesis and release of growth hormone (GH) from the anterior pituitary somatotrophs. Mutations in the gene encoding GHRHR have been connected to isolated growth hormone deficiency (IGHD), a short-stature condition caused by deficient production of GH or lack of GH action. GHRH is preferentially coupled to a stimulatory G(s) protein, which leads to the activation of adenylate cyclase and thereby increases in intracellular cAMP level. GHRHR is found in mammals as well as zebrafish and chicken, whereas the GHRHR type 2, an ortholog of the GHRHR, has only been identified in ray-finned fish, chicken and Xenopus. Xenopus laevis GHRHR2 has been shown to interact with both endogenous GHRH and PACAP-related peptide (PRP).

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 93204873  75 PSWDCVLQGALSTFANTSSFFWTVAIALYLylSIVRAARGPRTDRLLWAFHVVSWGVPLVITVAAVaLKKIGYDASDvsv 154
Cdd:cd15270  73 STVLCKVSVVFCHYCVMTNFFWLLVEAVYL--NCLLASSFPRGKRYFWWLVLLGWGLPTLCTGTWI-LCKLYFEDTE--- 146

                        90       100
                ....*....|....*....|
gi 93204873 155 gwCWiDLEaKDHVLWMLLTG 174
Cdd:cd15270 147 --CW-DIN-NDSPYWWIIKG 162

corticotropin-releasing factor receptor 1, member of the class B family of seven-transmembrane G protein-coupled receptors; The vertebrate corticotropin-releasing factor (CRF) receptors are predominantly expressed in central nervous system with high levels in cortex tissue, brain stem, and pituitary. They have two isoforms as a result of alternative splicing of the same receptor gene: CRF-R1 and CRF-R2, which differ in tissue distribution and ligand binding affinities. Recently, a third CRF receptor (CRF-R3) has been identified in catfish pituitary. The catfish CRF-R1 is highly homologous to CRF-R3. CRF is a 41-amino acid neuropeptide that plays a central role in coordinating neuroendocrine, behavioral, and autonomic responses to stress by acting as the primary neuroregulator of the hypothalamic-pituitary-adrenal axis, which controls the levels of cortisol and other stress related hormones. In addition, the CRF family of neuropeptides also includes structurally related peptides such as mammalian urocortin, fish urotensin I, and frog sauvagine. The actions of CRF and CRF-related peptides are mediated through specific binding to CRF-R1 and CRF-R2. CRF and urocortin 1 bind and activate mammalian CRF-R1 with similar high affinities. By contrast, urocortin 2 and urocortin 3 do not bind to CRF-R1 or stimulate CRF-R1-mediated cAMP formation. Urocortin 1 also shows high affinity for mammalian CRF-R2, whereas CRF has significantly lower affinity for this receptor. These evidence suggest that urocortin 1 is an endogenous ligand for CRF-R1 and CRF-R2. The CRF receptors are members of the B1 subfamily of class B GPCRs, also referred to as secretin-like receptor family, which includes receptors for polypeptide hormones of 27-141 amino-acid residues such as secretin, glucagon, glucagon-like peptide (GLP), calcitonin gene-related peptide, and parathyroid hormone (PTH). These receptors contain the large N-terminal extracellular domain (ECD), which plays a critical role in hormone recognition by binding to the C-terminal portion of the peptide. On the other hand, the N-terminal segment of the hormone induces receptor activation by interacting with the receptor transmembrane domains and connecting extracellular loops, triggering intracellular signaling pathways. All members of the B1 subfamily preferentially couple to G proteins of G(s) family, which positively stimulate adenylate cyclase, leading to increased intracellular cAMP formation and calcium influx. However, depending on its cellular location and function, CRF receptors can activate multiple G proteins, which can in turn stimulate different second messenger pathways.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 93204873  79 CVLQGALSTFANTSSFFWTVAIALYLYLSIVRAARgprTDRLL-WAFHVVSWGVPLVITVAAvALKKIGYDASDvsvgwC 157
Cdd:cd15445  73 CRLVTAAYNYFHVTNFFWMFGEGCYLHTAIVLTYS---TDKLRkWMFICIGWCIPFPIIVAW-AIGKLYYDNEK-----C 143

                .
gi 93204873 158 W 158
Cdd:cd15445 144 W 144

class F smoothened family membrane region, a homolog of frizzled receptors; This group represents smoothened (SMO), a transmembrane G protein-coupled receptor that acts as the transducer of the hedgehog (HH) signaling pathway. SMO is activated by the hedgehog (HH) family of proteins acting on the 12-transmembrane domain receptor patched (PTCH), which constitutively inhibits SMO. Thus, in the absence of HH proteins, PTCH inhibits SMO signaling. On the other hand, binding of HH to the PTCH receptor activates its internalization and degradation, thereby releasing the PTCH inhibition of SMO. This allows SMO to trigger intracellular signaling and the subsequent activation of the Gli family of zinc finger transcriptional factors and induction of HH target gene expression (PTCH, Gli1, cyclin, Bcl-2, etc). SMO is closely related to the frizzled (FZD) family of seven transmembrane-spanning proteins, which constitute a novel and separate family of G-protein coupled receptors. The FZDs are activated by the wingless/int-1 (WNT) family of secreted lipoglycoproteins and preferentially couple to stimulatory G proteins of the Gs family, which activate adenylate cyclase, but can also couple to G proteins of the Gi/Gq families. In the WNT/beta-catenin signaling pathway, the WNT ligand binds to FZD and a lipoprotein receptor-related protein (LRP) co-receptor. This leads to the stabilization and translocation of beta-catenin to the nucleus, where it induces the activation of TCF/LEF family transcription factors. The WNT and HH signaling pathways play critical roles in many developmental processes, such as cell-fate determination, cell proliferation, neural patterning, stem cell renewal, tissue homeostasis and repair, and tumorigenesis, among many others.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 93204873  73 AGPSWDCVLQGALSTFANTSSFFWTVAIALYLYLSIVraARGPRTDRLLWA---FHVVSWGVPLVITVAAVALKKIgyDA 149
Cdd:cd15030  89 AGENLSCVVIFVLVYYFLMAGCVWFVILTYAWHMSFK--ALGTIQDRLDKKtayFHLIAWSLPLVLTITIMALGQV--DG 164

                        90
                ....*....|
gi 93204873 150 SDVSvGWCWI 159
Cdd:cd15030 165 DSVS-GICFV 173

class F frizzled subfamily 3, member of 7-transmembrane G protein-coupled receptors; This group includes subfamily 3 of the frizzled (FZD) family of seven transmembrane-spanning proteins, which constitute a novel and separate class of GPCRs, and its closely related proteins. This class F protein family consists of 10 isoforms (FZD1-10) in mammals. The FZDs are activated by the wingless/int-1 (WNT) family of secreted lipoglycoproteins and preferentially couple to stimulatory G proteins of the Gs family, which activate adenylate cyclase, but can also couple to G proteins of the Gi/Gq families. In the WNT/beta-catenin signaling pathway, the WNT ligand binds to FZD and a lipoprotein receptor-related protein (LRP) co-receptor. This leads to the stabilization and translocation of beta-catenin to the nucleus, where it induces the activation of TCF/LEF family transcription factors. The conserved cytoplasmic motif of FZD, Lys-Thr-X-X-X-Trp, is required for activation of the WNT/beta-catenin pathway, and for membrane localization and phosphorylation of Dsh (dishevelled) protein, a key component of the WNT pathway that relays the WNT signals from the activated receptor to downstream effector proteins. The WNT pathway plays a critical role in many developmental processes, such as cell-fate determination, cell proliferation, neural patterning, stem cell renewal, tissue homeostasis and repair, and tumorigenesis, among many others.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 93204873  79 CVLQGALSTFANTSSFFWTVAIALYLYLSIVRAARGPRTDRLLWAFHVVSWGVPLVITVAAVALKKIgydASDVSVGWCW 158
Cdd:cd15033  95 CTMLFMVLYFFTMAGSVWWVILTITWFLAAVPKWGSEAIEKKALLFHASAWGIPGTLTIILLAMNKI---EGDNISGVCF 171

                        90
                ....*....|....
gi 93204873 159 IDLEAKDHVLWMLL 172
Cdd:cd15033 172 VGLYDVDALRYFVL 185