Transmembrane (cell surface) receptors

See also Membrane proteins.

Ion channel-linked (ionotropic) receptors

These receptors are typically the targets of fast neurotransmitters such as acetylcholine (nicotinic) and GABA; activation of these receptors results in changes in ion movement across a membrane.

• Ionotropic receptors
• 5-hydroxytryptamine receptor#Structural highlights/Specific Function of 5-HT3
• The extracellular subunit interface of the 5-HT3 receptors: a computational alanine scanning mutagenesis study^[1]
• 5-hydroxytryptamine receptor#5-HT3 receptor antagonists

5-HT3 receptor

The receptor is bullet-shaped and consists of 5 subunits (A-E) that form an oligomer. In the center of this pentamer of is a ligand-gated ion channel full of water, which the 5 subunits enclose pseudo-symmetrically. Each subunit of the 5-HT3 receptor consists of 3 regions; the extracellular region, the transmembrane region, and the intracellular region. The is relatively large compared to the other 2 regions, and contains a short C-terminus and a larger N-terminus. The N-terminus of the extracellular region is where the ligand binding occurs, and therefore deals with the agonists and antagonists. These are located between 2 bordering subunits, assembled from 3 α-helices of 1 subunit and 3 β-strands from the other subunit. Such connection creates a binding pocket with a small number of residues from each subunit pointed into the binding pocket, as opposed to the large number of residues that are pointing from the binding pocket. This binding pocket shrinks around agonists, encapsulating them, and widens around antagonists, repulsing them. The is within the C-terminus region, and contains 4 α-helical domains (M1-M4) that stretch the length of this inner, transmembrane area. These 4 α-helical domains conduct the channel openings via ion selectivity, depending on both charge and size. M2, the porous domain, contains rings of charged amino acids at both its start and its , accounting for M2 main contribution to ion selectivity. The M3 and M4 α-helices create a large with one another, thus assembling the .

• Nicotinic Acetylcholine Receptors in general

The receptor is a transmembrane pentameric glycoprotein. It cylindrical in appearance by electron microscopy approximately 16nm in length and 8nm in diameter. The main ion channel is composed of a water pore that runs through the entire length of the protein. If viewed from the synaptic cleft, the protein will look like a pseudo-symmetrical rosette shown in the picture below composed of 10 different alpha and 4 different beta subunits.

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• .
• .

• Alpha-bungarotoxin is a nicotinic cholinergic antagonist that is found within the venom of Bungarus multicinctus, a South-asian snake.
• Binding site of AChR
• Acetylcholine Receptor and its Reaction to Cobra Venom

When cobra venom is introduced into the body is moves along the bloodstream to a diaphragm muscle. It works as a postsynaptic neurotoxin binding to the receptor as an extracellular ligand by interacting with OH group leaving the acetylcholine channel open which releases ions used in creating an action potential. There must be 5 molecules of cobra toxin (red) to block the receptor (blue) as each molecule binds with an individual alpha chain on the acetylcholine receptor. The 2nd image depicts an individual toxin binding with one chain on the receptor, both in the same color. . This representation shows each molecule of the .

• AMPA glutamate receptor

Full view of the glutamate receptor shows the overall structure (N-terminal, ligand-binding and transmembrane domains) in and models. is a part of the extracellular domain. This domain is implicated in receptor assembly, trafficking, and localization.

• .
• . This domain widens in response to glutamate binding allowing for positive ions to pass through the post-synaptic membrane.
• .
• .

• Glutamate receptor (GluA2)

G protein-linked (metabotropic) receptors

This is the largest family of receptors and includes the receptors for several hormones and slow transmitters (dopamine, metabotropic glutamate). They are composed of 7 transmembrane α-helices. The loops connecting the α-helices form extracellular and intracellular domains. The binding-site for larger peptide ligands is usually located in the extracellular domain whereas the binding site for smaller non-peptide ligands is often located between the seven alpha helices and one extracellular loop. These receptors are coupled to different intracellular effector systems via G proteins

• G protein-coupled receptors
• Tutorial: The opioid receptor, a molecular switch

You can check out the in the window on the right. It shows the mu opioid receptor bound to a peptide ligand and a G protein . The G protein ("G" because it binds to GTP) consists of three parts A , B , and C ).

• Mu Opioid Receptor Bound to a Morphinan Antagonist

In this crystal structure of the μ opioid receptor it is (β-FNA), a close relative of morphine that is bound in the pocket.

• μ Opioid Receptors
• Mu Opioid Receptor

The binding of an opioid induces a in the μ-opioid receptor that activates an inhibitory G-protein (Gαi/o). This results in the dissociation of the G-protein complex. The Gα subunit then inhibits adenylyl cyclase. The Gβγ subunit acts to inhibit Ca2+ channels and activate K+ channels. .

• The κ-opioid receptor binds opium-type ligands

The κ-opioid receptor is a . The extracellular side is home to the proteins primary . These 2 units will span the length for the cell membrane to form the basis of the receptor molecule. , where helices I (in light blue) and helices VIII (in dark blue). This area will make up the basis for the intermembrane surface area. A distinguishing feature that separates the κ-opioid receptor from other receptors, is the large β-hairpin, , located near the main active site of the protein. It is believed that its function is to cap the active site of the receptor. Although in general, this protein is primarily composed of α-helices, not β-sheets (Compare to ). This evidence reinforces the idea that this protein is a transmembrane protein rather than one found inside the cytosol. In general transmembrane protein are composed almost entirely of α-helices (or β-sheet arranged in special fashion called a β-barrel), in order to have maximum stability inside the membrane. Interesting feature of the κ-opioid receptor is the formed by Cys131 and Cys210 which is conserved across all opioid receptors. of κ-opioid receptor. The human κ-opioid receptor ligand binding pocket displays a unique combination of key characteristics both shared with and distinct from those in the chemokine and aminergic receptor families.

• The δ-opioid receptor binds enkephalins

Opioid receptors typically have 2 big portions: the upper portion, zoomed in here with shown in indigo, that is ligand specific and recognizes a particular ligand, and the lower portion which is highly conserved amongst all receptors ^[2]. When approaches δ-opioid receptor, it is distinguished by the high hydrophobic interaction between the indole group on the ligand and leucine 300 on the receptor. As it glides deeper into the binding site facilitated by the hydrophobic interaction, the hydroxyl group of the tyrosine-like phenol group hydrogen bonds with water molecules which are hydrogen bound to a critical histidine 248. This holds the ligand by having both the phenol group and histidine anchored by a water molecule. The water molecules within the binding pocket flank both the ligand and receptor, serving almost as a scaffolding on which for both components to act. Adjacent to the phenol group, the oxygen of an ether is hydrogen bound to tyrosine 129 of the receptor. On the opposite side of the binding site, Asp128 forms a salt bridge with the charged amino group on the ligand. The rest of the ligand maintains hydrophobic contact with non-polar residues of the binding site. The phenol to water interaction is a conserved interaction between many opioid receptors and their respective ligands as evidenced by many natural antagonists having a tyrosine that interacts with a water molecule in a similar fashion ^[3].

• Neurotensin receptor

Like other G protein-coupled receptors, NTSR1 is composed of 3 distinct regions. An where neurotensin binds and causes a conformational change of the protein. A region containing (PDB code:4GRV) that transduce the signal from the extracellular side of the cell membrane to the intracellular side. Lastly, an intracellular region that when activated by a conformational change in the protein activates a G-protein associated with this receptor.

The in NTSR1 is located at the top of the protein (Figure 1). NTSR1 also contains an allosteric , which is located directly beneath the ligand binding pocket and the two pockets, which are separated by the residue ^[4]. NTSR1 has been mutated to exist in both and states.

• CXC chemokine receptor type 4

(PDB code 3oe0).

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.

• Orexin and Orexin receptor

The through a constricted solvent-accessible channel. A ^[5].

• Belsomra and Orexin receptors
• Hypocretin and receptors
• Human Follicle-Stimulating Hormone Complexed with its Receptor
• GPR40

hGPR40 contains (). hGPR40 and peptide-binding and opioid GPCRs, they share structural similarities such as a conserved motif on (ECL2). A conserved is formed between TM helix 3 (Cys 79) and the C-terminus of ECL2 (Cys170). A unique feature of hGPR40 is the presence of an additional 13 residues (Pro147-Gly159) on ECL2, which is absent on all the other peptide/opioid receptors. These extra residues form a separate between the B-sheet-like region and TM4. Together, the auxiliary loop and ECL2 of hGPR40 function as a over the canonical binding site covering it from the central extracellular region. The canonical binding pocket for many other GPCRs is solvent exposed and centrally located between the TM helices allowing ligands to directly bind from the extracellular space. However, because acts as a roof to this site, it inhibits ligands from entering directly from the extracellular region. Instead, the highly lipophilic nature of hGPRC40’s ligands allow it to enter a by moving through the lipid bilayer. FFAs bind to hGPR40 by coordinating its free carboxyl group to 3 amino acids , which are located close to the of hGPR40. The has been identified, but other binding sites were hypothesized. hGPR40 has a distinct binding pocket that is established by : , , , , , , , and (all individual residues shown in chartreuse). The importance of these residues for agonist binding was determined by alanine site-directed-mutagenesis studies. When the substrat/agonist enters the binding pocket, 4 of the 8 interact directly with the carboxylate moiety of the agonist by hydrogen bonding to it. These residues include 2 key arginines in the binding pocket, Arg183 and Arg258, and 2 key tyrosines, Tyr91 and Tyr240. Tyr240 is especially important for binding. hGPR40 contains a highly conserved hairpin extracellular loop () is the longest and most divergent of the extracellular loops found in proteins (). The loop is accompanied by a disulfide bond () that forms between TM4 and the C-terminus of the ECL2 loop. The only exception to the low flexibility is the tip of the auxiliary loop, which corresponds to residues Asp152-Asn155. This area of greater mobility allows for substrates to enter the binding site. is tested for the treatment of type 2 diabetes. The binding of TAK-875 to hGPR40 occurs by the ligand entering the binding site through the membrane bilayer. This membrane insertion is performed via a method similar to ligand binding to sphingosine 1-phosphate receptor 1, retinal loading of GPCR opsin, and the entry of anandamide in cannabinoid receptors, in which the block the binding from the extracellular matrix ^[6]. TAK-875 binds to the . The carboxylate of TAK-875 is buried within a very hydrophobic region and in a complex complex involving Glu172, Ser187, Asn241, and Asn 244 from hGPR40 forming ionic and polar interactions by coordinating TAK-875 with Arg183, Arg258, Tyr91, and Tyr240.

• Lysophosphatidic acid receptor

LPA₁ lies in the membrane as shown by the bound in the crystallization of LPA₁. Most (red) reside on the intracellular and extracellular areas of the receptor, while most residues positioned on the trans membrane helices inside the membrane are hydrophobic (blue). The intracellular region of this membrane protein is coupled to a heterotrimeric G protein. Three native in the extracellular region of this receptor provide fold stability. The 1st disulfide bond constrains the N terminal helix to extracellular loop (ECL) 2. The 2nd disulfide bond shapes ECL2, and the 3rd binds ECL3 to one of the TM α-helices. These disulfide bonds provide intramolecular stabilization along the extracellular region of the LPA₁ receptor, where the substrate enters into the binding pocket. The is a 6 turn α-helix and functions like a cap on the extracellular side of the protein, packing tightly against ECL1 and ECL2. The N-terminus helix also provides that interact with the ligand when bound. The biological ligand of the LPA₁ receptor is lysophosphatidic acid (LPA), a phospholipid that contains a long, nonpolar tail, a phosphate head, a chiral hydroxyl group, and an ester group. This receptor provides specificity for its ligand by the amphipathic binding pocket; the positive region on the left hand side of the pocket stabilizes the LPA's phosphate group, the nonpolar region at the bottom of the binding pocket stabilizes the hydrophobic tail of LPA, and the polar region at the top of the pocket stabilize binding of the ester and hydroxyl group. The for LPA consists of both polar and nonpolar residues. residues are located on the N terminus and within the binding pocket. A also interacts with the long acyl chain of LPA. The shape and polarity of the binding pocket makes it specific for molecules with a polar head and long hydrophobic tail shaped like LPA. ONO-9780307 (ON7) is an antagonist for LPA due to its large nonpolar region, chiral hydroxyl group, ester, and carboxylic acid which all resemble portions of the LPA molecule. 4 separate interactions with this antagonist of LPA₁ help demonstrate the key interactions that stabilize the binding of the LPA to this receptor. In the nonpolar region of the binding pocket, of LPA₁ stabilize the nonpolar group of ON7. At the polar region, the ligand binding is stabilized by forming ionic and polar interactions with the carboxylic acid and the hydroxyl group of ON7. Interplay between causes another stabilizing component with the ON7 antagonist. Glu293 forms polar interactions with Lys39, positioning it in close proximity to to the carboxylic acid of ON7, which then interactions with Lys39 via ionic bonding. While Lys39 is highly conserved among all 6 LPA receptors, a neighboring His residue is specific to the LPA₁ receptor. forms both ionic and polar interactions with the carboxylic acid of ON7.

• Sphingosine 1-Phosphate Receptor

Sphingosine-1-phosphate receptor (S1P₁) has altered ligand binding pathway (compared to LPA) includes global changes in the positioning of the extracellular loops and transmembrane helices. Specifically, a slight divergence of , which is positioned 3 Å closer to TMVII compared to S1P₁, and a repositioning of , resulting in a divergence of 8 Å from S1P₁ result in ligand access via the extracellular space. This narrowing of the gap between TMI and TMVII blocks membrane ligand access in LPA₁, while the greater distance between ECL3 and the other extracellular loops promotes extracellular access for LPA₁. Additionally, ECL0 is helical in S1P₁, but in LPA₁. This increased flexibility that results from ECL0 lack of secondary structure in LPA₁ further promotes favorable LPA access to the binding pocket from the extracellular space.

• Sphingosine 1-phosphate Receptor by Harish Srinivas
• Rhodopsin
• Rhodopsin Structure and Function
• Serotonin receptors, main page
• 3D structures of Serotonin receptors
• Adrenergic receptors in general
• Beta-1 Adrenergic receptor
• Dobutamine: Beta-1 Adrenergic receptor, 2y00, 2y01, 6h7l
• Isoprenaline: Beta-1 Adrenergic receptor, 2y03
• Carmoterol: 2y02
• Salbutamol: 2y04
• Adrenergic receptor page.
• Article Beta-2 Adrenergic Receptor by Wayne Decatur, David Canner, Dotan Shaniv, Joel L. Sussman, Michal Harel
• Article Beta-2 adrenergic receptor by Joel L. Sussman, Tala Curry, Michal Harel, Jaime Prilusky
• Group:SMART:A Physical Model of the beta-Adrenergic Receptor
• G_s: adenylate cyclase activated, cAMP up. For G_s see Beta2 adrenergic receptor-Gs protein complex updated
• Dopamine receptors 1 page
• Dopamine receptors 2 page
• Histamine H1 receptor
• 3rze - human histamine H1 receptor with an antagonist doxepin
• Adenosine A2A receptor
• Effect of Caffeine (Trimethylxanthine) on Human A2A Receptor
• Muscarinic acetylcholine receptor
• Glucose-dependent Insulinotropic Polypeptide Receptor
• Glucagon receptor
• Glucagon-like peptide 1 receptor
• Metabotropic Glutamate Receptors
• Ligand Binding N-Terminal of Metabotropic Glutamate Receptors
• Metabotropic glutamate receptor 5

Kinase-linked, enzyme-linked and related receptors

Receptor tyrosine kinases

Receptor tyrosine kinases (RTKs) are part of the larger family of protein tyrosine kinases. They are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones. Approximately 20 different RTK classes have been identified.^[7]

• RTK class I Epidermal Growth Factor Receptor family
• RTK class II Insulin receptor family
• RTK class III Platelet-derived growth factor receptor family
• RTK class IV Vascular Endothelial Growth Factor Receptor family
• RTK class V Fibroblast growth factor receptor family
• RTK class VIII Hepatocyte growth factor receptor family
• RTK class IX Ephrin receptor family
• RTK class XIII Epithelial discoidin domain-containing receptor (DDR receptor) family
• Neurotrophin: High affinity nerve growth factor receptor (or Tyrosine kinase receptor) and TrkB tyrosine kinase receptor
• Insulin-like growth factor receptor

Enzyme-linked receptor

• TGF-beta receptor
• Activin receptor
• Crystal structure of the extracellular domain of the receptor-like kinase TMK3 from Arabidopsis thaliana
• Bone Morphogenetic Protein 7 and receptor
• Neurotrophin
• High affinity nerve growth factor receptor TrkA
• TrkB tyrosine kinase receptor
• Toll-like Receptors

Immune receptors

Leukocyte immunoglobulin-like receptors

• Leukocyte immunoglobulin-like receptor

Cytokine receptors

TNF receptor superfamily

• Tumor necrosis factor receptor
  • TRAIL (TNF-related apoptosis-inducing ligand) or TNF ligand superfamily 10

Colony-stimulating factor receptor

Type I cytokine receptors

• Erythropoietin receptor
• Prolactin receptor

Type II cytokine receptors

Interferon receptors

• Interferon receptor
• Structural linkage between ligand discrimination and receptor activation by type I interferons
• Multiple sclerosis and Interferon Receptors

Interleukin receptors

• Interleukin receptor

Interleukin-20 receptor:

• Flexible regions govern promiscuous binding of IL-24 to receptors IL-20R1 and IL-22R1

Chemokine receptors, two of which acting as binding proteins for HIV (CXCR4 and CCR5). They are G protein-coupled receptors

T-cell receptors

• T-cell receptor
• SP3.4-TCR-HLA-DQ8-α-1-gliadin complex

TGF-beta receptor

• TGF-beta receptor

LDL receptor

• LDL receptor

Transferrin receptor

• Transferrin receptor

Intracellular receptors

Signal recognition particle receptor

• Signal recognition particle receptor

Receptor for activated C kinase 1

• Receptor for activated protein kinase C 1

Nuclear receptors

• Nuclear receptors
• Thyroid hormone receptor
• Retinoic acid receptor
• RA Mediated T-reg Differentiation; Retinoic acid acts as the ligand for the Retinoic Acid Receptor-α (RARα) / Retinoid X Receptor-α (RXRα) heterodimer
• PPAR-gamma
• Pioglitazone is a selective agonist for Peroxisome Proliferator-Activated Receptor Gamma
• Liver X receptor
• Bile acid receptor
• Vitamin D receptor
• Pregnane X receptor
• Retinoid X receptor
• Estrogen receptor
• Ivan Koutsopatriy estrogen receptor
• Estrogen receptor#Structure of estradiol metal chelate and estrogen receptor complex: The basis for designing a new class of SERMs ^[8]
• Estrogen receptor#Estrogen receptor α complexed with raloxifene and a corepressor peptide
• Tamoxifen and the Estrogen receptor
• Student Project 10 for UMass Chemistry 423 Spring 2015
• Estrogen-related receptor
• Glucocorticoid receptor
• Mineralocorticoid receptor
• Progesterone receptor
• Androgen receptor
• Liver receptor homolog-1?

Endoplasmic reticulum/Sarcoplasmic reticulum receptors

Ligand-gated Calcium channels

Inositol 1,4,5-Trisphosphate Receptor

Ryanodine receptor

SEE ALSO:

• Hormones and their receptors
• Hormone
• Growth factors
• Neurotransmitters