Receptor seven-helix structures

Many other receptors also have a seven-helix structure similar to that of the adrenergic receptors. These include receptors for the following glucagon,... 

Alchemical transformations have also been applied to the challenging case of G protein-coupled receptors (GPCRs), for which little structural information is available experimentally at the atomic level. Starting from a template of a seven-helix... 

G-Protein coupled receptors (GPCR) represent the start element in secondary messenger producing systems. They comprise a family of over 1000 structurally-related members. These membrane proteins are also called serpentine or seven-helix receptors due to their seven transmembrane domains with an a-helical conformation. Receptors belonging to this class respond to a variety of hormones and neurotransmitters, and they detect odorant molecules or light [3,4]. 

Integral membrane proteins. Membrane proteins are hard to crystallize178 and precise structures are known for only a few of them.179-181 A large fraction of all of the integral membrane proteins contain one or more membrane-spanning helices with loops of peptide chain between them. Folded domains in the cytoplasm or on the external membrane surface may also be present. The best-known structure of a transmembrane protein is that of the 248-residue bacteriorhodopsin. It consists of seven helical segments that span the plasma membrane (Fig. 23-45) and serves as a light-activated proton pump. Other proteins with similar structures act as hormone receptors in eukaryotic membranes. A seven-helix protein embedded in a membrane is depicted in Fig. 8-5 and also, in more detail, in Fig. 11-6. 

Initially, we attempted to build a 3-D homology model of the transmembrane regions of the human thrombin receptor from the structure of IBRD, which is available from the Brookhaven Protein Database. Unfortunately, the suboptimal placement of several amino acid side chains resulted in severe deviations from structural standards for membrane-bound receptors with a seven-helix bundle topology (Figure 2). For example, carboxylate and ammonium groups on amino acid side chains at a mid-helix location were directed into the membrane, rather than toward the inside of the helix bundle, and the hydrophobic packing between some helices was either poor or nonexistent. 

With the seven-helix bundle construct in hand, we turned our attention to incorporation of the three extracellular (EC) loops. The cytoplasmic loops were disregarded because they would not be involved in molecular recognition between the receptor and the SFLLRN ligand. Extracellular loop 3, the smallest of the three EC loops, was added first via the loop-search routine in the Biopolymer mode of Sybyl. The loop backbone choices found in the Brookhaven PDB were examined in 3-D and selected on the basis of their fit to the overall protein structure. After the side chains were added to EC3, some of them had to be rotated to avoid unfavorable steric interactions with other parts of the protein. Then, the entire protein was energy minimized. Extracellular loop 1 was then added, followed by EC2, and each time the loop selection was made after analyzing the protein in 3-D. Side chains of amino... 

Etzkorn M, Martell S, Andronesi OC, Seidel K, Engelhard M, Baldus M. Secondary structure, dynamics, and topology of a seven-helix receptor in native membranes, studied by solid-state NMR spectroscopy. Angew. Chem. Int. Ed. Engl. 2007 46 459-462. 

After a brief overview of signal transduction, the text describes the structure of the seven-helix transmembrane P-adrenergic receptor and indicates how it transmits to the intracellular side of the plasma membrane a signal arising from binding the hormone epinephrine on the extracellular surface of the cell. The common features of the G proteins are presented next. The description of the information-transmission pathway from hormone stimulus to G proteins to adenylate cyclase is completed by a discussion of how cAMP activates specific protein kinases to modulate the activities of the phosphorylated target proteins. A small number of hormone molecules outside the cell results in an amplified response because each activated enzyme in the triggered cascade forms numerous products. There are many distinct seven-helix transmembrane hormone receptors. 

Gautier A, Mott HR, Bostock MJ et al (2010) Structure determination of the seven-helix transmembrane receptor sensory rhodopsin II by solution NMR spectroscopy. Nat Struct Mol Biol 17 768-774... 

Gautier A, Kirkpatrick JP, Nietlispach D (2008) Solution-state NMR spectroscopy of a seven-helix transmembrane protein receptor backbone assignment, secondary structure, and dynamics. Angew Chem Int Ed 47 7297-7300... 

Donnelly, D. and Findlay, J.B.C. (1994) Seven-helix receptors structure and modelhng, Curr. Opin. Struct. Biol. 4, 582-589. 

The seven helix region of the serotonin receptor is the site of serotonin binding. The serotonin receptor is a member of the G-coupled receptor protein family. These proteins have similar structures. Their different specificities depend upon differences in primary structure at the ligand bindings site. 

The muscarinic receptor is a seven-helix membrane-spanning G-protein-type receptor and the associated second messenger is either activation of inositol-3-phosphate (IPS) or inhibition of cyclic adenosine monophosphate (cAMP). There are five subtypes of the muscarinic receptors (M1-M5). All share the same general structure and the active site, but they differ in their tissue distribution, their second messenger mechanism and the associated physiological response. ... 

FIGURE 2.1 A side view of the structure of the prototype G-protein-coupled, 7TM receptor rhodopsin. The x-ray structure of bovine rhodopsin is shown with horizontal gray lines, indicating the limits of the cellular lipid membrane. The retinal ligand is shown in a space-filling model as the cloud in the middle of the structure. The seven transmembrane (7TM) helices are shown in solid ribbon form. Note that TM-III is rather tilted (see TM-III at the extracellular and intracellular end of the helix) and that kinks are present in several of the other helices, such as TM-V (to the left), TM-VI (in front of the retinal), and TM-VII. In all of these cases, these kinks are due to the presence of a well-conserved proline residue, which creates a weak point in the helical structure. These kinks are believed to be of functional importance in the activation mechanism for 7TM receptors in general. Also note the amphipathic helix-VIII which is located parallel to the membrane at the membrane interface. 

In the x-ray structure of rhodopsin, an amphipathic helix runs parallel to the membrane from the intracellular end of TM-VII beneath the seven-helical bundle to the other side of TM-I and TM-II. At this point, one or more Cys residues are often found and are known to be subject to a dynamic posttranslational modification with palmitic acid residues. Like the phosphorylation event, the palmitoylation process appears to be dynamically regulated by receptor occupancy and is also involved in the desensitization phenomenon. The two posttranslational modifications can influence each other. For example, the conformational constraint induced by palmitoylation may alter the accessibility of certain phosphorylation sites. Like the phosphorylation process, the functional consequences of palmitoylation also appear to vary from receptor to receptor. 

The currently accepted structural models of the G-protein coupled receptor tend strongly towards the well established structure of bacteriorhodopsin (Fig. 5.4) that is also a 7-helix transmembrane protein. The model assumes that the seven helices are bedded bimdle-wise in the membrane. Detailed structural information on the conformation of the extracellular and intracellular structural portions is still lacking. 

Fig. 2. Structure of a heptahelical receptor. Cartoon model of dark (inactive) bovine rhodopsin (1U19), showing the seven transmembrane-spanning a helices (red to blue) and 11-r/Vi ctinal (gray spheres). Conserved residues important for receptor and G protein activation are shown (magenta spheres), including the DRY motif on helix III (yellow) and NpxxYx5F motif on helices VII and VIII blue and purple). The extracellular and intracellular faces of rhodopsin are shown. Receptor activation results in an outward movement of helix VI yellow arrow), which opens a gap in the cytoplasmic face of the receptor, exposing residues critical for G protein activation, such as the DRY motif on helix III (yellow).

The rhodopsin structure places the il loop adjacent to the short eighth membrane-embedded a-helix. With the exception of the 5-HT4A receptor (six residues), the 5-HT receptors have the same length as the rhodopsin loop (seven residues). Interestingly a XKKLXXX motif is conserved between the rhodopsin sequence and the majority of the 5-HT sequences, suggesting that the il loops of rhodopsin and the 5-HT receptors could have a common structure. Systematic mutagenesis studies have not been conducted. 

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