The Cationminus signpi Interaction.

Model for the structure of bacteriorhodopsin based on high-resolution electron cryo-microscopy.

Four adenosine receptors have been cloned and characterized from several mammalian species. The receptors are named adenosine A(1), A(2A), A(2B), and A(3). The A(2A) and A(2B) receptors preferably interact with members of the G(s) family of G proteins and the A(1) and A(3) receptors with G(i/o) proteins. However, other G protein interactions have also been described. Adenosine is the preferred endogenous agonist at all these receptors, but inosine can also activate the A(3) receptor. The levels of adenosine seen under basal conditions are sufficient to cause some activation of all the receptors, at least where they are abundantly expressed. Adenosine levels during, e.g., ischemia can activate all receptors even when expressed in low abundance. Accordingly, experiments with receptor antagonists and mice with targeted disruption of adenosine A(1), A(2A), and A(3) expression reveal roles for these receptors under physiological and particularly pathophysiological conditions. There are pharmacological tools that can be used to classify A(1), A(2A), and A(3) receptors but few drugs that interact selectively with A(2B) receptors. Testable models of the interaction of these drugs with their receptors have been generated by site-directed mutagenesis and homology-based modelling. Both agonists and antagonists are being developed as potential drugs.

https://pharmrev.aspetjournals.org/content/pharmrev/53/4/527.full.pdf

International Union of Pharmacology. XXV. Nomenclature and Classification of Adenosine Receptors

Publisher Summary This chapter discusses the integrated methods for the construction of three-dimensional models and computational probing of structure–function relations in G protein-coupled receptors (GPCR). The rapid pace of cloning and expression of G protein-coupled receptors offers attractive opportunities to probe the structural basis of signal transduction mechanisms at the level of these cell-surface receptors. Major insights have emerged from comparisons and classifications of the amino acid sequences of GPCRs into families defined by evolutionary developments and adapted to perform selective functions. Structural data on GPCRs, based on biochemical, immunological, and biophysical approaches have validated consensus architecture of GPCRs with an extracellular N-terminus, a cytoplasmic C-terminus, and a transmembrane portion comprised of seven-transmembrane helical domains connected by loops. Developments in the molecular modeling and computational exploration of GPCR proteins indicate a tantalizing potential to alleviate some of these difficulties. These expectations are based on the increased rate of success achieved by molecular modeling and computational simulation methods in providing structural insights relevant to the functions of biological molecules.

[19] Integrated methods for the construction of three-dimensional models and computational probing of structure-function relations in G protein-coupled receptors

Microbial-type rhodopsins are found in archaea, prokaryotes, and eukaryotes. Some of them represent membrane ion transport proteins such as bacteriorhodopsin, a light-driven proton pump, or channelrhodopsin-1 (ChR1), a recently identified light-gated proton channel from the green alga Chlamydomonas reinhardtii. ChR1 and ChR2, a related microbial-type rhodopsin from C. reinhardtii, were shown to be involved in generation of photocurrents of this green alga. We demonstrate by functional expression, both in oocytes of Xenopus laevis and mammalian cells, that ChR2 is a directly light-switched cation-selective ion channel. This channel opens rapidly after absorption of a photon to generate a large permeability for monovalent and divalent cations. ChR2 desensitizes in continuous light to a smaller steady-state conductance. Recovery from desensitization is accelerated by extracellular H+ and negative membrane potential, whereas closing of the ChR2 ion channel is decelerated by intracellular H+. ChR2 is expressed mainly in C. reinhardtii under low-light conditions, suggesting involvement in photoreception in dark-adapted cells. The predicted seven-transmembrane α helices of ChR2 are characteristic for G protein-coupled receptors but reflect a different motif for a cation-selective ion channel. Finally, we demonstrate that ChR2 may be used to depolarize small or large cells, simply by illumination.

/pdf/channelrhodopsin-2-a-directly-light-gated-cation-selective-3kd1b2nbr7.pdf

Channelrhodopsin-2, a directly light-gated cation-selective membrane channel.

Analysis of high-resolution electron diffraction patterns from purple membrane labelled with heavy-atoms

A map of the structure o f bacteriorhodopsin has been obtained using electron microscopy and diffraction [ 11. It shows a resolution of 3.5 A in a direction parallel to the membrane plane, but poorer than this perpendicular. The map shows many features well resolved from the main density of the seven a-helices, which we interpret as the bulky aromatic side-chains of phenylalanine, tyrosine and tryptophan as well as a very dense feature which is the Bionone ring of the retinal chromophore. Using these bulky side-chains as guide points and taking account of bulges in the helices that indicate smaller side-chains such as leucine, a complete atomic model for bacteriorhodopsin between amino acids 8 and 225 has been built. Twenty-one amino acids contributed by all seven helices surround the retinal, and 26 amino acids contributed by five helices form the proton channel. Ten of the amino acids in the middle of the Fig. 1. Artistic impression showing the relutionship between the key residiies Asp-tIS, Asp-%, Asp-212, Lys-216 and A%82 and the retinul binding site, the proton chunnel unrl overall molecrrlur hoiindury for the ground stute of hucteriorhodopsiri . _ proton channel are also part of the retinal-building site. The model also Drovides a useful basis for consideration of the The cytoplasm is at the top of the diab Tram.

T. A. Ceska

Papers

Model for the structure of bacteriorhodopsin based on high-resolution electron cryo-microscopy.

Analysis of high-resolution electron diffraction patterns from purple membrane labelled with heavy-atoms

An atomic model for the structure of bacteriorhodopsin.

Electron diffraction analysis of the M412 intermediate of bacteriorhodopsin

Structure of bacteriorhodopsin