Publication Date:
2011-03-11
Description:
G-protein-coupled receptors (GPCRs) are seven transmembrane helix (TM) proteins that transduce signals into living cells by binding extracellular ligands and coupling to intracellular heterotrimeric G proteins (Galphabetagamma). The photoreceptor rhodopsin couples to transducin and bears its ligand 11-cis-retinal covalently bound via a protonated Schiff base to the opsin apoprotein. Absorption of a photon causes retinal cis/trans isomerization and generates the agonist all-trans-retinal in situ. After early photoproducts, the active G-protein-binding intermediate metarhodopsin II (Meta II) is formed, in which the retinal Schiff base is still intact but deprotonated. Dissociation of the proton from the Schiff base breaks a major constraint in the protein and enables further activating steps, including an outward tilt of TM6 and formation of a large cytoplasmic crevice for uptake of the interacting C terminus of the Galpha subunit. Owing to Schiff base hydrolysis, Meta II is short-lived and notoriously difficult to crystallize. We therefore soaked opsin crystals with all-trans-retinal to form Meta II, presuming that the crystal's high concentration of opsin in an active conformation (Ops*) may facilitate all-trans-retinal uptake and Schiff base formation. Here we present the 3.0 A and 2.85 A crystal structures, respectively, of Meta II alone or in complex with an 11-amino-acid C-terminal fragment derived from Galpha (GalphaCT2). GalphaCT2 binds in a large crevice at the cytoplasmic side, akin to the binding of a similar Galpha-derived peptide to Ops* (ref. 7). In the Meta II structures, the electron density from the retinal ligand seamlessly continues into the Lys 296 side chain, reflecting proper formation of the Schiff base linkage. The retinal is in a relaxed conformation and almost undistorted compared with pure crystalline all-trans-retinal. By comparison with early photoproducts we propose how retinal translocation and rotation induce the gross conformational changes characteristic for Meta II. The structures can now serve as models for the large GPCR family.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Choe, Hui-Woog -- Kim, Yong Ju -- Park, Jung Hee -- Morizumi, Takefumi -- Pai, Emil F -- Krauss, Norbert -- Hofmann, Klaus Peter -- Scheerer, Patrick -- Ernst, Oliver P -- England -- Nature. 2011 Mar 31;471(7340):651-5. doi: 10.1038/nature09789. Epub 2011 Mar 9.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Institut fur Medizinische Physik und Biophysik - CC2, Charite - Universitatsmedizin Berlin, Chariteplatz 1, D-10117 Berlin, Germany. hwchoe@jbnu.ac.kr〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/21389988" target="_blank"〉PubMed〈/a〉
Keywords:
Binding Sites
;
Conserved Sequence
;
Crystallization
;
Crystallography, X-Ray
;
GTP-Binding Protein alpha Subunits/chemistry/metabolism
;
Ligands
;
Models, Molecular
;
Opsins/chemistry
;
Peptide Fragments/chemistry/metabolism
;
Protein Conformation
;
Retinaldehyde/chemistry/metabolism
;
Rhodopsin/*chemistry/*metabolism
;
Schiff Bases/chemistry
;
Static Electricity
Print ISSN:
0028-0836
Electronic ISSN:
1476-4687
Topics:
Biology
,
Chemistry and Pharmacology
,
Medicine
,
Natural Sciences in General
,
Physics
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