title: The first transmembrane segment of the dopamine D2 receptor: accessibility in the binding-site crevice and position in the transmembrane bundle.
abstract: The binding site of the dopamine D2 receptor, like that of homologous G-protein-coupled receptors (GPCRs), is contained within a water-accessible crevice formed among its seven transmembrane segments (TMs). Using the substituted-cysteine-accessibility method (SCAM), we are mapping the residues that contribute to the surface of this binding-site crevice. We have now mutated to cysteine, one at a time, 21 consecutive residues in TM1. Six of these mutants reacted with charged sulfhydryl reagents, whereas bound antagonist only protected N52(1.50)C from reaction. Except for A38(1.36)C, none of the substituted cysteine mutants in the extracellular half of TM1 appeared to be accessible. Pro(1.48) is highly conserved in opsins, but absent in catecholamine receptors, and the high-resolution rhodopsin structure showed that Pro(1.48) bends the extracellular portion of TM1 inward toward TM2 and TM7. Analysis of the conversation of residues in the extracellular portion of TM1 of opsins showed a pattern consistent with alpha-helical structure with a conserved face. In contrast, this region in catecholamine receptors is poorly conserved, suggesting a lack of critical contacts. Thus, in catecholamine receptors in the absence of Pro(1.48), TM1 may be straighter and therefore further from the helix bundle, consistent with the apparent lack of conserved contact residues. When examined in the context of a model of the D2 receptor, the accessible residues in the cytoplasmic half of TM1 are at the interface with TM7 and with helix 8 (H8). We propose the existence of critical contacts of TM1, TM7, and H8 that may stabilize the inactive state of the receptor.
authors.affiliation: Center for Molecular Recognition, Department of Pharmacology, College of Physicians and Surgeons, Columbia University, 630 West 168th Street, New York, New York 10032, USA.
authors.forename: L
authors.initials: L
authors.lastname: Shi
authors.forename: M M
authors.initials: MM
authors.lastname: Simpson
authors.forename: J A
authors.initials: JA
authors.lastname: Ballesteros
authors.forename: J A
authors.initials: JA
authors.lastname: Javitch
mesh_terms: D019943:Amino Acid Substitution; D001665:Binding Sites / Q000235:genetics; D002460:Cell Line; D003545:Cysteine / Q000737:chemistry; D006801:Humans; D066298:In Vitro Techniques; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed; D017434:Protein Structure, Tertiary; D017448:Receptors, Dopamine D2 / Q000737:chemistry* / Q000235:genetics; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics; D013439:Sulfhydryl Reagents; D014867:Water
chemical_list: D017448:Receptors, Dopamine D2; D011994:Recombinant Proteins; D013439:Sulfhydryl Reagents; D014867:Water; D003545:Cysteine