title: Partial agonist activity of 11-cis-retinal in rhodopsin mutants.
abstract: Rhodopsin, the photoreceptor molecule of the vertebrate rod cell, is a G protein-coupled receptor. Rhodopsin consists of the opsin apoprotein and its 11-cis-retinal chromophore, which is covalently bound to a specific lysine residue by a stable protonated Schiff base linkage. Rhodopsin activation occurs when light causes photoisomerization of the 11-cis chromophore to its all-trans form. The all-trans chromophore is the receptor agonist. The 11-cis-retinylidene chromophore is analogous pharmacologically to a potent inverse agonist of the receptor. We report here that replacement of a highly conserved glycine residue (Gly121) causes 11-cis-retinal to become a pharmacologic partial agonist. Although the mutant apoproteins do not display constitutive activity, they are active in the dark when bound to an 11-cis-retinylidene chromophore, or to a "locked" chromophore analogue, Ret-7. The degree of partial agonism is directly related to the size of the amino acid replacement at position 121, and it can be reversed by a specific second-site replacement of Phe261. Thus, mutation of Gly121 in rhodopsin causes 11-cis-retinal to act as a partial agonist rather than an inverse agonist, allowing the mutant pigment to activate transducin in the dark.
authors.affiliation: Laboratory of Molecular Biology and Biochemistry, Rockefeller University, New York, New York 10021, USA.
authors.forename: M
authors.initials: M
authors.lastname: Han
authors.forename: J
authors.initials: J
authors.lastname: Lou
authors.forename: K
authors.initials: K
authors.lastname: Nakanishi
authors.forename: T P
authors.initials: TP
authors.lastname: Sakmar
authors.forename: S O
authors.initials: SO
authors.lastname: Smith
mesh_terms: D000818:Animals; D019556:COS Cells; D003624:Darkness; D019204:GTP-Binding Proteins / Q000378:metabolism; D005998:Glycine / Q000235:genetics; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000378:metabolism; D007536:Isomerism; D008027:Light; D009154:Mutation; D011994:Recombinant Proteins / Q000819:agonists; D012172:Retinaldehyde / Q000494:pharmacology*; D012243:Rhodopsin / Q000819:agonists* / Q000235:genetics*; D013053:Spectrophotometry
chemical_list: D011994:Recombinant Proteins; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D012243:Rhodopsin; D019204:GTP-Binding Proteins; D012172:Retinaldehyde; D005998:Glycine