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M. Kono; The Role of the Conserved Counterion Residue in a UV Cone Pigment . Invest. Ophthalmol. Vis. Sci. 2005;46(13):2987.
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© ARVO (1962-2015); The Authors (2016-present)
In rhodopsin, the 11–cis retinal chromophore is bound to Lys296 via a protonated Schiff base linkage. The resulting positive charge in the transmembrane domain is offset by the highly conserved negatively charged Glu113 in helix 3. Recent work indicates that UV cone pigments have retinal bound as an unprotonated Schiff base, and the absence of a positive charge yields the blue–shifted UV absorption properties. Nevertheless, Glu is conserved in all vertebrate UV pigments cloned to date. Purpose:This study aims to ascertain the role of this conserved Glu. Methods: The wild–type and mutant salamander UV cone pigments were expressed in COS cells. In the UV pigment, Glu108 corresponds to Glu113 of bovine rhodopsin. This residue was replaced with Gln, and the absorption spectrum and the protein’s ability to activate transducin was assessed. Results: The absorption maximum of the Glu to Gln mutant is blue–shifted about 6 nm relative to the wild–type. The apoprotein of the mutant is more constitutively active than the wild–type opsin and is inactivated by 11–cis retinal. Conclusions: The data suggest that Glu108 is conserved not to be a counterion to the Schiff base but to maintain a relatively inactive aprotein.
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