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A. K. Gross, S. Madabushi, O. Lichtarge, T. G. Wensel; Rhodopsin Residues Controlling Regeneration Kinetics. Invest. Ophthalmol. Vis. Sci. 2007;48(13):590.
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Rhodopsin’s ligand, 11-cis retinal, is almost insoluble in water, but is quite soluble in the hydrocarbon phase of lipid membranes. It is therefore likely that retinal entry occurs between alpha-helices in rhodopsin’s transmembrane domain. We have sought to identify functionally important residues in the transmembrane domain that might include those which control retinal binding kinetics.
We have used the data mining technique of Evolutionary Tracing to identify residue positions commonly important in diverse GPCRs or within the rhodopsin family. We expressed variants of bovine rhodopsin bearing mutations at these sites in COS cells, purified the mutant rhodopsins and measured retinal binding spectrophotometrically. To confer sufficient stability to allow the opsins to be purified in the absence of 11-cis retinal, a thermostable mutation (N2C, D282C) was introduced as well.
We found that residues C167 and E122 located in the transmembrane region alter the kinetics of retinal binding. Whereas wild-type opsin binds retinal in our conditions with a t1/2 of 14 s, E122S binds with a t1/2 of 28 s and C167S binds with a t1/2 of 122 s. A mutation found in a subset of patients with automsomal dominant retinitis pigmentosa, A164V, is located near C167 and E122, and has previously been reported to slow retinal binding kinetics.
The region of rhodopsin where C167, E122, and A164 are located is important in controlling the kinetics of rhodopsin regeneration. This region or adjacent portions of the transmembrane domain may be the site of retinal entry.
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