Title

Signal transduction by short-wavelength opsins

Date of Award

5-2006

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Advisor(s)

Robert R. Birge

Keywords

Signal transduction, Short-wavelength, Opsins, Color vision

Subject Categories

Biophysics

Abstract

Daytime color vision is mediated by cone opsin proteins, which belong to the family of GPCRs and are found in the cone photoreceptor cells of the retina. The light sensitive part of the opsin is an 11- cis retinal chromophore attached via a protonated Schiff base linkage to the protein. Absorption of a photon causes the 11- cis to all- trans isomerization of the chromophore, which in turn drives the protein through a sequence of thermally trappable conformations before forming the physiologically active state. The chromophore binding pocket plays a major role in tuning the spectral sensitivity of the chromophore in different opsin groups. These spectral tuning mechanisms are not clearly understood, especially for the opsin group that mediates color sensitivity in the violet and ultraviolet region of the energy spectrum. In the study presented in this thesis, several residues from the 2 nd and 3 rd transmembrane domains that are important for regulating spectral tuning of the dark as well as the primary photoproduct and signal transduction properties of a violet sensitive cone opsin have been identified. Based on the spectroscopic results combined with theoretical analysis, a spectral tuning mechanism unique to short-wavelength sensitive opsins is proposed. The ultraviolet sensitive opsins are unique since it is believed that these are the only visual pigments that have an unprotonated chromophore-retinylidene Schiff base linkage in the dark state. The photobleaching pathway of an ultraviolet sensitive opsin has been characterized using cryogenic and semi-low temperature spectroscopy. Evidence is provided to show that a conserved counterion mediated protonation of the chromophore-protein retinylidene Schiff base linkage is necessary for normal signal transduction in ultraviolet sensitive visual pigments.

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