Cone Opsins

Chen, C. K., Zhang, K., Church-Kopish, J., et al. (2001) Characterization of human GRK7 as a potential cone opsin kinase. Mol. Vision. 7, 305-313. 

A key feature of both vertebrate and invertebrate opsin molecules is their ability to interact with a G protein, typically transducin, to initiate phototransduction (Ebrey Koutalos 2001). The third cytoplasmic loop that connects a-hehces V and VI contributes to the G protein binding and activation and is highly conserved amongst the rod and cone opsins (Fig. 1, Table 2). This conservation extends to the P opsin family. However, the third cytoplasmic loop of the VA... 

The opsins probably perform a variety of different tasks, but their known roles are as photosensors or photoisomerases (Foster Bellingham 2002). Photosensory opsins such as the rod and cone opsins, P opsin and VA opsin use light to activate a phototransduction cascade that ultimately results in a change in membrane potential of the photoreceptor cell. By contrast, photoisomerases are involved in photopigment regeneration. The best described photoisomerase is... 

Any one cone cell contains only one type of opsin and is sensitive to only one color of light. Color blindness results from loss or mutation of one or the other of the cone opsins. The combination of 11-c/s-retinaldehyde with cone opsin is sometimes called iodopsin, with rhodopsin meaning more specifically the holo-protein of rod opsin. Most studies of the mechanisms of vision shown in Figure 2.5 have been performed using rods by extrapolation, it is assumed that the same mechanisms are involved in cone vision. 

Opsins. Cone opsins are 40% homologous to rhodopsin, S cone opsins are 40% homologous with M and L cone opsins, but M and L cone opsins are 97% homologous with one another. The differences in spectral absorbance among different opsins result from differences in a small number of amino acid residues that alter the position of hydroxyl groups close to ll-cw-retinal. The 30nm difference in spectral absorbance between primate M and L cones is determined primarily by three amino acids alanine vs. serine at position 180 ( 4nm), phenylalanine vs. tyrosine at position 277 (-10 nm) and alanine vs. threonine at position 285 ( 16nm) (Deeb, 2005). 

Stenkamp DL, Hisatomi O, Barthel LK, Tokunaga F, Raymond PA. 1996. Temporal expression of rod and cone opsins in embryonic goldfish retina predicts the spatial organization of the cone mosaic. Invest Ophthalmol Vis... 

Fig. 4.24 Normalized absorption spectra of rhodopsin from human retinal rod cells (curve 2), and cone-opsins from human blue , green and red cone cells (curves 1, 3 and 4, respectively). The spectra are drawn as described by Stavenga et al. [151]...

Figure 5. The role of p-ionone on the activation of rod and cone opsins.

Recently, a new mouse model was developed that allows us to examine photoresponses in an entirely opsin-free mouse model. Opsins require their chromophore, 11-cis-retinal, for function, and therefore, one obvious approach to create an opsin-free model would be to deprive mice of dietary vitamin A, rendering aU opsins nonfunctional, including rod and cone opsins as well as known and yet to be discovered novel opsins. This approach is not feasible in normal mice, because vitamin A is required during development, and adult mice have stored sufficient vitamin A in their Hver to last for the lifetime of the animals, even if they were put on a vitamin-A-free diet. The creation of the plasma retinol-binding protein mutant rbp) enables us to deplete ocular retinal in mice. The Rbp protein transports retinal from storage in the Hver to peripheral tissues. With the Rbp mutation, peripheral tissues such as the eye can be vitamin A depleted in 6 to 8 months. Therefore, we used the vitamin-A-depleted rhp mouse as an opsin-free model for circadian photoreception. 

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