Amino acid replacements and wavelength absorption of visual pigments in vertebrates

Regeneration of ultraviolet pigments of vertebrates

We report here the regeneration of the visual pigments of mouse, rat, goldfish and pigeon, which have wavelengths of maximal absorption at 359 nm, 358 nm, 359 nm, and 393 nm, respectively. The construction and functional assays of the ultraviolet or near-ultraviolet pigments from a wide range of vertebrate species will allow us to study the molecular bases of ultraviolet vision for the first time.

Molecular genetic basis of adaptive selection: examples from color vision in vertebrates

A central unanswered question in phototransduction is how photosensitive molecules, visual pigments, regulate their absorption spectra. In nature, there exist various types of visual pigments that are adapted to diverse photic environments. To elucidate the molecular mechanisms involved in the adaptive selection of these pigments, we have to identify amino acid changes of pigments that are potentially important in changing the wavelength of maximal absorption (lambda max) and then determine the effects of these mutations on the shift in lambda max. The desired mutants can be constructed using site-directed mutagenesis, expressed in tissue culture cells, and the functional effect of virtually any such mutant can be rigorously determined. The availability of these cell/molecular methods makes vision an ideal model system in studying adaptive mechanisms at the molecular level. The identification of potentially important amino acid changes using evolutionary biological means is an indispensable step in elucidating the molecular mechanisms that underlie the spectral tuning of visual pigments.

Functional characterization of visual and nonvisual pigments of American chameleon (Anolis carolinensis)

Using only 11-cis 3, 4-dehydroretinal as a chromophore in the pure-cone retina, American chameleon (Anolis carolinensis) detects a wide range of color from ultraviolet (UV) to infrared. We previously characterized its visual opsin genes sws1Ac, sws2Ac, rh1Ac, rh2Ac, and LwsAc that encode SWS1Ac, SWS2Ac, RH1Ac, RH2Ac, and LWSAc opsins, respectively, and the pineal gland-specific opsin (PAc) gene. Here we present the light absorption profiles of the visual pigments obtained by expressing these opsins and reconstituting them with 11-cis retinal using the COS1 cell cDNA expression system. The purified SWS1Ac, SWS2Ac, RH1Ac, RH2Ac, LWSAc, and PAc pigments have the wavelengths of maximal absorption at 358, 437, 491, 495, 560, and 482 nm, respectively. SWS1Ac is the first vertebrate UV opsin whose spectral sensitivity has been directly evaluated. RH1 pigments, orthologous to the rod pigments of other vertebrates, are sensitive to hydroxylamine in the dark, exhibiting a cone pigment-like characteristic, probably reflecting their adaptation to the pure cone retina. Interestingly, the blue-sensitive SWS2Ac pigment shows an exceptionally low level of sensitivity to hydroxylamine, possessing a rod pigment-like characteristic.

Cloning and characterization of the pineal gland-specific opsin gene of marine lamprey (Petromyzon marinus)

We have cloned and sequenced the pineal gland-specific opsin (P opsin) gene, p(Pm), of marine lamprey, P. marinus. The deduced P opsin, P(Pm), consisting of 444 aa, is about 90 aa longer than the orthologous opsins of chicken, pigeon, and American chameleon. The evolution of P(Pm) is characterized not only by a significantly higher rate of aa replacement than the other P opsins but also by a larger number of unique aa replacements at highly conserved sites in the nonTM region. These changes in P(Pm) might have been important in achieving marine lamprey-specific interaction between P(Pm) and other proteins in the pineal gland.

Cloning and expression of the red visual pigment gene of goat (Capra hircus)

We have cloned and sequenced the red opsin gene, rCh, of goat (Capra hircus). When rCh is expressed in cultured cells and reconstituted with 11-cis retinal, the resulting visual pigment has a wavelength of maximal absorption (lambda max) of 553 nm. This result and the deduced aa sequence of the goat red opsin suggest that the lambda max values of the red pigments of goat and human are based mainly on Y277 and T285. The slightly lower lambda max value of the red pigment in goat than in human (approximately 560 nm) can be explained by a single aa difference between the goat (A180) and human (S180) red pigments.

Expression of visual and nonvisual opsins in American chameleon

We previously characterized five visual opsin genes of American chameleon (Anolis carolinensis). Here we report its nonvisual opsin gene orthologous to the chicken pineal gland-specific opsin (p-opsin) gene. In the pure-cone American chameleon retina, all visual opsins including rod opsin are expressed. In both pineal and parietal eye, three visual opsins as well as P-opsin are expressed. Although opsins are detected in the pineal glands of a wide variety of vertebrates, Southern analysis suggests that the P-opsin gene is used mainly by birds and reptiles.

Threonine-89 participates in the active site of bacteriorhodopsin: evidence for a role in color regulation and Schiff base proton transfer

Bacteriorhodopsin (bR) functions as a light-driven proton pump in the purple membrane of Halobacterium salinarium. A major feature of bR is the existence of an active site which includes a retinylidene Schiff base and amino acid residues Asp-85, Asp-212, and Arg-82. This active site participates in proton transfers and regulates the visible absorption of bacteriorhodopsin and its photointermediates. In this work we find evidence that Thr-89 also participates in this active site. The substitution Thr-89 --> Asn (T89N) results in changes in the properties of the all-trans retinylidene chromophore of light-adapted bR including a redshift of the visible lambda(max) and a downshift in C=N and C=C stretch frequencies. Changes are also found in the M and N intermediates of the T89N photocycle including shifts in lambda(max), a downshift of the Asp-85 carboxylic acid C=O stretch frequency by 10 cm(-1), and a 3-5-fold decrease in the rate of formation of the M intermediate. In contrast, the properties of the 13-cis retinylidene chromophore of dark-adapted T89N as well as the K and L intermediates of the T89N photocycle are similar to the wild-type bacteriorhodopsin. These results are consistent with an interaction of the hydroxyl group of Thr-89 with the protonated Schiff base of light-adapted bR and possibly the N intermediate but not the 13-cis chromophore of dark-adapted bR or the K and L intermediates. Thr-89 also appears to influence the rate of Schiff base proton transfer to Asp-85 during formation of the M intermediate, possibly through an interaction with Asp-85. In contrast, the hydroxyl group of Thr-89 is not obligatory for proton transfer from Asp-96 to the Schiff base during formation of the N intermediate.

Molecular evolution of the rhodopsin gene of marine lamprey, Petromyzon marinus

Visual pigment genes have been isolated from a marine lamprey, Petromyzon marinus. We report here the rhodopsin gene, spanning 21.2 kb from start to stop codons, making it the longest opsin gene known in vertebrates. Southern analysis suggests that the lamprey genome contains a single rhodopsin gene. The amino acid (aa) sequence deduced from this gene has 92% sequence similarity with that of the river lamprey rhodopsin. The data reveal that aa substitutions occurred more often in the transmembrane region than in the non-transmembrane region, possibly reflecting functional adaptation of the rhodopsin during the last 500 million years of the jawless fish evolution.

Activation of transducin by a Xenopus short wavelength visual pigment

Phototransduction in cones differs significantly from that in rods in sensitivity, kinetics, and recovery following exposure to light. The contribution that the visual pigment makes in determining the cone response was investigated biochemically by expressing a Xenopus violet cone opsin (VCOP) cDNA in COS1 cells and assaying the light-dependent activation of transducin. Light-exposed VCOP stimulated [35S]guanosine 5'-(gamma-thio)triphosphate nucleotide exchange on bovine rod transducin in a time-dependent manner with a half-time for activation of 0.75 min, similar to that of bovine rhodopsin. In exhaustive binding assays, VCOP and rhodopsin activity showed similar concentration dependence with half-maximal activation occurring at 0.02 mol of pigment/mol of transducin. Although VCOP was able to activate as many as 12 transducins per photoisomerization, rhodopsin catalyzed significantly more. When assays were performed with lambda > 420 nm illumination, VCOP exhibited rapid regeneration and high affinity for the photoregenerated 11-cis-retinal. Recycling of the chromophore and reactivation of the pigment resulted in multiple activations of transducin, whereas a maximum of 1 transducin per VCOP was activated under brief illumination. The decay of the active species formed following photobleaching was complete in <5 min, approximately 10-fold faster than that of rhodopsin. In vitro, VCOP activated rod transducin with kinetics and affinity similar to those of rhodopsin, but the active conformation decayed more rapidly and the apoprotein regenerated more efficiently with VCOP than with rhodopsin. These properties of the violet pigment may account for much of the difference in response kinetics between rods and cones.

Phylogenetic relationships among short wavelength-sensitive opsins of American chameleon (Anolis carolinensis) and other vertebrates

The vertebrate opsins have been classified into four major phylogenetic groups. One of them, a short wavelength-sensitive (SWS) opsin group, is further divided into two subgroups, SWS-I and SWS-II, having the wavelengths of maximal absorption of about 420 and 450 nm, respectively. Here we report the DNA sequences of the SWS-I and SWS-II genes from the lizard Anolis carolinensis. The shorter wavelengths of absorption by the two SWS subgroup opsins seem to be achieved by different sets of amino acid replacements in the transmembrane regions.

Spectral tuning and molecular evolution of rod visual pigments in the species flock of cottoid fish in Lake Baikal

Lake Baikal in Eastern Siberia is the deepest and one of the largest and most ancient lakes in the world. However, even in the deepest regions, oxygenation levels do not fall below 75-80% of the surface levels. This has enabled a remarkable flock of largely endemic teleost fish of the sub-order Cottoidei to colonize all depth habitats. We have previously shown that species that occupy progressively deeper habitats show a blue shift in the peak wavelength of absorbance (lambda max) of both their rod and cone visual pigments; for the rod pigments, a number of stepwise shifts occur from about 516 nm in littoral species to about 484 nm in abyssal species. By sequencing the rod opsin gene from 11 species of Baikal cottoids that include representatives from all depth habitats, we have been able to identify four amino acid substitutions that would account for these shifts. The effect of each substitution on lambda max is approximately additive and each corresponds to a particular lineage of evolution.