Primary structure of frog rhodopsin

Evolutionary analyses of visual opsin genes in frogs and toads: Diversity, duplication, and positive selection

Among major vertebrate groups, anurans (frogs and toads) are understudied with regard to their visual systems, and little is known about variation among species that differ in ecology. We sampled North American anurans representing diverse evolutionary and life histories that likely possess visual systems adapted to meet different ecological needs. Using standard molecular techniques, visual opsin genes, which encode the protein component of visual pigments, were obtained from anuran retinas. Additionally, we extracted the visual opsins from publicly available genome and transcriptome assemblies, further increasing the phylogenetic and ecological diversity of our dataset to 33 species in total. We found that anurans consistently express four visual opsin genes (RH1, LWS, SWS1, and SWS2, but not RH2) even though reported photoreceptor complements vary widely among species. The proteins encoded by these genes showed considerable sequence variation among species, including at sites known to shift the spectral sensitivity of visual pigments in other vertebrates and had conserved substitutions that may be related to dim-light adaptation. Using molecular evolutionary analyses of selection (dN/dS) we found significant evidence for positive selection at a subset of sites in the dim-light rod opsin gene RH1 and the long wavelength sensitive cone opsin LWS. The function of sites inferred to be under positive selection are largely unknown, but a few are likely to affect spectral sensitivity and other visual pigment functions based on proximity to previously identified sites in other vertebrates. We also found the first evidence of visual opsin duplication in an amphibian with the duplication of the LWS gene in the African bullfrog, which had distinct LWS copies on the sex chromosomes suggesting the possibility of sex-specific visual adaptation. Taken together, our results indicate that ecological factors, such as habitat and life history, as well as behavior, may be driving changes to anuran visual systems.

New views on phototransduction from atomic force microscopy and single molecule force spectroscopy on native rods

By combining atomic force microscopy (AFM) imaging and single-molecule force spectroscopy (SMFS), we analyzed membrane proteins of the rod outer segments (OS). With this combined approach we were able to study the membrane proteins in their natural environment. In the plasma membrane we identified native cyclic nucleotide-gated (CNG) channels which are organized in single file strings. We also identified rhodopsin located both in the discs and in the plasma membrane. SMFS reveals strikingly different mechanical properties of rhodopsin unfolding in the two environments. Molecular dynamic simulations suggest that this difference is likely to be related to the higher hydrophobicity of the plasma membrane, due to the higher cholesterol concentration. This increases rhodopsin mechanical stability lowering the rate of transition towards its active form, hindering, in this manner, phototransduction.

Differences in the pharmacological activation of visual opsins

Opsins, like many other G-protein-coupled receptors, sustain constitutive activity in the absence of ligand. In partially bleached rods and cones, opsin's activity closes cGMP-gated channels and produces a state of "pigment adaptation" with reduced sensitivity to light and accelerated flash response kinetics. The truncated retinal analogue, beta-ionone, further desensitizes partially bleached green-sensitive salamander rods, but enables partially bleached red-sensitive cones to recover dark-adapted physiology. Structural differences between rod and cone opsins were proposed to explain the effect. Rods and cones, however, also contain different transducins, raising the possibility that G-protein type determines the photoreceptor-specific effects of beta-ionone. To test the two hypotheses, we applied beta-ionone to partially bleached blue-sensitive rods and cones of salamander, two cells that couple the same cone-like opsin to either rod or cone transducin, respectively. Immunocytochemistry confirmed that all salamander rods contain one form of transducin, whereas all cones contain another. beta-Ionone enhanced pigment adaptation in blue-sensitive rods, but it also did so in blue- and UV-sensitive cones. Furthermore, all recombinant salamander rod and cone opsins, with the exception of the red-sensitive cone opsin, activated rod transducin upon the addition of beta-ionone. Thus opsin structure determines the identity of beta-ionone as an agonist or an inverse agonist and in that respect distinguishes the red-sensitive cone opsin from all others.

Rhodopsin C terminus, the site of mutations causing retinal disease, regulates trafficking by binding to ADP-ribosylation factor 4 (ARF4)

The maintenance of photoreceptor cell polarity is compromised by the rhodopsin mutations causing the human disease autosomal dominant retinitis pigmentosa. The severe form mutations occur in the C-terminal sorting signal of rhodopsin, VXPX-COOH. Here, we report that this sorting motif binds specifically to the small GTPase ARF4, a member of the ARF family of membrane budding and protein sorting regulators. The effects of blocking ARF4 action were functionally equivalent to the effects of blocking the rhodopsin C-terminal sorting signal. ARF4 was essential for the generation of post-Golgi carriers targeted to the rod outer segments of retinal photoreceptors. Thus, the severe retinitis pigmentosa alleles that affect the rhodopsin sorting signal interfere with interactions between ARF4 and rhodopsin, leading to aberrant trafficking and initiation of retinal degeneration.

Evaluation of the 17-kDa prenyl-binding protein as a regulatory protein for phototransduction in retinal photoreceptors

The mammalian rod photoreceptor phosphodiesterase (PDE6) holoenzyme is isolated in both a membrane-associated and a soluble form. Membrane binding is a consequence of prenylation of PDE6 catalytic subunits, whereas soluble PDE6 is purified with a 17-kDa prenyl-binding protein (PDEdelta) tightly bound. This protein, here termed PrBP/delta, has been hypothesized to reduce activation of PDE6 by transducin, thereby desensitizing the photoresponse. To test the potential role of PrBP/delta in regulating phototransduction, we examined the abundance, localization, and potential binding partners of PrBP/delta in retina and in purified rod outer segment (ROS) suspensions whose physiological and biochemical properties are well characterized. The amphibian homologue of PrBP/delta was cloned and sequenced and found to have 82% amino acid sequence identity with mammalian PrBP/delta. In contrast to bovine ROS, all of the PDE6 in purified frog ROS is membrane-associated. However, addition of recombinant frog PrBP/delta can solubilize PDE6 and prevent its activation by transducin. PrBP/delta also binds other prenylated photoreceptor proteins in vitro, including opsin kinase (GRK1/GRK7) and rab8. Quantitative immunoblot analysis of the PrBP/delta content of purified ROS reveals insufficient amounts of PrBP/delta (<0.1 PrBP/delta per PDE6) to serve as a subunit of PDE6 in either mammalian or amphibian photoreceptors. The immunolocalization of PrBP/delta in frog and bovine retina shows greatest PrBP/delta immunolabeling outside the photoreceptor cell layer. Within photoreceptors, only the inner segments of frog double cones are strongly labeled, whereas bovine photoreceptors reveal more PrBP/delta labeling near the junction of the inner and outer segments (connecting cilium) of photoreceptors. Together, these results rule out PrBP/delta as a PDE6 subunit and implicate PrBP/delta in the transport and membrane targeting of prenylated proteins (including PDE6) from their site of synthesis in the inner segment to their final destination in the outer segment of rods and cones.

Recreating a functional ancestral archosaur visual pigment

The ancestors of the archosaurs, a major branch of the diapsid reptiles, originated more than 240 MYA near the dawn of the Triassic Period. We used maximum likelihood phylogenetic ancestral reconstruction methods and explored different models of evolution for inferring the amino acid sequence of a putative ancestral archosaur visual pigment. Three different types of maximum likelihood models were used: nucleotide-based, amino acid-based, and codon-based models. Where possible, within each type of model, likelihood ratio tests were used to determine which model best fit the data. Ancestral reconstructions of the ancestral archosaur node using the best-fitting models of each type were found to be in agreement, except for three amino acid residues at which one reconstruction differed from the other two. To determine if these ancestral pigments would be functionally active, the corresponding genes were chemically synthesized and then expressed in a mammalian cell line in tissue culture. The expressed artificial genes were all found to bind to 11-cis-retinal to yield stable photoactive pigments with lambda(max) values of about 508 nm, which is slightly redshifted relative to that of extant vertebrate pigments. The ancestral archosaur pigments also activated the retinal G protein transducin, as measured in a fluorescence assay. Our results show that ancestral genes from ancient organisms can be reconstructed de novo and tested for function using a combination of phylogenetic and biochemical methods.

A structural role for Asp83 in the photoactivation of rhodopsin

Asp83 is a highly conserved residue in the second transmembrane domain of visual pigments and many members of other G protein-coupled receptor subfamilies. Upon illumination, the rod visual pigment rhodopsin proceeds through various intermediate states (Batho<-->BSI<-->Lumi<-->Meta I<-->Meta II). Meta II represents the active state of rhodopsin, which binds and activates the G protein transducin. Evidence has been presented that Asp83 participates in the formation of Meta II and undergoes a change in H-bonding. To investigate whether this role of Asp83 requires its proton-donating capacity and/or its H-bonding capability, we constructed the mutants D83C and D83N. Both mutants appear to effectively activate transducin, indicating that Asp83 is not essential for signal transduction. Differential effects of the mutations D83C and D83N are observed in the spectral properties and the pH sensitivity of the Meta I-->Meta II transition. In general, D83C behaves much more like wild-type than D83N. We conclude that the structural role of Asp83 also involves the acidic nature of its carboxyl group. In addition, the participation in Meta II formation of Cys83 in D83C manifests itself as a change in the vibrational properties of the sulfhydryl group, demonstrating that the -SH group can be used as a non-invasive probe for local structural changes.

A simple method for classifying genes and a bootstrap test for classifications

A new simple method for classifying genes is proposed based on Klastorin's method. This method classifies genes into monophyletic groups which are made distinct from each other by evolutionary changes. The method is applicable as long as the phylogenetic tree of genes is obtained. There is a fast algorithm for obtaining the classification. A bootstrap test of a classification is also presented. As an example, we classified opsin genes. The classification obtained by this method is the same as the previous classification based on the function of opsins.

Molecular evolution of vertebrate visual pigments

Dramatic improvement of our understanding of the genetic basis of vision was brought by the molecular characterization of the bovine rhodopsin gene and the human rhodopsin and color opsin genes (Nathans and Hogness, 1983; Nathans et al., 1984, 1986a,b). The availability of cDNA clones from these studies has facilitated the isolation of retinal and nonretinal opsin genes and cDNA clones from a large variety of species. Today, the number of genomic and cDNA clones of opsin genes isolated from different vertebrate species exceeds 100 and is increasing rapidly. The opsin gene sequences reveal the importance of the origin and differentiation of various opsins and visual pigments. To understand the molecular genetic basis of spectral tuning of visual pigments, it is essential to establish correlations between a series of the sequences of visual pigments and their lambda(max) values. The potentially important amino acid changes identified in this way have to be tested whether they are in fact responsible for the lambda(max)-shifts using site-directed mutagenesis and cultured cells. A major goal of molecular evolutionary genetics is to understand the molecular mechanisms involved in functional adaptations of organisms to different environments, including the mechanisms of the regulation of the spectral absorption. Therefore, both molecular evolutionary analyses of visual pigments and vision science have an important common goal.

Regulation of sorting and post-Golgi trafficking of rhodopsin by its C-terminal sequence QVS(A)PA

Several mutations that cause severe forms of the human disease autosomal dominant retinitis pigmentosa cluster in the C-terminal region of rhodopsin. Recent studies have implicated the C-terminal domain of rhodopsin in its trafficking on specialized post-Golgi membranes to the rod outer segment of the photoreceptor cell. Here we used synthetic peptides as competitive inhibitors of rhodopsin trafficking in the frog retinal cell-free system to delineate the potential regulatory sequence within the C terminus of rhodopsin and model the effects of severe retinitis pigmentosa alleles on rhodopsin sorting. The rhodopsin C-terminal sequence QVS(A)PA is highly conserved among different species. Peptides that correspond to the C terminus of bovine (amino acids 324-348) and frog (amino acids 330-354) rhodopsin inhibited post-Golgi trafficking by 50% and 60%, respectively, and arrested newly synthesized rhodopsin in the trans-Golgi network. Peptides corresponding to the cytoplasmic loops of rhodopsin and other control peptides had no effect. When three naturally occurring mutations: Q344ter (lacking the last five amino acids QVAPA), V345M, and P347S were introduced into the frog C-terminal peptide, the inhibitory activity of the peptides was no longer detectable. These observations suggest that the amino acids QVS(A)PA comprise a signal that is recognized by specific factors in the trans-Golgi network. A lack of recognition of this sequence, because of mutations in the last five amino acids causing autosomal dominant retinitis pigmentosa, most likely results in abnormal post-Golgi membrane formation and in an aberrant subcellular localization of rhodopsin.

Rhodopsins from three frog and toad species: sequences and functional comparisons

The frequency of thermal 'dark events' in the membrane current of rhodopsin rods of the bullfrog, Rana catesbeiana, is considerably lower than observed in rods of two toad species, even though all three rhodopsins have approximately the same absorbance characteristics. In order to map amino acid substitutions possibly associated with thermal stability in the genus Rana, the cDNA's coding for the rhodopsins of Bufo bufo, B. marinus and R. temporaria were sequenced and the conceptually translated protein sequences aligned to the previously sequenced rhodopsins of R. catesbeiana, R. pipiens and Xenopus laevis. Across the six anuran species studied, there are sixteen non-conserved substitutions and six changes that include gain or loss of a hydroxyl group. Serine or threonine at position 220 is unique to the three Rana species, phenylalanine at position 270 is unique to all three Ranas and to X. laevis, and phenylalanine at position 274 is unique to both species of the genus Bufo. This investigation produces a list of substitutions that are candidates for future studies of thermal stability. In addition, a number of amino acids are identified that apparently do not influence absorbance characteristics, at least not cumulatively.

Retinol kinetics in the isolated retina determined by retinoid extraction and HPLC

Suzuki et al. [Vis. Res. 26, 425-9 (1986); Vis. Res. 28, 1061-70 (1988)] have described a formaldehyde-based (HCHO-based) extraction procedure that efficiently recovers 11-cis retinal initially present as rhodopsin chromophore in photoreceptor membranes. Using the isolated retina of the toad (Bufo marinus), we tested whether this procedure ('HCHO' method), in combination with a formaldehyde-free extraction procedure ('i/h' method) and the analysis of extracted retinoids by high performance liquid chromatography (HPLC), can account quantitatively for light-induced changes in retinoid levels and thus serve as an alternative to spectrophotometry for tracking the formation of all-trans retinol in this intact rod preparation. Initially dark-adapted retinas were incubated in bright light or in darkness and then analysed by homogenization and extraction using the HCHO and i/h methods. Combined data obtained using the two extraction procedures indicated a near-conservation of total retinoid recovered from dark-incubated and illuminated retinas, and thus accounted for light-induced changes in retinoid levels. The HCHO procedure, employing formaldehyde, isopropanol and hexane, was similar to that described by Suzuki et al. and recovered retinaldehydes including chromophoric 11-cis retinal. The i/h procedure utilized isopropanol and hexane and, unlike the HCHO method, efficiently recovered all-trans retinol. Illumination (onset at time zero) that produced an approximately exponential decline of 11-cis retinal (time constant of 24 s) led to an increase and then a gradual decline in all-trans retinal. The normalized peak level of all-trans retinal, representing about 0.54 of the total molar quantity of recovered retinoid, developed with illumination periods of 10-80 s. The normalized level of all-trans retinol reached approximately 0.3 in retinas illuminated for 1 min and, with longer illuminations (up to 30 min), exhibited an approximately exponential further growth to approximately 0.9 with a time constant of 9.2 min. The results indicate the workability of the HCHO and i/h extraction procedures for tracking the in situ conversion of all-trans retinal to all-trans retinol, a reaction thought to be important for both operation of the retinoid visual cycle and shut-off of the phototransduction cascade.

A second type of rod opsin cDNA from the common carp (Cyprinus carpio)

A second type of rhodopsin cDNA from carp (cRh-II) shared 97.2% polynucleotide identity with the previously reported cRh-I. The deduced amino acid sequences of cRh-I and cRh-II exhibited 98.6% identity. The key difference between these two types of cRh is that valine at position 169 of cRh-I was replaced by glutamic acid in cRh-II. Southern blot analysis of genomic DNA showed that there were two types of cRh gene. These two rod opsin genes were proven to be expressed in carp retinas by using RT-PCR with type-specific primers.

Point mutations in bovine opsin can be classified in four groups with respect to their effect on the biosynthetic pathway of opsin

Expression in vitro with the recombinant baculovirus expression system showed correct biosynthesis and post-translational processing of "wild-type' bovine opsin with regard to translocation, glycosylation, palmitoylation and targeting. However, several of these processes were severely affected by point mutations. From the overall results of 16 mutants reported here, four groups were distinguished. One group significantly affected neither biosynthesis nor folding of opsin (D83N, P291A, A299C-V300A-P303G). A second group produced a truncated protein (R69H, Y301F), suggesting that these positions are essential for a correct translational process. A third group affected membrane translocation as well as glycosylation, which can be interpreted as interference with the function of a transfer signal. Substitutions at positions Glu-113, Glu-122, Glu-134, Arg-135 and Lys-248 belong to this category. A fourth group induced structural changes in the protein that led to heterogeneous distribution in the plasma membrane (E113Q/D, W265F, Y268S). Taking any functional consequences of these mutations into consideration, it seems that point mutations can have mosaic effects and therefore should be examined at several levels (folding, targeting, functional parameters).

Absorbance spectra and molecular structure of the blue-sensitive rod visual pigment in the conger eel (Conger conger)

The conger eel (Conger conger) is a nocturnal fish that can be found living in shallow coastal water and deep water down to 1000 m. The conger eel has a pure rod retina with a visual pigment maximally sensitive to blue light around 487 nm. We have cloned and sequenced the opsin cDNA which is presumed to code for this visual pigment and have found it to be highly homologous to the form of opsin that is expressed in mature, deep-living European eels (Anguilla anguilla). The opsin sequence information presented here provides additional evidence that specific amino acid sites are involved in the spectral tuning of this class of blue-sensitive visual pigments.

Cytoplasmic domain of rhodopsin is essential for post-Golgi vesicle formation in a retinal cell-free system

In retinal photoreceptors, highly polarized organization of the light-sensitive organelle, the rod outer segment, is maintained by the sorting of rhodopsin and its associated proteins into distinct post-Golgi vesicles that bud from the trans-Golgi network (TGN) and by their vectorial transport toward the rod outer segment. We have developed an assay that reconstitutes the formation of these vesicles in a retinal cell-free system. Vesicle formation in this cell-free assay is ATP-, GTP-, and cytosol-dependent. In frog retinas vesicle budding also proceeds at 0 degrees C, both in vivo and in vitro. Vesicles formed in vitro are indistinguishable from the vesicles formed in vivo by their buoyant density, protein composition, topology, and morphology. In addition to the previously identified G-proteins, these vesicles also contain rab11. Concurrently with vesicle budding, resident proteins are retained in the TGN. Collectively these data suggest that rhodopsin and its associated proteins are sorted upon exit from the TGN in this cell-free system. Removal of membrane-bound GTP-binding proteins of the rab family by rab GDP dissociation inhibitor completely abolishes formation of these vesicles and results in the retention of rhodopsin in the Golgi. A monoclonal antibody to the cytoplasmic (carboxy-terminal) domain of rhodopsin and its Fab fragments strongly inhibit vesicle formation and arrest newly synthesized rhodopsin in the TGN rather than the Golgi. Therefore rhodopsin sorting at the exit from the TGN is mediated by the interaction of its cytoplasmic domain with the intracellular sorting machinery.

The molecular basis for the green-blue sensitivity shift in the rod visual pigments of the European eel

When the European eel matures sexually and migrates back to deep sea breeding grounds the visual pigments in its rod photoreceptors change from being maximally sensitive to green light to being maximally sensitive to blue light. In part, this change in sensitivity is due to a change in the opsin component of the visual pigment molecule. We used hormone injection to induce these developmental changes in a group of eels and from these animals an opsin coding region was cloned and sequenced using cDNA made from retinal mRNA. From the retinae of hormone-injected eels and those not injected with hormones, distinct opsin mRNAs were isolated. These mRNAs encode two rod opsin proteins that are very similar but have significant amino acid substitutions in key positions that are likely to be involved in spectral tuning of the eel green and blue sensitive rod visual pigment molecules.

Projection structure of frog rhodopsin in two crystal forms

Rhodopsin is the G protein-coupled receptor that upon light activation triggers the visual transduction cascade. Rod cell outer segment disc membranes were isolated from dark-adapted frog retinas and were extracted with Tween detergents to obtain two-dimensional rhodopsin crystals for electron crystallography. When Tween 80 was used, tubular structures with a p2 lattice (a = 32 A, b = 83 A, gamma = 91 degrees) were formed. The use of a Tween 80/Tween 20 mixture favored the formation of larger p22(1)2(1) lattices (a = 40 A, b = 146 A, gamma = 90 degrees). Micrographs from frozen hydrated frog rhodopsin crystals were processed, and projection structures to 7-A resolution for the p22(1)2(1) form and to 6-A resolution for the p2 form were calculated. The maps of frog rhodopsin in both crystal forms are very similar to the 9-A map obtained previously for bovine rhodopsin and show that the arrangement of the helices is the same. In a tentative topographic model, helices 4, 6, and 7 are nearly perpendicular to the plane of the membrane. In the higher-resolution projection maps of frog rhodopsin, helix 5 looks more tilted than it appeared previously. The quality of the two frog rhodopsin crystals suggests that they would be suitable to obtain a three-dimensional structure in which all helices would be resolved.

Molecular basis for tetrachromatic color vision

Determination of the primary structures of six kinds of vertebrate visual pigments enabled us to classify them into four groups of cone-type pigments. The phylogenetic tree demonstrated that an ancestor of vertebrate visual pigments evolved into four kinds of cone-type pigments, from one of which rhodopsins diverged. Tetrachromatic color vision of chicken is discussed on the basis of both the absorption spectra of purified cone pigments and the filtering effect of colored oil-droplets.

Effect of carboxyl mutations on functional properties of bovine rhodopsin

Bovine rod rhodopsin and membrane-carboxyl group mutants are expressed using the recombinant baculovirus expression system. Biosynthesis of wild-type and the mutant D83N is normal. The mutations E122L and E134D/R affect glycosylation and translocation. After regeneration, purification and reconstitution in retina lipids a wild-type photosensitive pigment with spectral and photolytic properties identical to native bovine rod rhodopsin is generated. Only the mutations D83N and E122L affect the spectral properties and then only slightly. All mutations induce a shift in the Meta I<==>Meta II equilibrium towards Meta I (E134D/R) or Meta II (D83N, E122L). FT-IR analysis shows that the mutation E134D/R does not significantly affect the carboxyl-vibration region but, in particular in the case of E134R, affects secondary structural changes upon Meta II formation. E122L also has an effect on secondary structural changes and in addition eliminates a negative band at 1728 cm-1. The mutation D83N removes a pair of negative/positive bands from the carboxyl-vibration region, indicating that Asp83 stays protonated upon formation of Meta II but undergoes a change in hydrogen bonding.

Cloning and expression of frog rhodopsin cDNA

The cDNA encoding the putative rhodopsin of frog (Rana catesbeiana) was cloned and expressed in cultured cells. The deduced amino acid sequence (354 residues) has more than 90% identity with the rhodopsins of two other frogs (Rana pipiens and Xenopus laevis) and 80% identity with other vertebrate rhodopsins. The isoelectric point calculated from the sequence was about 8.2, which is intermediate between rhodopsins and the cone visual pigments of higher vertebrates. The cloned cDNA was expressed in cultured mammalian cells. The difference absorbance maximum before and after photobleaching was about 500 nm, the same as that observed in the retina, demonstrating that the cloned cDNA does indeed encode functional rhodopsin.

The primary structure of mantid opsin

The sequence encoding opsin from the mantid Sphodromantis sp. has been determined by dideoxynucleotide sequencing of PCR products from a cDNA derived from eye cup tissue. The 376-amino-acid (aa) residues show approx. 56% identity and 85% similarity to known insect opsins (Drosophila melanogaster and Calliphora erythrocephala). The predicted protein structure, based on the hydropathy profile and placement of key aa residues, reveals a seven-transmembrane structure typical of a rhodopsin. Unlike the previously characterised insect visual pigments which have 3-hydroxy retinal in their binding sites, mantid rhodopsin contains 11-cis retinal. Comparison of transmembrane sequences from the opsin family was performed in order to identify any specific aa substitutions which are able to account for the selection of retinal or its 3-hydroxy derivative by insect opsins.

Amino acid sequence surrounding the retinal-binding site in retinochrome of the squid, Todarodes pacificus

Squid (Todarodes pacificus) retinochrome was reduced to N-retinyl protein with borane dimethylamine and cleaved by CNBr. The retinyl peptide was then isolated by chromatography while being monitored for absorbances at 215 and 330 nm, and the N-terminal amino acid sequence was determined to be Ser-Lys-Thr-Gly-X-Ala-Leu-Phe-Pro. This sequence was the same that we had observed at the 7th transmembrane domain of retinochrome whose structure was reported previously. During Edman degradation of the retinyl peptide, the yield of the PTH-lysine at the second cycle was lower than those of the other PTH-amino acids, proving that the lysine residue forms a Schiff's base with retinal (Lys-275 in retinochrome). The amino acid sequence surrounding the retinal-binding lysine in retinochrome greatly differed from those in a variety of known visual pigments. This fact would be associated with the difference in the photoisomerization of chromophore between retinochrome and rhodopsin. The protein structure of retinochrome is also compared with that of rhodopsin in Todarodes.