Rhodopsin (RHO) in Sepia officinalis is a rhabdomeric-type (r-opsin) visual photopigment that functions as a G protein-coupled receptor activated by light. Also referred to as Sof_r-opsin1 in the literature (PMID:33117186), it binds 11-cis-retinal via a Schiff base at Lys305 and, upon photon absorption, isomerizes to all-trans-retinal triggering a Gq-mediated phototransduction cascade involving phospholipase C activation and TRP channel opening, resulting in photoreceptor cell depolarization. Originally characterized from retinal tissue (PMID:9662500), S. officinalis rhodopsin has a characteristic proline-rich C-terminus shared by cephalopod rhodopsins. The retinal rhodopsin has a lambda-max of approximately 492 nm (blue-green), and S. officinalis is monochromatic with a single retinal opsin, compensating for color-blindness with polarization sensitivity via orthogonal microvillar arrangements (PMID:20392722). Retinochrome works in tandem with rhodopsin to regenerate 11-cis-retinal from all-trans-retinal, sustaining continuous phototransduction (PMID:33117186). Critically, rhodopsin transcripts are also expressed in dermal tissues including chromatophores of S. officinalis skin (PMID:25994635, PMID:20392722), suggesting a dual role in both ocular vision and extraocular/dermal photoreception. Functional evidence from the closely related Octopus bimaculoides demonstrates that excised skin undergoes light-activated chromatophore expansion (LACE) via the same r-opsin phototransduction cascade used in eyes, with an action spectrum lambda-max of 480 nm matching retinal rhodopsin (PMID:25994633). Rhodopsin and associated phototransduction components are expressed across diverse cephalopod non-eye tissues including skin, suckers, arm ganglia, fin muscles, and even optic lobes (PMID:26351853, PMID:34571813, PMID:40511715), establishing a widespread distributed photosensory system that likely contributes to camouflage and body patterning. Comparative genomics across 80 mollusk genomes shows cephalopods have the fewest opsins of any mollusk lineage (~5 per species), having lost ciliary opsins and Go-opsins entirely (PMID:38039155). S. officinalis has 6 opsin genes including a second rhabdomeric opsin paralog (r-opsin2), a xenopsin, and two retinochrome genes (PMID:33117186). Rhodopsin expression in the developing eye begins at embryonic stage 23 and increases significantly through stage 28, coinciding with the onset of light perception in embryos (PMID:33117186).
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004930 G protein-coupled receptor activity | IEA GO_REF:0000002 | ACCEPT | Summary: Rhodopsin is the canonical invertebrate rhabdomeric G protein-coupled receptor. The InterPro-based IEA annotation is strongly supported: S. officinalis rhodopsin belongs to the opsin subfamily of GPCRs (InterPro IPR000276, IPR001760) and functions by activating Gq-alpha upon light-induced retinal isomerization (PMID:9662500). UniProt function annotation states it "activates signaling via G-proteins" and that "Signaling mediates the activation of phospholipase C." Reason: This is a core molecular function of rhodopsin. As a 7-transmembrane rhabdomeric opsin, it couples to Gq-alpha to initiate the invertebrate phototransduction cascade. The InterPro domain match (IPR000276) and CDD classification (cd15337, 7tmA_Opsin_Gq_invertebrates) unambiguously place this protein in the GPCR superfamily. Experimental evidence from related cephalopods confirms Gq-alpha coupling (PMID:25994635). Biochemical studies in squid demonstrated that light-activated rhodopsin catalyzes GTP binding to a 42 kDa Gq protein, and a ~130 kDa PLC was purified and shown to be activated by Gq (PMID:1445212, PMID:9636052). Supporting Evidence: PMID:9662500 The cDNA sequence of the rhodopsin gene of the cuttlefish Sepia officinalis (L.) (Sub-class Coleoidea, Order Sepiida) is presented, together with an analysis of the structure of the gene. PMID:25994635 Rhodopsin, retinochrome and Gqalpha transcripts were also found in RNA extracts from dissociated chromatophores isolated from D. pealeii dermal tissues. PMID:1445212 Photoaffinity labelling by a GTP analogue has been used to identify a 42 kDa band as the major G alpha subunit in squid photoreceptor membranes, recently identified by partial sequence analysis to be a member of the Gq sub-group of GTP-binding proteins PMID:9636052 Invertebrate visual signal transduction is initiated by rhodopsin activation of a guanine nucleotide binding protein, Gq, which stimulates phospholipase C (PLC) activity. |
| GO:0007186 G protein-coupled receptor signaling pathway | IEA GO_REF:0000002 | MODIFY | Summary: GPCR signaling pathway is correct but too general. GO has a precise term that captures both the GPCR and phototransduction aspects: GO:0030265 (phospholipase C-activating opsin-mediated signaling pathway), defined as "Gq-mediated activation of phospholipase C... Typical examples are rhabdomeric photoreceptors in the eyes of protostomes." This term is a descendant of BOTH GO:0007186 (GPCR signaling) AND GO:0007602 (phototransduction), making it the most informative single BP annotation for cephalopod rhodopsin. S. officinalis rhodopsin signals through Gq->PLC->PIP2->IP3/DAG->TRP, which is precisely what GO:0030265 describes (PMID:1445212, PMID:9636052). Reason: GO:0030265 (phospholipase C-activating opsin-mediated signaling pathway) is a child of both GPCR signaling (GO:0007186) and phototransduction (GO:0007602), capturing both aspects in a single specific term. Its definition explicitly describes the Gq-PLC-PIP2 rhabdomeric cascade used by protostome photoreceptors including cephalopods. Proposed replacements: phospholipase C-activating opsin-mediated signaling pathway Supporting Evidence: PMID:1445212 Photoaffinity labelling by a GTP analogue has been used to identify a 42 kDa band as the major G alpha subunit in squid photoreceptor membranes, recently identified by partial sequence analysis to be a member of the Gq sub-group of GTP-binding proteins PMID:9636052 cephalopod phototransduction is mediated by Gq activation of more than one cytosolic PLC enzyme |
| GO:0007601 visual perception | IEA GO_REF:0000002 | ACCEPT | Summary: The visual perception annotation from InterPro (IPR001760, Opsin) is appropriate for the retinal function of rhodopsin. Bellingham et al. (1998) cloned this rhodopsin from retinal tissue and described it as a "photoreceptor required for image-forming vision" (PMID:9662500). Cuttlefish are highly visual animals with well-developed camera-type eyes. Reason: Visual perception is a core biological process for rhodopsin in the retina. The gene was cloned from retinal cDNA (PMID:9662500), and UniProt annotates it as required for "image-forming vision at low light intensity." S. officinalis is monochromatic with a single retinal rhodopsin (lambda-max ~492 nm), but compensates for color-blindness through polarization sensitivity conferred by the orthogonal microvillar arrangement in rhabdomeric photoreceptors (PMID:20392722). This annotation only captures the ocular function -- the dermal/extraocular photoreception role is not covered and is proposed as a NEW annotation below. Supporting Evidence: PMID:9662500 The cDNA sequence of the rhodopsin gene of the cuttlefish Sepia officinalis (L.) (Sub-class Coleoidea, Order Sepiida) is presented, together with an analysis of the structure of the gene. A proline-rich C terminus is present; this structure is characteristic of cephalopod rhodopsins. PMID:20392722 Since the opsin in the fin is identical to that of the retina (Ξ»max=492 nm), and the ventral transcripts are also unlikely to be spectrally different, colour discrimination by the skin opsins is unlikely. |
| GO:0016020 membrane | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: The membrane annotation from InterPro (IPR000276, IPR001760, IPR017452) is correct but non-specific. Rhodopsin is a multi-pass integral membrane protein with seven transmembrane helices, as confirmed by sequence analysis and UniProt topology annotation (PMID:9662500). Reason: This is a correct but generic cellular component annotation. As a 7TM receptor, rhodopsin is by definition an integral membrane protein. However, the more specific annotations (plasma membrane, rhabdomere membrane) are more informative. This IEA annotation is redundant with the more specific ISS annotations below, but is not incorrect. |
| GO:0005886 plasma membrane | ISS GO_REF:0000024 | MODIFY | Summary: The plasma membrane annotation is based on sequence similarity to human rhodopsin (P08100). While technically the protein resides in a membrane, for invertebrate rhabdomeric photoreceptors the more specific location is the rhabdomere membrane (GO:0033583), a specialized microvillar extension of the photoreceptor cell. UniProt annotates this protein at "Cell projection, rhabdomere membrane" based on similarity to Todarodes pacificus rhodopsin (P31356). Reason: The ISS transfer from vertebrate rhodopsin (P08100) maps to plasma membrane, but this misses the specific localization. Invertebrate rhabdomeric opsins localize to rhabdomere membranes, the microvillar structures of invertebrate photoreceptor cells. UniProt itself annotates this as "Cell projection, rhabdomere membrane." The more specific GO:0033583 rhabdomere membrane is the appropriate term. Proposed replacements: rhabdomere membrane |
| GO:0016020 membrane | ISS GO_REF:0000024 | KEEP AS NON CORE | Summary: This duplicate membrane annotation is via ISS transfer from Todarodes pacificus rhodopsin (P31356). It is correct but redundant with the IEA membrane annotation above. Reason: This is a correct but non-specific annotation that is already captured by the IEA-derived membrane annotation. The ISS evidence from T. pacificus rhodopsin provides independent support but adds no additional specificity. The more specific rhabdomere membrane annotation proposed above is more informative. |
| GO:0016918 retinal binding | ISS GO_REF:0000024 | ACCEPT | Summary: Retinal binding is a core molecular function of rhodopsin. The ISS annotation is based on similarity to T. pacificus rhodopsin (P31356). S. officinalis rhodopsin contains the conserved lysine at position 305 (equivalent to K296 in bovine rhodopsin) that forms the Schiff base linkage with 11-cis-retinal (PMID:9662500). UniProt annotates the retinylidene modification at this position. Reason: Retinal binding is fundamental to rhodopsin function. Three lines of evidence support this for S. officinalis specifically: (1) Sequence analysis shows the conserved retinal-binding lysine (K305, equivalent to K296 in bovine rhodopsin) that forms the Schiff base with 11-cis-retinal β present in S. officinalis and all cephalopod opsins (PMID:9662500); (2) The protein has a measured lambda-max of ~492 nm (PMID:20392722), which can only arise from a retinal chromophore bound in the opsin binding pocket β the spectral tuning model shows that substitutions at just three amino acid sites in the binding pocket explain the lambda-max shifts between S. officinalis, squid, and octopus rhodopsins (PMID:9662500); (3) S. officinalis co-expresses retinochrome, which regenerates 11-cis-retinal from all-trans-retinal after photoisomerization, demonstrating a functional retinal cycling system that requires retinal binding by rhodopsin (PMID:33117186). Additionally, the invertebrate-specific counterion site (aromatic amino acid replacing E113 of vertebrate rhodopsin) is conserved (PMID:9662500), confirming a structurally competent retinal binding pocket. No direct crystallography or retinal-extraction assay has been performed on S. officinalis rhodopsin specifically, so the evidence is structural/evolutionary rather than direct biochemical demonstration in this species. Supporting Evidence: PMID:9662500 A spectral tuning model involving substitutions at only three amino acid sites is proposed for the spectral shifts between the rhodopsins of Sepia officinalis, three species of squid and Paroctopus defleini. PMID:9662500 In common with all invertebrate opsins studied so far, the equivalent site to the counterion in vertebrate opsins is occupied by an aromatic amino acid. PMID:20392722 the opsin in the fin is identical to that of the retina (Ξ»max=492 nm), and the ventral transcripts are also unlikely to be spectrally different |
| GO:0009881 photoreceptor activity | ISS PMID:9662500 The rhodopsin gene of the cuttlefish Sepia officinalis: sequ... | NEW | Summary: Photoreceptor activity (GO:0009881) is listed in UniProt DR lines for this protein (IEA:UniProtKB-KW) but is absent from the GOA TSV. This is a core molecular function annotation. As a rhabdomeric opsin, S. officinalis rhodopsin absorbs photons via its retinal chromophore and transduces light into a cellular signal via Gq-alpha activation and PLC signaling. The more specific child term GO:0008020 (G protein-coupled photoreceptor activity) would be even more appropriate. Reason: This is arguably the most important MF annotation for this gene and is missing from the GOA file. Rhodopsin is the defining photoreceptor molecule. The protein absorbs light through its covalently bound 11-cis-retinal chromophore (lambda-max ~492 nm in S. officinalis retina; PMID:20392722), undergoes conformational change, and activates Gq-alpha signaling which stimulates PLC-beta, hydrolyzes PIP2 to IP3 and DAG, and opens TRP channels. This function is documented both in the retina (PMID:9662500) and in extraocular tissues (PMID:25994635, PMID:25994633). The child term GO:0008020 (G protein-coupled photoreceptor activity) would be the most precise annotation. Proposed replacements: G protein-coupled photoreceptor activity Supporting Evidence: PMID:9662500 The cDNA sequence of the rhodopsin gene of the cuttlefish Sepia officinalis (L.) (Sub-class Coleoidea, Order Sepiida) is presented, together with an analysis of the structure of the gene. PMID:25994635 RT-PCR revealed the presence of transcripts encoding rhodopsin and retinochrome within the retinas and skin of the squid Doryteuthis pealeii, and the cuttlefish Sepia officinalis and Sepia latimanus. PMID:20392722 the mRNA coding for opsin from various body regions was amplified and sequenced, and gene expression was detected in fin and ventral skin samples. |
| GO:0030265 phospholipase C-activating opsin-mediated signaling pathway | ISS PMID:9662500 The rhodopsin gene of the cuttlefish Sepia officinalis: sequ... | NEW | Summary: GO:0030265 (phospholipase C-activating opsin-mediated signaling pathway) is the most precise BP term for cephalopod rhodopsin. It is a child of both GO:0007186 (GPCR signaling) and GO:0007602 (phototransduction) via GO:0016056 (G protein-coupled opsin signaling pathway). Its definition explicitly describes "Gq-mediated activation of phospholipase C... PIP2 into IP3 and DAG... Typical examples are rhabdomeric photoreceptors in the eyes of protostomes" β exactly what S. officinalis rhodopsin does. The Gq-PLC cascade was biochemically characterized in squid: a 42 kDa Gq-alpha is activated by photoexcited rhodopsin (PMID:1445212), and PLC-beta was purified and shown to be Gq-activated (PMID:9636052). This pathway is functional in both ocular and extraocular contexts (PMID:25994633, PMID:25994635). Retinochrome regenerates 11-cis-retinal (PMID:33117186). Signal termination involves GRK1 and visual arrestin (PMID:33117186). Reason: This is the most specific and accurate BP term available. GO:0030265 subsumes both the GPCR signaling and phototransduction parent terms, capturing that rhodopsin signals through a Gq-PLC rhabdomeric cascade. Evidence from Kingston et al. (2015) directly shows rhodopsin, retinochrome, and Gq-alpha co-expression in S. officinalis skin (PMID:25994635). LACE in octopus skin has an action spectrum matching rhodopsin (lambda-max 480 nm, PMID:25994633). S. officinalis retinal rhodopsin has lambda-max ~492 nm (PMID:20392722). Supporting Evidence: PMID:25994635 This is the first evidence that cephalopod dermal tissues, and specifically chromatophores, may possess the requisite combination of molecules required to respond to light. PMID:25994633 the maximum sensitivity of the light sensors underlying LACE closely matches the known spectral sensitivity of opsin from octopus eyes. PMID:1445212 Binding was not detected above background in the dark, but was rapidly activated by light. PMID:9636052 We have previously purified a 140-kDa PLC enzyme from squid photoreceptors that is regulated by squid Gq. |
| GO:0009583 detection of light stimulus | ISS PMID:25994635 Visual phototransduction components in cephalopod chromatoph... | NEW | Summary: Detection of light stimulus is a key biological process for rhodopsin, relevant to both ocular and extraocular photoreception. Kingston et al. (2015) demonstrated rhodopsin, retinochrome, and Gq-alpha transcripts and protein in S. officinalis skin and chromatophores by RT-PCR and immunohistochemistry (PMID:25994635). The complete phototransduction cascade in dermal tissues indicates rhodopsin participates in detection of light stimulus beyond the eye. Functional validation comes from LACE in O. bimaculoides (PMID:25994633) and electrophysiological recordings of light-evoked neural activity in octopus arm nerve cords (PMID:40067259). Reason: This annotation captures the sensory detection function of rhodopsin in cephalopod tissues. The evidence is strong across multiple species: rhodopsin transcripts and protein are present in S. officinalis skin (PMID:25994635), LACE demonstrates functional light detection in excised octopus skin (PMID:25994633), and electrophysiology confirms light-evoked neural activity in octopus arms (PMID:40067259). This annotation encompasses both ocular and extraocular photoreception roles, which is particularly valuable because it does not carry the implicit eye-specific connotation of "visual perception." Rhodopsin expression in developing S. officinalis eyes begins at embryonic stage 23 and increases significantly through stage 28, coinciding with the onset of light perception: near-hatching embryos sense light through the egg capsule and change body orientation in response to illumination (PMID:33117186). Supporting Evidence: PMID:25994635 RT-PCR revealed the presence of transcripts encoding rhodopsin and retinochrome within the retinas and skin of the squid Doryteuthis pealeii, and the cuttlefish Sepia officinalis and Sepia latimanus. PMID:25994633 LACE in isolated preparations suggests that octopus skin is intrinsically light sensitive and that this dispersed light sense might contribute to their unique and novel patterning abilities. PMID:40067259 The results showed that the axial nerve cord is strongly responsive to light stimulation of the arm and that the response travels along the length of the axial nerve cord. Blue light generated the strongest neural activity while red and green light also induced responses. |
| GO:0071482 cellular response to light stimulus | ISS PMID:25994633 Eye-independent, light-activated chromatophore expansion (LA... | NEW | Summary: Cellular response to light stimulus captures the downstream cellular effects of rhodopsin activation in non-eye tissues. LACE in O. bimaculoides demonstrates that individual chromatophore cells respond to light by expanding, independently of the CNS (PMID:25994633). The spectral sensitivity matches rhodopsin (lambda-max 480 nm). The r-opsin phototransduction cascade in chromatophore cells drives this cellular response. Maselli et al. (2025) further showed rhodopsin expression in O. vulgaris skin, suckers, and optic lobes with evidence of functional light detection (PMID:40511715). Reason: This annotation specifically highlights the cellular-level response to light mediated by rhodopsin in dermal/extraocular tissues. The LACE phenotype -- chromatophore expansion in response to light in excised skin -- is a direct cellular response to light stimulus that is independent of the eye and CNS (PMID:25994633). While the direct evidence is from O. bimaculoides, the molecular machinery (rhodopsin + retinochrome + Gq-alpha) is demonstrated in S. officinalis skin (PMID:25994635), making ISS annotation justified. Supporting Evidence: PMID:25994633 we found that light causes chromatophores to expand in excised pieces of Octopus bimaculoides skin. We call this behavior light-activated chromatophore expansion (or LACE). PMID:25994635 This is the first evidence that cephalopod dermal tissues, and specifically chromatophores, may possess the requisite combination of molecules required to respond to light. |
| GO:0033583 rhabdomere membrane | ISS GO_REF:0000024 | NEW | Summary: UniProt annotates this protein as localized to "Cell projection, rhabdomere membrane" based on similarity to T. pacificus rhodopsin (P31356). This is the appropriate specific CC annotation for invertebrate rhabdomeric photoreceptors. The rhabdomere is the microvillar photoreceptive structure of invertebrate photoreceptor cells, where rhodopsin is concentrated at high density. Reason: This is the most specific and accurate CC annotation for invertebrate rhodopsin. UniProt already annotates this localization for O16005. It is more informative than the generic "plasma membrane" or "membrane" annotations currently in the GOA file. The ISS evidence from T. pacificus rhodopsin (P31356) is strong given the high conservation of rhabdomeric photoreceptor ultrastructure across cephalopods. |
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Download this section (compressed HTML)Q: What is the spectral sensitivity (lambda-max) of S. officinalis skin rhodopsin versus retinal rhodopsin? Are they identical, as demonstrated in squid (PMID:26351853), or has the dermal copy acquired distinct spectral tuning?
Q: Does S. officinalis exhibit LACE (light-activated chromatophore expansion) similar to O. bimaculoides? The molecular components are present (PMID:25994635) but functional LACE has not been directly demonstrated in cuttlefish.
Q: Is rhodopsin expression in S. officinalis chromatophores regulated by circadian or environmental light conditions, potentially modulating the sensitivity of dermal photoreception?
Q: What is the downstream signaling cascade in chromatophore cells versus retinal cells? Do both utilize the same TRP channel or are there tissue-specific effectors?
Q: What is the expression pattern and function of the r-opsin2 paralog in S. officinalis? Bonade et al. (2020) could not detect r-opsin2 in embryonic eyes or skin (PMID:33117186), and Mathger et al. (2010) found a one-amino-acid variant in ventral skin that could represent r-opsin2 (PMID:20392722). Is r-opsin2 expressed in adult-specific tissues or under particular environmental conditions?
Experiment: Perform LACE assays on excised S. officinalis skin to test whether cuttlefish chromatophores expand in response to light independently of neural input, as demonstrated in O. bimaculoides. Generate an action spectrum to compare with the retinal rhodopsin lambda-max.
Hypothesis: S. officinalis chromatophores will exhibit light-activated expansion with spectral sensitivity matching retinal rhodopsin, given that the complete phototransduction machinery (rhodopsin, retinochrome, Gq-alpha) is present in cuttlefish skin.
Experiment: Use scRNA-seq on S. officinalis skin to identify cell types expressing rhodopsin and characterize co-expression with other phototransduction components (retinochrome, Gq-alpha, TRP channels, GRK1) at single-cell resolution.
Hypothesis: A distinct photoreceptor cell cluster will be identified in S. officinalis skin, analogous to the Cl32 photoreceptor cell cluster found in L. vulgaris scRNA-seq data, co-expressing rhodopsin with the complete Gq phototransduction cascade.
Experiment: Perform in situ hybridization or immunofluorescence on S. officinalis arm suckers and optic lobes to determine whether rhodopsin is expressed in these tissues, as shown in O. vulgaris (PMID:40511715) and D. pealeii (PMID:26351853).
Hypothesis: S. officinalis suckers and optic lobes will express rhodopsin, consistent with a distributed extraocular photoreception system conserved across coleoid cephalopods.
Experiment: Test whether CRISPR-mediated knockdown of rhodopsin in S. officinalis skin affects chromatophore responses to local light stimulation, to establish causality between rhodopsin expression and dermal photoreception.
Hypothesis: Rhodopsin knockdown in skin will abolish or reduce LACE and other dermal light responses, confirming that rhodopsin is the primary photopigment mediating extraocular photoreception in chromatophore cells.
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