Symplectin is a 60 kDa bioluminescence photoprotein from the purpleback flying squid Sthenoteuthis oualaniensis (formerly Symplectoteuthis oualaniensis). It is a member of the carbon-nitrogen hydrolase superfamily (biotinidase/pantetheinase family) that has been neofunctionalized for light production. The protein binds dehydrocoelenterazine (dhCtz) covalently via a thioether bond to Cys-390 (PMID:18997450). In the presence of monovalent cations (K+ or Na+) and molecular oxygen at physiological pH, it catalyzes the oxidation of dhCtz with release of CO2 and emission of blue light at 470 nm (PMID:12054553, PMID:18997450). Symplectin is selectively expressed in the photogenic organ (photophore) on the squid mantle, where it is contained in granules (PMID:12054553, PMID:18997450). It exists mainly as oligomers (~200 kDa or more) with a minor monomeric form of ~60 kDa (PMID:12054553). The protein retains the conserved catalytic triad (E60, K163, C196) of the ancestral CN hydrolase/nitrilase domain, and may retain hydrolase activity via this domain, while the bioluminescence function is carried out by the base domain around Cys-390 (PMID:28785521). Phylogenetic analysis shows that symplectin evolved via multiple gene duplications within cephalopods, forming one of four paralog groups in the pantetheinase/biotinidase protein family, and represents a novel bioluminescence system unrelated to any other known luciferase or photoprotein (PMID:28785521). Symplectin homologs are found in both luminous and non-luminous cephalopod species, indicating that only specific amino acid changes in the symplectin lineage conferred photoprotein activity (PMID:28785521). A symplectin-like 60 kDa photoprotein has also been identified in the related Humboldt squid Dosidicus gigas, which similarly uses dhCtz and emits 470 nm light (PMID:30963583).
Definition: Catalysis of the reaction: dehydrocoelenterazine + O2 = coelenteramide + CO2 + light. The substrate is linked to the enzyme via a thioether bond to a cysteine residue, and the reaction requires monovalent cations (K+ or Na+).
Justification: GO contains species-specific luciferin monooxygenase terms for Cypridina (GO:0047712), Renilla (GO:0050248), Oplophorus (GO:0033756), Photinus (GO:0047077), and Watasenia (GO:0050397) systems, but no term exists for the dehydrocoelenterazine-based system used by symplectin and related squid photoproteins. This system is mechanistically distinct from all of these: it uses dehydrocoelenterazine rather than coelenterazine, the substrate is covalently bound via thioether (not peroxide), and the reaction is triggered by monovalent cations rather than calcium. The related Humboldt squid D. gigas also uses this system (PMID:30963583).
Parent term: luciferin monooxygenase activity
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0008218 bioluminescence | IDA PMID:12054553 A novel photoprotein from oceanic squid (Symplectoteuthis ou... | ACCEPT | Summary: Symplectin is the bioluminescence photoprotein of S. oualaniensis, directly demonstrated by extraction from photogenic organ with luminescence activity assays (PMID:12054553) and detailed mechanistic characterization of the light-emitting reaction (PMID:18997450). The protein was extracted from the photogenic organ with 0.6 M KCl, emitted blue light at 470 nm upon pH adjustment to 7.8, and was shown to have dehydrocoelenterazine covalently bound at Cys-390 (PMID:18997450). This is the core biological process annotation for this protein. Reason: Bioluminescence is the defining and core function of symplectin, supported by extensive direct experimental evidence. Fujii et al. (2002) originally purified a 60 kDa photoprotein from the photogenic organ and demonstrated its luminescence activity (PMID:12054553). Isobe et al. (2008) further proved the molecular mechanism, identifying Cys-390 as the dehydrocoelenterazine binding site via thioether linkage and demonstrating the loss of 12 mass units (one carbon atom as CO2) during the light-emitting reaction (PMID:18997450). This is the only IDA annotation and it is well-supported. Supporting Evidence: PMID:12054553 A 60-kDa photoprotein was selectively extracted from squid photogenic organ with 0.6 M KCl solution at pH 6 as luminescence-active forms. PMID:18997450 Symplectin is a photoprotein of luminous squid, Symplectoteuthis oualaniensis ...Symplectin is contained in the granules and emits blue light (470 nm), when stimulated PMID:18997450 we conclude that Cys-390 among 11 cysteine residues in symplectin is the binding site for F-DCZ (9) through a thioether (-S-) bond to form an adduct with structure 8. PMID:28785521 The other protein comes from the purpleback flying squid Sthenoteuthis oualaniensis, where a 500-amino acid photoprotein has been cloned and characterized (Isobe et al., 2008) and was named symplectin. |
| GO:0045289 luciferin monooxygenase activity | IDA PMID:18997450 Cysteine-390 is the binding site of luminous substance with ... | NEW | Summary: Symplectin catalyzes the oxidation of dehydrocoelenterazine (a luciferin) with molecular oxygen, releasing CO2 and emitting light. This is the generalized luciferin monooxygenase reaction (luciferin + O2 = oxidized luciferin + CO2 + light). Isobe et al. (2008) provided the molecular mechanism showing intramolecular oxidation of dhCtz bound at Cys-390 in the presence of O2 and monovalent cation, with loss of one carbon atom (as CO2) during light emission (PMID:18997450). This MF annotation captures the enzymatic activity underlying the bioluminescence BP annotation. No existing species-specific luciferin monooxygenase term exists for squid dehydrocoelenterazine-based systems in GO. Reason: The existing GO annotations lack a molecular function term for symplectin. GO:0045289 (luciferin monooxygenase activity) is defined as "Catalysis of the generalized reaction: luciferin + O2 = oxidized luciferin + CO2 + light." This precisely describes what symplectin does: it binds dhCtz via thioether at Cys-390, and in the presence of O2 and K+/Na+ at pH 7.8, the chromophore undergoes oxidation with release of CO2 (loss of 12 mass units) and emission of blue light at 470 nm. Although symplectin uses dehydrocoelenterazine rather than coelenterazine itself, the reaction chemistry (oxidation of a luciferin substrate with O2 and CO2 release to produce light) falls squarely under this GO term. Supporting Evidence: PMID:18997450 The chromophore 2 is rapidly oxidized at its optimum pH 7.8 in the presence of dioxygen and mono-valent cation (K+ or Na+) to form a short lifetime intermediate ...The intermediate collapses with releasing carbon dioxide to form an amide (4) having electrons in the singlet excited state orbital. Blue light luminescence is emitted while the excited electrons drop to the ground state PMID:18997450 loss of 12 mass units or 1 carbon atom from the chromo-peptide during luminescence, which coincides with the expected structural change PMID:12054553 A 60-kDa photoprotein was selectively extracted from squid photogenic organ with 0.6 M KCl solution at pH 6 as luminescence-active forms. PMID:21656688 Symplectin is a photoprotein containing the dehydrocoelenterazine (DCL) chromophore, which links to a cysteine residue through a covalent bond with the emission of blue light. |
| GO:0016810 hydrolase activity, acting on carbon-nitrogen (but not peptide) bonds | ISS PMID:28785521 Symplectin evolved from multiple duplications in bioluminesc... | NEW | Summary: Symplectin belongs to the biotinidase/pantetheinase family of carbon-nitrogen hydrolases and retains the conserved catalytic triad (E60-K163-C196) of the nitrilase domain (PMID:28785521). Francis et al. (2017) noted that active site residues involved in pantetheinase catalysis are conserved across essentially all cephalopod members of this family, suggesting symplectin may retain hydrolase activity in addition to its photoprotein function. The catalytic triad is in the nitrilase domain (residues 1-290), which is spatially separate from the bioluminescence-active base domain (residues 291-465). UniProt also lists EC 3.5.-.- for this protein. However, no direct assay of hydrolase activity has been performed on symplectin itself. Reason: Although no hydrolase assay has been performed directly on symplectin, the conservation of the complete catalytic triad (E60-K163-C196) in the nitrilase domain, combined with the structural separation from the bioluminescence domain, provides strong sequence-based evidence that this function is retained. The closest characterized homologs, human vanin-1 (30% identity) and biotinidase (31% identity), are both CN hydrolases (PMID:28785521). GO:0016810 is more appropriate than the generic GO:0016787 (hydrolase activity) assigned by IEA, as the protein specifically belongs to the CN hydrolase superfamily. The ISS evidence code is appropriate given the conserved catalytic triad and family membership. Supporting Evidence: PMID:28785521 active site residues involved in pantetheinase catalysis are also conserved across essentially all of these proteins, suggesting that symplectin may have multiple functions including hydrolase activity, and that the evolution of the luminous phenotype required other changes in the protein outside of the main binding pocket. PMID:28785521 the conserved catalytic triad is located in the nitrilase domain of symplectin (E60, K163, C196), while the cysteine for the thioether linkage is found in the base domain. Therefore, catalytic binding pocket of nitrilase domain is not responsible for the bioluminescence activity of symplectin PMID:28785521 one or many of the cephalopod proteins may still have biotinidase activity or act as a hydrolase in other contexts via the nitrilase domain. Since symplectin is therefore predicted to have two separate functional domains, it is possible that symplectin performs multiple functions, and may even still have a role in biotin metabolism. |
| GO:0030955 potassium ion binding | IDA PMID:18997450 Cysteine-390 is the binding site of luminous substance with ... | NEW | Summary: Symplectin is a monovalent ion-dependent photoprotein. Light emission requires the presence of K+ or Na+ ions (PMID:12054553, PMID:18997450). The protein was originally extracted with 0.6 M KCl, and luminescence was triggered by K+ at the optimal pH (PMID:18997450). UniProt lists potassium and sodium as keywords. While K+ functions as a cofactor/activator rather than a classical binding partner, potassium ion binding captures the functional dependency on K+. Reason: The protein requires monovalent cations (K+ or Na+) for its bioluminescence activity. This is a key aspect of its mechanism not captured by existing GO annotations. The extraction and reconstitution experiments demonstrate that K+ triggers the light-emitting reaction (PMID:18997450). The KCl-dependent extraction procedure itself reflects the ionic requirements of the protein. Supporting Evidence: PMID:18997450 The chromophore 2 is rapidly oxidized at its optimum pH 7.8 in the presence of dioxygen and mono-valent cation (K+ or Na+) to form a short lifetime intermediate PMID:18997450 the reconstituted F-DCZ-symplectin was incubated in the presence of 50 mM K+ for 20 min at pH 8.0 so as to emit strong blue light PMID:12054553 A 60-kDa photoprotein was selectively extracted from squid photogenic organ with 0.6 M KCl solution at pH 6 as luminescence-active forms. |
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Download this section (compressed HTML)Q: Does symplectin retain functional hydrolase (pantetheinase/biotinidase) activity via its conserved nitrilase domain catalytic triad (E60-K163-C196), or has this activity been lost during evolution of the bioluminescence function? Francis et al. (2017) noted that the catalytic triad is conserved and the nitrilase domain is spatially separate from the bioluminescence-active base domain, raising the possibility of dual function (PMID:28785521).
Q: What structural features distinguish luminous symplectin-group proteins from non-luminous homologs found in non-bioluminescent cephalopods (Sepia pharaonis, Loligo vulgaris)? Symplectin homologs exist in non-luminous species, suggesting that only specific amino acid changes confer photoprotein activity. Key candidate residues near Cys-390 include F316, F321, F323, Y359, D314, K325, and E357 (PMID:28785521).
Q: Is the native oligomeric state (~200 kDa or larger) functionally significant for bioluminescence, or can the monomer produce light independently? Fujii et al. (2002) found that symplectin exists mainly as oligomers with trace monomer, and vanin-1 (the closest structural homolog) is a homodimer. Francis et al. (2017) suggested a head-to-tail dimer arrangement could allow the nitrilase domain of one monomer to catalyze dhCtz oxidation bound to the other (PMID:28785521).
Experiment: Express recombinant symplectin and test for pantetheine/biotinidase hydrolase activity in vitro to determine whether the ancestral CN hydrolase function is retained alongside photoprotein activity.
Experiment: Perform site-directed mutagenesis of the candidate residues near Cys-390 (F316, F321, F323, Y359, D314, K325, E357) individually and in combination to identify the minimal set of changes required for bioluminescence in the symplectin lineage.
Experiment: Solve the crystal structure of symplectin (or use cryo-EM of the native oligomer) with bound dehydrocoelenterazine to elucidate the structural basis of the light-emitting reaction and the role of the base domain.
Experiment: Clone and characterize symplectin-group homologs from non-luminous species (S. pharaonis, D. pealei) for both hydrolase and photoprotein activity to test whether bioluminescence evolved once or multiple times within this paralog group.
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