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Ca2+-regulated photoproteins of ctenophores lose bioluminescence activity when exposed to visible light. Little is known about the chemical nature of chromophore photoinactivation. Using a total synthesis strategy, we have established the structures of two unusual coelenterazine products, isolated from recombinant berovin of the ctenophore Beroe abyssicola, which are Z/E isomers. We propose that during light irradiation, these derivatives are formed from 2-hydroperoxycoelenterazine via the intermediate 8a-peroxide by a mechanism reminiscent of that previously described for the auto-oxidation of green-fluorescent-protein-like chromophores.

The bright bioluminescence of ctenophores inhabiting the oceans worldwide is caused by light-sensitive Ca

Hydromedusan photoproteins responsible for the bioluminescence of a variety of marine jellyfish and hydroids are a unique biochemical system recognized as a stable enzyme-substrate complex consisting of apoprotein and preoxygenated coelenterazine, which is tightly bound in the protein inner cavity. The binding of calcium ions to the photoprotein molecule is only required to initiate the light emission reaction. Although numerous experimental and theoretical studies on the bioluminescence of these photoproteins were performed, many features of their functioning are yet unclear. In particular, which ionic state of dioxetanone intermediate decomposes to yield a coelenteramide in an excited state and the role of the water molecule residing in a proximity to the N1 atom of 2-hydroperoxycoelenterazine in the bioluminescence reaction are still under discussion. With the aim to elucidate the function of this water molecule as well as to pinpoint the amino acid residues presumably involved in the protonation of the primarily formed dioxetanone anion, we constructed a set of single and double obelin and aequorin mutants with substitutions of His, Trp, Tyr, and Ser to residues with different properties of side chains and investigated their bioluminescence properties (specific activity, bioluminescence spectra, stopped-flow kinetics, and fluorescence spectra of Ca2+-discharged photoproteins). Moreover, we determined the spatial structure of the obelin mutant with a substitution of His64, the key residue of the presumable proton transfer, to Phe. On the ground of the bioluminescence properties of the obelin and aequorin mutants as well as the spatial structures of the obelin mutants with the replacements of His64 and Tyr138, the conclusion was made that, in fact, His residue of the Tyr-His-Trp triad and the water molecule perform the “catalytic function” by transferring the proton from solvent to the dioxetanone anion to generate its neutral ionic state in complex with water, as only the decomposition of this form of dioxetanone can provide the highest light output in the light-emitting reaction of the hydromedusan photoproteins.

Ca

Active hydromedusan and ctenophore Ca

Light-sensitive Ca

Color variants of Ca2+ -regulated photoprotein obelin were shown to be an important tool for dual-analyte binding assay. To provide site-directed conjugation with biospecific molecules, several obelin color mutants carrying unique cysteine residues were obtained and characterized for their novel properties. A pair of obelins Y138F,A5C and W92F,H22E,D12C was found to be most suitable (in terms of high bioluminescent activity and stability) as reporters in simultaneous assay of two targets in a sample. Availability of SH-groups, accessible for chemical modification, essentially simplifies the synthesis of biospecific conjugates, increases their yield and conserves obelins' bioluminescence activity. Conjugates with immunoglobulin and oligonucleotide were produced and successfully applied in single nucleotide polymorphism genotyping.

A one-step bioluminescent immunoassay for tick-borne encephalitis virus (TBEV) in natural ticks based on the hybrid protein 14D5a-Rm7 was developed. Recombinant Ca2+-dependent coelenterazine-binding protein was shown to be a more convenient substrate form for the Rm7 domain than coelenterazine. Over 600 samples of natural ticks were analyzed and shown to have essential differences in the discrimination factor D for TBEV-positive (2.77 ± 0.81) and TBEV-negative (1.15 ± 0.28) samples.

The full-length cDNA genes encoding five new isoforms of Ca2 +-regulated photoprotein mitrocomin from a small tissue sample of the outer bell margin containing photocytes of only one specimen of the luminous jellyfish Mitrocoma cellularia were cloned, sequenced, and characterized after their expression in Escherichia coli and subsequent purification. The analysis of cDNA nucleotide sequences encoding mitrocomin isoforms allowed suggestion that two isoforms might be the products of two allelic genes differing in one amino acid residue (64R/Q) whereas other isotypes appear as a result of transcriptional mutations. In addition, the crystal structure of mitrocomin was determined at 1.30 A resolution which expectedly revealed a high similarity with the structures of other hydromedusan photoproteins. Although mitrocomin isoforms reveal a high degree of identity of amino acid sequences, they vary in specific bioluminescence activities. At that, all isotypes displayed the identical bioluminescence spectra (473–474 nm with no shoulder at 400 nm). Fluorescence spectra of Ca2 +-discharged mitrocomins were almost identical to their light emission spectra similar to the case of Ca2 +-discharged aequorin, but different from Ca2 +-discharged obelins and clytin which fluorescence is red-shifted by 25–30 nm from bioluminescence spectra. The main distinction of mitrocomin from other hydromedusan photoproteins is an additional Tyr at the C-terminus. Using site-directed mutagenesis, we showed that this Tyr is not important for bioluminescence because its deletion even increases specific activity and efficiency of apo-mitrocomin conversion into active photoprotein, in contrast to C-terminal Pro of other photoproteins. Since genes in a population generally exist as different isoforms, it makes us anticipate the cloning of even more isoforms of mitrocomin and other hydromedusan photoproteins with different bioluminescence properties.

To facilitate the detection of the tick-borne encephalitis virus (TBEV), the causative agent of one of the most severe human neuroinfections, we have developed an immunoassay based on bioluminescent hybrid protein 14D5a-Rm7 as a detection probe. The protein containing Renilla luciferase as a reporter and a single-chain variable fragment (scFv) of murine immunoglobulin to TBEV as a recognition element was constructed, produced by bacterial expression, purified, and tested. Both domains were shown to reveal their specific biological properties-affinity to the target antigen and bioluminescent activity. Hybrid protein was applied as a label for solid-phase immunoassay of the antigens, associated with the tick-borne encephalitis virus (native glycoprotein E or extracts of the infected strain of lab ticks). The assay demonstrates high sensitivity (0.056 ng of glycoprotein E; 10(4)-10(5) virus particles or 0.1 pg virions) and simplicity and is competitive with conventional methods for detection of TBEV.

Coelenterazine-dependent copepod luciferases containing natural signal peptide for secretion are a very convenient analytical tool as they enable monitoring of intracellular events with high sensitivity, without destroying cells or tissues. This property is well suited for application in biomedical research and development of cell-based assays for high throughput screening. We report the cloning of cDNA gene encoding a novel secreted non-allelic 16.5-kDa isoform (MLuc7) of Metridia longa luciferase, which, in fact, is the smallest natural luciferase of known for today. Despite the small size, isoform contains 10 conservative Cys residues suggesting the presence of up to 5 SS bonds. This hampers the efficient production of functionally active recombinant luciferase in bacterial expression systems. With the use of the baculovirus expression system, we produced substantial amounts of the proper folded MLuc7 luciferase with a yield of ∼3 mg/L of a high purity protein. We demonstrate that MLuc7 produced in insect cells is highly active and extremely thermostable, and is well suited as a secreted reporter when expressed in mammalian cells ensuring higher sensitivity of detection as compared to another Metridia luciferase isoform (MLuc164) which is widely employed in real-time imaging.

Calcium ion is a ubiquitous intracellular messenger, performing this function in many eukaryotic cells. To understand calcium regulation mechanisms and how disturbances of these mechanisms are associated with disease states, it is necessary to measure calcium inside cells. Ca(2+)-regulated photoproteins have been successfully used for this purpose for many years. Here we report the results of comparative studies on the properties of recombinant aequorin from Aequorea victoria, recombinant obelins from Obelia geniculata and Obelia longissima, recombinant mitrocomin from Mitrocoma cellularia, and recombinant clytin from Clytia gregaria as intracellular calcium indicators in a set of identical in vitro and in vivo experiments. Although photoproteins reveal a high degree of identity of amino acid sequences and spatial structures, and, apparently, have a common mechanism for the bioluminescence reaction, they were found to differ in the Ca(2+) concentration detection limit, the sensitivity of bioluminescence to Mg(2+), and the rates of the rise of the luminescence signal with a sudden change of Ca(2+) concentration. In addition, the bioluminescence activities of Chinese hamster ovary cells expressing wild-type photoproteins also differed. The light signals of cells expressing mitrocomin, for example, slightly exceeded the background, suggesting that mitrocomin may be hardly used to detect intracellular Ca(2+) without modifications improving its properties. On the basis of experiments on the activation of endogenous P2Y2 receptor in Chinese hamster ovary cells by ATP, we suggest that wild-type aequorin and obelin from O. longissima are more suitable for calcium detection in cytoplasm, whereas clytin and obelin from O. geniculata can be used for calcium measurement in cell compartments with high Ca(2+) concentration.

Previous studies have stated that aequorin loses most of its bioluminescence activity upon modification of the C-terminus, thus limiting the production of photoprotein fusion proteins at its N-terminus. In the present work, we investigate the importance of the C-terminal proline and the hydrogen bonds it forms for photoprotein active complex formation, stability and functional activity. According to the crystal structures of obelin and aequorin, two Ca(2+)-regulated photoproteins, the carboxyl group of the C-terminal Pro forms two hydrogen bonds with the side chain of Arg21 (Arg15 in aequorin case) situated in the first α-helix. Whereas, deletion or substitution of the C-terminal proline could noticeably change the bioluminescence activity, stability or the yield of an active photoprotein complex. Therefore, modifications of the first α-helix Arg has a clear destructive effect on the main photoprotein properties. A C-terminal hydrogen-bond network is proposed to be important for the stability of photoprotein molecules towards external disturbances, when taking part in the formation of locked protein conformations and isolation of coelenterazine-binding cavities.

The bright bioluminescence of ctenophores, found in oceans worldwide, is determined by Ca 2 + -regulated photoproteins, functionally identical to and sharing many properties of hydromedusan photoproteins. In contrast, however, the ctenophore photoproteins are extremely sensitive to UV and visible light over the range of their absorption spectrum. The spatial structure of a novel light-sensitive photoprotein from the ctenophore Beroe abyssicola in its apoform bound with three calcium ions is determined at 2.0 A. We demonstrate that the apoberovin is a slightly asymmetrical compact globular protein formed by two domains with a cavity in the center, which exactly retains the fold architecture characteristic of hydromedusan photoproteins despite their low amino acid sequence identity. However, the structural alignment of these two photoprotein classes clearly shows that despite the high similarity of shape and geometry of their coelenterazine-binding cavities, their interiors differ drastically. The key residues appearing to be crucial for stabilizing the 2-hydroperoxycoelenterazine and for formation of the emitter in hydromedusan photoproteins, are replaced in berovin by amino acid residues having completely different side chain properties. Evidently, these replacements must be responsible for the distinct properties of ctenophore photoproteins such as sensitivity to light or the fact that the formation of active photoprotein from apophotoprotein, coelenterazine, and oxygen is more effective at alkaline pH.

Color variants of Ca

Bright bioluminescence of ctenophores is caused by Ca(2+)-regulated photoproteins. Although these photoproteins are functionally identical to and share many properties of cnidarian photoproteins, like aequorin and obelin, and retain the same spatial architecture, they are extremely sensitive to light, i.e. lose the ability to bioluminesce on exposure to light over the entire absorption spectrum. In addition, the degree of identity of their amino acid sequences with those of cnidarian photoproteins is only 29.4%. This suggests that the residues involved in bioluminescence of ctenophore and cnidarian photoproteins significantly differ. Here we describe the bioluminescent properties of berovin mutants with substitution of the residues located in the photoprotein internal cavity. Since the spatial structure of berovin bound with a substrate is not determined yet, to identify these residues we have modeled it with an accommodated substrate using the structures of some cnidarian Ca(2+)-regulated photoproteins with bound coelenterazine or coelenteramide as templates in order to obtain an adequate sampling and to take into account all possible conformers and variants for ligand-protein docking. Based on the impact of substitutions on the bioluminescent properties and model structures we speculate that within the internal cavity of ctenophore photoproteins, coelenterazine is bound as a 2-peroxy anion adduct which is stabilized owing to Coulomb interaction with a positively charged guanidinium group of Arg41 paired with Tyr204. In this case, the bioluminescence reaction is triggered by only calcium-induced conformational changes leading to the disturbance of charge-charge interaction.

A method for single nucleotide polymorphism identification was developed, which was based on the primer extension reaction (PEXT) followed by bioluminescent solid-phase microassay. Recombinant Ca2+-regulated photoprotein obelin and coelenterazine-dependent Renilla muelleri luciferase were used as reporters. The study was performed as an example of SNP genotyping of the human F5 gene encoding human Factor V Leiden polymorphism 1691 G→A (R506Q). Genomic DNA was amplified by PCR using primers flanking polymorphic site of 140 base pairs. PCR products were used as templates for two PEXT reactions using two primers containing 3′-terminal nucleotides, which were complementary to either normal or mutant alleles. If the template and allele-specific primer were completely complementary, the latter was elongated with DNA polymerase. The resulting extension product contained biotin residue due to the presence of biotinylated deoxyuridine triphosphate (B-dUTP) in the reaction mixture. The products were analyzed using obelin-streptavidin conjugates. The optimal PEXT-reaction conditions were found, which ensured a high reliability of SNP genotyping. A new approach to simultaneously revealing both alleles in one well was developed using two bioluminescent reporters. The efficiency of the proposed approach was shown in the study of clinical DNA samples.

Light-sensitive Ca2+-regulated photoproteins are responsible for the bright bioluminescence of ctenophores. Using functional screening, four full-size cDNA genes encoding the same 208-amino-acid polypeptide were isolated from two independent cDNA libraries prepared from two Beroe abyssicola specimens. Sequence analysis revealed three canonical EF-hand calcium-binding sites characteristic of Ca2+-regulated photoproteins, but a very low degree of sequence identity (27–29%) with aequorin-type photoproteins, despite functional similarities. Recombinant berovin was expressed in Escherichia coli cells, purified, converted to active photoprotein and characterized. Active berovin has absorption maxima at 280 and 437 nm. The Ca2+-discharged protein loses visible absorption, but exhibits a new absorption maximum at 335 nm. The berovin bioluminescence is blue (λmax = 491 nm) and a change in pH over the range 6.0–9.5 has no significant effect on the light emission spectrum. By contrast, the fluorescence of Ca2+-discharged protein (λex = 350 nm) is pH sensitive: at neutral pH the maximum is at 420 nm and at alkaline pH there are two maxima at 410 and 485 nm. Like native ctenophore photoproteins, recombinant berovin is also inactivated by light. The Ca2+ concentration–effect curve is a sigmoid with a slope on a log–log plot of ∼ 2.5. Although this curve for berovin is very similar to those obtained for obelin and aequorin, there are evident distinctions: berovin responds to calcium changes at lower concentrations than jellyfish photoproteins and its Ca2+-independent luminescence is low. Recombinant berovin was successfully expressed in mammalian cells, thereby demonstrating potential for monitoring intracellular calcium. Database The nucleotide sequences have been deposited in the GenBankTM/EBI Data Bank with accession numbers: apoberovin cDNA genes, JN673813 (BA1), JN673814 (BA2), JN673815 (BA3), JN673816 (BA4); fragment 18S rRNA, JN673817 (BA-rRNA5).

The bioluminescent systems of many marine organisms are comprised of two proteins--the Ca(2+)-regulated photoprotein and green-fluorescent protein (GFP). This work reports the cloning of the full-size cDNA encoding GFP (cgreGFP) from jellyfish Clytia gregaria, its expression and properties of the recombinant protein. The overall degree of identity between the amino acid sequence of the novel cgreGFP and the sequence of GFP (avGFP) from Aequorea victoria is 42% (similarity--64%) despite these GFPs originating from jellyfish that both belong to the same class, Hydrozoa. However although the degree of identity is low, three residues, Ser-Tyr-Gly, which form the chromophore are identical in both GFPs. The cgreGFP displayed two absorption peaks at 278 and 485 nm, and the fluorescence maximum at 500 nm. The fluorescence quantum yield was determined to be 0.86, the brightness to be 54 mM(-1) cm(-1). For the first time we have also demonstrated an efficient radiationless energy transfer in vitro between clytin and cgreGFP in solution at micromolar concentrations. The cgreGFP may be a useful intracellular fluorescent marker, as it was able to be expressed in mammalian cells.

The recombinant coelenterazine-dependent luciferases (isoforms MLuc164 and MLuc39) from the marine copepod Metridia longa were expressed as inclusion bodies in E. coli cells, dissolved in 6 M guanidinium chloride and folded in conditions developed for proteins containing intramolecular disulfide bonds. One of them (MLuc39) was obtained in an active monomeric form with a high yield. The luciferase bioluminescence is found to be initiated not only by free coelenterazine, but also by Ca2+-dependent coelenterazine-binding protein (CBP) of Renilla muelleri on Ca2+ addition. The use of CBP as a "substrate" provides higher light emission and simultaneously the lower level of background. The high purity MLuc39 can be detected down to attomol with a linear range extending over 5 orders of magnitude. The MLuc39 reveals also a high stability towards heating and chemical modification; the chemically synthesized biotinylated derivatives of the luciferase preserve 35-40% of the initial activity. The luciferase applicability as an in vitro bioluminescent reporter is demonstrated in model tandem bioluminescent solid-phase microassay combining the Ca2+-regulated photoprotein obelin and the Metridia luciferase.

International audience; The main analytical use of Ca(2+)-regulated photoproteins from luminous coelenterates is for real-time non-invasive visualization of intracellular calcium concentration ([Ca(2+)]i) dynamics in cells and whole organisms. A limitation of this approach for in vivo deep tissue imaging is the fact that blue light emitted by the photoprotein is highly absorbed by tissue. Seven novel coelenterazine analogues were synthesized and their effects on the bioluminescent properties of recombinant obelin from Obelia longissima and aequorin from Aequorea victoria were evaluated. Only analogues having electron-donating groups (m-OCH3 and m-OH) on the C6 phenol moiety or an extended resonance system at the C8 position (1-naphthyl and α-styryl analogues) showed a significant red shift of light emission. Of these, only the α-styryl analogue displayed a sufficiently high light intensity to allow eventual tissue penetration. The possible suitability of this compound for in vivo assays was corroborated by studies with aequorin which allowed the monitoring of [Ca(2+)]i dynamics in cultured CHO cells and in hippocampal brain slices. Thus, the α-styryl coelenterazine analogue might be potentially useful for non-invasive, in vivo bioluminescence imaging in deep tissues of small animals.

Light-sensitive Ca(2+) -regulated photoproteins are responsible for the bright bioluminescence of ctenophores. Using functional screening, four full-size cDNA genes encoding the same 208-amino-acid polypeptide were isolated from two independent cDNA libraries prepared from two Beroe abyssicola specimens. Sequence analysis revealed three canonical EF-hand calcium-binding sites characteristic of Ca(2+) -regulated photoproteins, but a very low degree of sequence identity (27-29%) with aequorin-type photoproteins, despite functional similarities. Recombinant berovin was expressed in Escherichia coli cells, purified, converted to active photoprotein and characterized. Active berovin has absorption maxima at 280 and 437 nm. The Ca(2+) -discharged protein loses visible absorption, but exhibits a new absorption maximum at 335 nm. The berovin bioluminescence is blue (λ(max) = 491 nm) and a change in pH over the range 6.0-9.5 has no significant effect on the light emission spectrum. By contrast, the fluorescence of Ca(2+) -discharged protein (λ(ex) = 350 nm) is pH sensitive: at neutral pH the maximum is at 420 nm and at alkaline pH there are two maxima at 410 and 485 nm. Like native ctenophore photoproteins, recombinant berovin is also inactivated by light. The Ca(2+) concentration-effect curve is a sigmoid with a slope on a log-log plot of ∼ 2.5. Although this curve for berovin is very similar to those obtained for obelin and aequorin, there are evident distinctions: berovin responds to calcium changes at lower concentrations than jellyfish photoproteins and its Ca(2+) -independent luminescence is low. Recombinant berovin was successfully expressed in mammalian cells, thereby demonstrating potential for monitoring intracellular calcium. Database The nucleotide sequences have been deposited in the GenBankTM/EBI Data Bank with accession numbers: apoberovin cDNA genes, JN673813 (BA1), JN673814 (BA2), JN673815 (BA3), JN673816 (BA4); fragment 18S rRNA, JN673817 (BA-rRNA5).

The technology of real-time imaging in living cells is crucial for understanding of intracellular events. For this purpose, bioluminescent reporters have been introduced as sensitive and convenient tools. Metridia luciferase (MLuc) from the copepod Metridia longa is a coelenterazine-dependent luciferase containing a natural signal peptide for secretion. We report the high-active MLuc mutants with deletion of the N-terminal variable part of amino acid sequence. The MLuc variants were produced in Escherichia coli cells, converted to an active protein, and characterized. We demonstrate that the truncated MLucs have significantly increased bioluminescent activity as against the wild type enzyme but substantially retain other properties. One of the truncated variants of MLuc was transiently expressed in HEK 293 cells. The results clearly suggest that the truncated Metridia luciferase is well suited as a secreted reporter ensuring higher detection sensitivity in comparison with a wild type enzyme.

Nanodiamonds synthesized by detonation have been found not to immobilize the ring form of pUC19 plasmid DNA. Linear pUC19 molecules with blunt ends, prepared by restriction of the initial ring form of pUC19 DNA, and linear 0.25-10 kb DNA fragments are adsorbed on nanodiamonds. The amount of adsorbed linear DNA molecules depends on the size of the molecules and the size of the nanodiamond clusters.

A cDNA encoding the Ca2+-regulated photoprotein of the bioluminescent marine hydroid Obelia geniculata was cloned and sequenced. The cDNA is a 774 bp fragment containing two overlapping open reading frames, one of which contained 585 bp encoding a 195 amino acid polypeptide which obviously has the primary structure of the apoprotein of a calcium-regulated photoprotein. Many of the residues are identical to those in other Ca2+-regulated photoproteins: 86% compared with that from Obelia longissima, 76% with that from Clytia (Phialidium), 64% with that from Aequorea, and 64% with that from Mitrocoma(Halistaura). The obelin from O. geniculata was overexpressed in Escherichia coli, refolded from inclusion bodies, and purified. The yield of highly purified recombinant protein was 55-80 mg/L of LB medium. O. geniculata obelin has absorption maxima at 280 and 460 nm and a shoulder at approximately 310 nm. The calcium-discharged protein loses visible absorption but exhibits a new absorption maximum at 343 nm. The bioluminescence of the obelin from O. geniculata is blue (lambda(max) = 495 nm). In contrast, the fluorescence of the calcium-discharged protein is yellow-green (lambda(max) = 520 nm; excitation at 340 nm). This is in sharp contrast to aequorin in which the bioluminescence and fluorescence emission spectra of the calcium-discharged protein are almost identical (lambda(max) = 465 nm). The Ca2+ concentration-effect curve for O. geniculata obelin is similar to those of many other photoproteins: at [Ca2+] below approximately 10(-8) M, calcium-independent luminescence is observed, and at [Ca2+] approximately 10(-3) M, the luminescence reaches a maximum. Between these extremes, the curve spans a vertical range of almost 8 log units with a maximum slope on a log-log plot of about 2.5. In the absence of Mg2+ the rate constant for the rise of bioluminescence determined by the stopped-flow technique is about 450 s(-1). The effects of Mg2+ on the kinetics of bioluminescence are complicated, but at all concentrations studied they are relatively small compared to the corresponding effects on aequorin luminescence. At least with respect to speed and sensitivity to Mg2+, the obelins from both O. longissima and O. geniculata would appear to be more suitable than aequorin for use as intracellular Ca2+ indicators.