Molecular mechanisms governing the evolutionary conservation of Glycine in the 6th position of loops ΙΙΙ and ΙV in photoprotein mnemiopsin 2

In vitro reversible photoinactivation in a novel variant of Mnemiopsin 2

Incubation of Mnemiopsin with coelenterazine in a dark medium in the presence of oxygen molecules leads to the formation of functional bioluminescent complexes, which initiate light emission upon coordination of calcium ions. However, the functional complex is inhibited when exposed to environmental light. The photoinactivation is reversible in vivo by restoring the live organism to a dark medium, but it is irreversible in Mnemiopsin extracts in vitro. It has been suggested that the photoinactivation of Mnemiopsin results from the dissociation of coelenterazine and oxygen from the photoprotein. Accordingly, the dissociated chromophore differs from free coelenterazine due to the coordination of oxygen in its structure. In this study, while working on several mutants of Mnemiopsin 2, we accidentally observed that a mutant of Mnemiopsin 2, P181D, can recover its light-emitting ability after being treated with light. Compared with the wild-type Mnemiopsin, which completely loses its luminescence activity after 1 min of exposure to light, under similar conditions, the mutant exhibited 71 % of its original activity. Further studies showed that its activity after 60 min of exposure to light was 20.3 % of the original activity under standard conditions. To elucidate this observation, we extended our study and found that replacing Proline, a neutral residue with limited conformational space, with Aspartic acid, a charged residue with greater conformational space, increased the cooperativity of interactions within the photoprotein molecule and enhanced the affinity of the core structure for coelenterazine and oxygen.

Velamins: green-light-emitting calcium-regulated photoproteins isolated from the ctenophore Velamen parallelum

Ca2+-regulated photoproteins (CaPhs) consist of single-chain globular proteins to which coelenterazine, a widely distributed marine luminogenic substrate (the luciferin), binds along with molecular oxygen, producing a stable peroxide. Upon Ca2+ addition, CaPhs undergo conformational changes leading to the cyclization of the peroxide and the formation of a high-energy intermediate. Subsequently, its decomposition yields coelenteramide in an excited state and results in the emission of a flash of light. To date, most known CaPh systems emit blue light (λmax 465-495 nm), except for two bolinopsin isospecies that emit green light (λmax 500 nm). Here, we report the cloning and functional characterization of wild-type CaPhs capable of emitting green light: velamins, isolated from the bioluminescent ctenophore Velamen parallelum. Ten unique photoprotein-like sequences were recovered and grouped in three main clusters. Representative sequences were cloned, expressed, purified, and regenerated into the active His-tagged α-, β-, and γ-velamins. Upon injection of a calcium-containing buffer into the velamin, a flash of green light (λmax 500-508 nm) was observed across pH values ranging from 7 to 9. Whilst α-velamin isoforms exhibited the highest light emission activity, β- and γ-velamins were found to be more thermostable at higher temperatures. Velamins are the wild-type CaPhs with the longest-wavelength light emission yet reported, making them an excellent model for investigating spectral modulation mechanisms in photoproteins.

Site-specific mutagenesis on Mnemiopsin 2: Calcium coordination and substrate binding properties of new variants

We used site-specific mutagenesis by targeting E179 and F190 on the structure of photoprotein Mnemiopsin 2 (Mn2) from Mnemiopsis leidyi. The tertiary structure of E179S and F190L mutants was made by the MODELLER program. Far-ultraviolet circular dichroism data showed that the overall secondary structural content of photoprotein is not changed upon mutation, however the helicity and stabilizing interactions in helical structure decreases in mutants as compared with the wild-type (WT) photoprotein. Fluorescence spectra data revealed that the tertiary structure of the mutants is more compact than that of WT Mn2. According to the heat-induced denaturation experiments data, the melting temperature (Tm ) for the unfolding of tertiary structure of the F190L variant increases by 3°C compared with that of the WT and E179S mutant. Interestingly, the conformational enthalpy of the F190L mutant (86 kcal mol-1 ) is considerably lower than those in the WT photoprotein (102 kcal mol-1 ) and E179S mutant (106 kcal mol-1 ). The significant difference in the enthalpy of the thermal unfolding process could be explained by considering that the thermally denatured state of the F190L mutant is structurally less expanded than the WT and E179S variants. Bioluminescence activity data showed that the maximum characteristic wavelengths of the mutants undergo blue shift as compared with the WT protein. Initial intensity of the F190L and E179S variants was recorded to be 137.5% and 55.9% of the WT protein, respectively.

Brief History of Ctenophora

Ctenophores are the descendants of the earliest surviving lineage of ancestral metazoans, predating the branch leading to sponges (Ctenophore-first phylogeny). Emerging genomic, ultrastructural, cellular, and systemic data indicate that virtually every aspect of ctenophore biology as well as ctenophore development are remarkably different from what is described in representatives of other 32 animal phyla. The outcome of this reconstruction is that most system-level components associated with the ctenophore organization result from convergent evolution. In other words, the ctenophore lineage independently evolved as high animal complexities with the astonishing diversity of cell types and structures as bilaterians and cnidarians. Specifically, neurons, synapses, muscles, mesoderm, through gut, sensory, and integrative systems evolved independently in Ctenophora. Rapid parallel evolution of complex traits is associated with a broad spectrum of unique ctenophore-specific molecular innovations, including alternative toolkits for making an animal. However, the systematic studies of ctenophores are in their infancy, and deciphering their remarkable morphological and functional diversity is one of the hot topics in biological research, with many anticipated surprises.

An in silico analysis on the photoproteins Mnemiopsin 1 and Mnemiopsin 2 to explain the experimental results

Mnemiopsin 1 (Mn1) and Mnemiopsin 2 (Mn2) are photoproteins found in Mnemiopsis leidyi. We have tried to answer the question of whether the structural features of photoproteins can explain the observed activity data. According to the activity measurements data, they have the same characteristic wavelength. However, the initial intensity of Mn2 is significantly higher than that of Mn1, and decay time of Mn1 (0.92 s-1 ) is lower than that of Mn2 (1.46 s-1 ). The phylogenetic analysis demonstrates that, compared with Obelin and Aequorin from Obelia longissima and Aequorea victoria, respectively, a gene modification event may have caused the expansion of the N-terminal side of all photoproteins from M. leidyi. An in silico study has shown that the stability of the photoprotein-substrate complex of Mn2 is higher than that of Mn1, indicating a higher affinity of the substrate for Mn2 compared with Mn1. It was revealed that the active EF-hand loops 1 and III in Mn2 is locally more rigid compared with those in Mn1. We concluded that different stability of the photoprotein complexes leads to different initial intensity. While different patterns of the local dynamics of loops I and III may influence the decay rate.

Longer characteristic wavelength in a novel engineered photoprotein Mnemiopsin 2

We designed two mutants of photoprotein Mnemiopsin 2 (Mn2) including M52I and V144I, where the mutations were applied in the EF-hand loops I and III. Far-UV CD measurements demonstrated that the stability of the helices in the wild-type (WT) protein is greater compared with the mutants. Heat-induced denaturation experiments in the apo-form of photoproteins showed that WT Mn2 has higher value of the enthalpy change for the unfolding process, indicating that it has more stabilizing interaction compared with mutants. According to the activity measurement data, both mutants, particularly V144I have lower initial intensity as well as slower decay rate as compared with the WT photoprotein. Importantly, it was found that V144I variant shows 25 nm of red shift in the characteristic wavelengths as compared with the WT photoprotein. This finding can be considered as an advantage for in vivo application of photoprotein for imaging purposes. It concluded that this position on loop III of Mn2 is a hotspot point for characteristic wavelength determination. However, further research on this mutant is needed for making stable variants of Mn2 with novel optical features.

Recombinant Ca2+-regulated photoproteins of ctenophores: current knowledge and application prospects

Bright bioluminescence of ctenophores is conditioned by Ca²⁺-regulated photoproteins. Although they share many properties characteristic of hydromedusan Ca²⁺-regulated photoproteins responsible for light emission of marine animals belonging to phylum Cnidaria, a substantial distinction still exists. The ctenophore photoproteins appeared to be extremely sensitive to light-they lose the ability for bioluminescence on exposure to light over the entire absorption spectrum. Inactivation is irreversible because keeping the inactivated photoprotein in the dark does not recover its activity. The capability to emit light can be restored only by incubation of inactivated photoprotein with coelenterazine in the dark at alkaline pH in the presence of oxygen. Although these photoproteins were discovered many years ago, only the cloning of cDNAs encoding these unique bioluminescent proteins in the early 2000s has provided a new impetus for their studies. To date, cDNAs encoding Ca²⁺-regulated photoproteins from four different species of luminous ctenophores have been cloned. The amino acid sequences of ctenophore photoproteins turned out to completely differ from those of hydromedusan photoproteins (identity less than 29%) though also similar to them having three EF-hand Ca²⁺-binding sites. At the same time, these photoproteins reveal the same two-domain scaffold characteristic of hydromedusan photoproteins. This review is an attempt to systemize and critically evaluate the data scattered through various articles regarding the structural features of recombinant light-sensitive Ca²⁺-regulated photoproteins of ctenophores and their bioluminescent and physicochemical properties as well as to compare them with those of hydromedusan photoproteins. In addition, we also discuss the prospects of their biotechnology applications.