Flavoprotein miniSOG as a genetically encoded photosensitizer for cancer cells.

BACKGROUND: Genetically encoded photosensitizers are a promising optogenetic instrument for light-induced production of reactive oxygen species in desired locations within cells in vitro or whole body in vivo. Only two such photosensitizers are currently known, GFP-like protein KillerRed and FMN-binding protein miniSOG. In this work we studied phototoxic effects of miniSOG in cancer cells. METHODS: HeLa Kyoto cell lines stably expressing miniSOG in different localizations, namely, plasma membrane, mitochondria or chromatin (fused with histone H2B) were created. Phototoxicity of miniSOG was tested on the cells in vitro and tumor xenografts in vivo. RESULTS: Blue light induced pronounced cell death in all three cell lines in a dose-dependent manner. Caspase 3 activation was characteristic of illuminated cells with mitochondria- and chromatin-localized miniSOG, but not with miniSOG in the plasma membrane. In addition, H2B-miniSOG-expressing cells demonstrated light-induced activation of DNA repair machinery, which indicates massive damage of genomic DNA. In contrast to these in vitro data, no detectable phototoxicity was observed on tumor xenografts with HeLa Kyoto cell lines expressing mitochondria- or chromatin-localized miniSOG. CONCLUSIONS: miniSOG is an excellent genetically encoded photosensitizer for mammalian cells in vitro, but it is inferior to KillerRed in the HeLa tumor. GENERAL SIGNIFICANCE: This is the first study to assess phototoxicity of miniSOG in cancer cells. The results suggest an effective ontogenetic tool and may be of interest for molecular and cell biology and biomedical applications.

Analysis of alternative splicing of cassette exons at single-cell level using two fluorescent proteins.

Alternative splicing plays a major role in increasing proteome complexity and regulating gene expression. Here, we developed a new fluorescent protein-based approach to quantitatively analyze the alternative splicing of a target cassette exon (skipping or inclusion), which results in an open-reading frame shift. A fragment of a gene of interest is cloned between red and green fluorescent protein (RFP and GFP)-encoding sequences in such a way that translation of the normally spliced full-length transcript results in expression of both RFP and GFP. In contrast, alternative exon skipping results in the synthesis of RFP only. Green and red fluorescence intensities can be used to estimate the proportions of normal and alternative transcripts in each cell. The new method was successfully tested for human PIG3 (p53-inducible gene 3) cassette exon 4. Expected pattern of alternative splicing of PIG3 minigene was observed, including previously characterized effects of UV light irradiation and specific mutations. Interestingly, we observed a broad distribution of normal to alternative transcript ratio in individual cells with at least two distinct populations with ∼45% and >95% alternative transcript. We believe that this method is useful for fluorescence-based quantitative analysis of alternative splicing of target genes in a variety of biological models.

Structure of the red fluorescent protein from a lancelet (Branchiostoma lanceolatum): a novel GYG chromophore covalently bound to a nearby tyrosine.

A key property of proteins of the green fluorescent protein (GFP) family is their ability to form a chromophore group by post-translational modifications of internal amino acids, e.g. Ser65-Tyr66-Gly67 in GFP from the jellyfish Aequorea victoria (Cnidaria). Numerous structural studies have demonstrated that the green GFP-like chromophore represents the `core' structure, which can be extended in red-shifted proteins owing to modifications of the protein backbone at the first chromophore-forming position. Here, the three-dimensional structures of green laGFP (λex/λem = 502/511 nm) and red laRFP (λex/λem ≃ 521/592 nm), which are fluorescent proteins (FPs) from the lancelet Branchiostoma lanceolatum (Chordata), were determined together with the structure of a red variant laRFP-ΔS83 (deletion of Ser83) with improved folding. Lancelet FPs are evolutionarily distant and share only ∼20% sequence identity with cnidarian FPs, which have been extensively characterized and widely used as genetically encoded probes. The structure of red-emitting laRFP revealed three exceptional features that have not been observed in wild-type fluorescent proteins from Cnidaria reported to date: (i) an unusual chromophore-forming sequence Gly58-Tyr59-Gly60, (ii) the presence of Gln211 at the position of the conserved catalytic Glu (Glu222 in Aequorea GFP), which proved to be crucial for chromophore formation, and (iii) the absence of modifications typical of known red chromophores and the presence of an extremely unusual covalent bond between the Tyr59 C(ß) atom and the hydroxyl of the proximal Tyr62. The impact of this covalent bond on the red emission and the large Stokes shift (∼70 nm) of laRFP was verified by extensive structure-based site-directed mutagenesis.

Engineering of a monomeric green-to-red photoactivatable fluorescent protein induced by blue light.

Green fluorescent protein (GFP) and GFP-like proteins represent invaluable genetically encoded fluorescent probes. In the last few years a new class of photoactivatable fluorescent proteins (PAFPs) capable of pronounced light-induced spectral changes have been developed. Except for tetrameric KFP1 (ref. 4), all known PAFPs, including PA-GFP, Kaede, EosFP, PS-CFP, Dronpa, PA-mRFP1 and KikGR require light in the UV-violet spectral region for activation through one-photon excitation--such light can be phototoxic to some biological systems. Here, we report a monomeric PAFP, Dendra, derived from octocoral Dendronephthya sp. and capable of 1,000- to 4,500-fold photoconversion from green to red fluorescent states in response to either visible blue or UV-violet light. Dendra represents the first PAFP, which is simultaneously monomeric, efficiently matures at 37 degrees C, demonstrates high photostability of the activated state, and can be photoactivated by a common, marginally phototoxic, 488-nm laser line. We demonstrate the suitability of Dendra for protein labeling and tracking to quantitatively study dynamics of fibrillarin and vimentin in mammalian cells.

KV1.2 channel-specific blocker from Mesobuthus eupeus scorpion venom: Structural basis of selectivity.

Scorpion venom is an unmatched source of selective high-affinity ligands of potassium channels. There is a high demand for such compounds to identify and manipulate the activity of particular channel isoforms. The objective of this study was to obtain and characterize a specific ligand of voltage-gated potassium channel KV1.2. As a result, we report the remarkable selectivity of the peptide MeKTx11-1 (α-KTx 1.16) from Mesobuthus eupeus scorpion venom to this channel isoform. MeKTx11-1 is a high-affinity blocker of KV1.2 (IC50 ∼0.2 nM), while its activity against KV1.1, KV1.3, and KV1.6 is 10 000, 330 and 45 000 fold lower, respectively, as measured using the voltage-clamp technique on mammalian channels expressed in Xenopus oocytes. Two substitutions, G9V and P37S, convert MeKTx11-1 to its natural analog MeKTx11-3 (α-KTx 1.17) having 15 times lower activity and reduced selectivity to KV1.2. We produced MeKTx11-1 and MeKTx11-3 as well as their mutants MeKTx11-1(G9V) and MeKTx11-1(P37S) recombinantly and demonstrated that point mutations provide an intermediate effect on selectivity. Key structural elements that explain MeKTx11-1 specificity were identified by molecular modeling of the toxin-channel complexes. Confirming our molecular modeling predictions, site-directed transfer of these elements from the pore region of KV1.2 to KV1.3 resulted in the enhanced sensitivity of mutant KV1.3 channels to MeKTx11-1. We conclude that MeKTx11-1 may be used as a selective tool in neurobiology.

A strategy for the generation of non-aggregating mutants of Anthozoa fluorescent proteins.

Recently, we cloned several fluorescent proteins of different colors homologous to Aequorea victoria green fluorescent protein, which have great biotechnological potential as in vivo markers of gene expression. However, later investigations revealed severe drawbacks in the use of novel fluorescent proteins (FPs), in particular, the formation of tetramers (tetramerization) and high molecular weight aggregates (aggregation). In this report, we employ a mutagenic approach to resolve the problem of aggregation. The elimination of basic residues located near the N-termini of FPs results in the generation of non-aggregating versions of several FPs, specifically, drFP583 (DsRed), DsRed-Timer, ds/drFP616, zFP506, zFP538, amFP486, and asFP595.

A strong strand displacement activity of thermostable DNA polymerase markedly improves the results of DNA amplification.

The sensitivity and robustness of various DNA detection and amplification techniques are to a large extent determined by the properties of the DNA polymerase used. We have compared the performance of conventional Taq and Bst DNA polymerases to a novel Taq DNA polymerase mutant (SD DNA polymerase), which has a strong strand displacement activity, in PCR (including amplification of GC-rich and complex secondary structure templates), long-range PCR (LR PCR), loop-mediated amplification (LAMP), and polymerase chain displacement reaction (PCDR). Our results demonstrate that the strand displacement activity of SD DNA polymerase, in combination with the robust polymerase activity, provides a notable improvement in the sensitivity and efficiency of all these methods.

Imaging H2O2 microdomains in receptor tyrosine kinases signaling.

HyPer, a ratiometric genetically encoded fluorescent sensor, is a popular tool for intracellular hydrogen peroxide detection. When expressed in cultured cells, the freely diffusing version of the sensor (HyPer-cyto) detects temporal patterns of H2O2 generation. However, rapid diffusion of the probe within the nucleocytoplasmic compartment averages the H2O2 signal even in cases of local oxidant production. Consequently, we immobilized the sensor within specific subcellular compartments allowing it to monitor local increases in H2O2. Here, we provide a protocol of ratiometric imaging and ImageJ-based quantification of H2O2 microdomains produced by cells upon physiological stimulation.

Bright monomeric red fluorescent protein with an extended fluorescence lifetime.

Fluorescent proteins have become extremely popular tools for in vivo imaging and especially for the study of localization, motility and interaction of proteins in living cells. Here we report TagRFP, a monomeric red fluorescent protein, which is characterized by high brightness, complete chromophore maturation, prolonged fluorescence lifetime and high pH-stability. These properties make TagRFP an excellent tag for protein localization studies and fluorescence resonance energy transfer (FRET) applications.

Bright far-red fluorescent protein for whole-body imaging.

For deep imaging of animal tissues, the optical window favorable for light penetration is in near-infrared wavelengths, which requires proteins with emission spectra in the far-red wavelengths. Here we report a far-red fluorescent protein, named Katushka, which is seven- to tenfold brighter compared to the spectrally close HcRed or mPlum, and is characterized by fast maturation as well as a high pH-stability and photostability. These unique characteristics make Katushka the protein of choice for visualization in living tissues. We demonstrate superiority of Katushka for whole-body imaging by direct comparison with other red and far-red fluorescent proteins. We also describe a monomeric version of Katushka, named mKate, which is characterized by high brightness and photostability, and should be an excellent fluorescent label for protein tagging in the far-red part of the spectrum.

Genetically encoded fluorescent indicator for intracellular hydrogen peroxide.

We developed a genetically encoded, highly specific fluorescent probe for detecting hydrogen peroxide (H(2)O(2)) inside living cells. This probe, named HyPer, consists of circularly permuted yellow fluorescent protein (cpYFP) inserted into the regulatory domain of the prokaryotic H(2)O(2)-sensing protein, OxyR. Using HyPer we monitored H(2)O(2) production at the single-cell level in the cytoplasm and mitochondria of HeLa cells treated with Apo2L/TRAIL. We found that an increase in H(2)O(2) occurs in the cytoplasm in parallel with a drop in the mitochondrial transmembrane potential (DeltaPsi) and a change in cell shape. We also observed local bursts in mitochondrial H(2)O(2) production during DeltaPsi oscillations in apoptotic HeLa cells. Moreover, sensitivity of the probe was sufficient to observe H(2)O(2) increase upon physiological stimulation. Using HyPer we detected temporal increase in H(2)O(2) in the cytoplasm of PC-12 cells stimulated with nerve growth factor.

Analysis of DsRed Mutants. Space around the fluorophore accelerates fluorescence development.

Earlier mutagenesis of the red fluorescent protein drFP583, also called DsRed, resulted in a mutant named Fluorescent Timer (Terskikh, A., Fradkov, A., Ermakova, G., Zaraisky, A., Tan, P., Kajava, A. V., Zhao, X., Lukyanov, S., Matz, M., Kim, S., Weissman, I., and Siebert, P. (2000) Science 290, 1585--1588). Further mutagenesis generated variants with novel and improved fluorescent properties. The mutant called AG4 exhibits only green fluorescence. The mutant, called E5up (V105A), shows complete fluorophore maturation, eventually eliminating residual green fluorescence present in DsRed. Finally, the mutant, called E57 (V105A, I161T, S197A), matures faster than DsRed as demonstrated in vitro with purified protein and in vivo with recombinant protein expressed in Escherichia coli and Xenopus leavis. Comparative analysis of the mutants in the context of the crystal structure of DsRed suggests that mutants with free space around the fluorophore mature faster and more completely.

Far-red fluorescent tag for protein labelling.

Practical applications of green fluorescent protein ('GFP')-like fluorescent proteins (FPs) from species of the class Anthozoa (sea anemones, corals and sea pens) are strongly restricted owing to their oligomeric nature. Here we suggest a strategy to overcome this problem by the use of two covalently linked identical red FPs as non-oligomerizing fusion tags. We have applied this approach to the dimeric far-red fluorescent protein HcRed1 and have demonstrated superiority of the tandem tag in the in vivo labelling of fine cytoskeletal structures and tiny nucleoli. In addition, a possibility of effective fluorescence resonance energy transfer ('FRET') between enhanced yellow FP mutant ('EYFP') and tandem HcRed1 was demonstrated in a protease assay.

A colourless green fluorescent protein homologue from the non-fluorescent hydromedusa Aequorea coerulescens and its fluorescent mutants.

We have cloned an unusual colourless green fluorescent protein (GFP)-like protein from Aequorea coerulescens (acGFPL). The A. coerulescens specimens displayed blue (not green) luminescence, and no fluorescence was detected in these medusae. Escherichia coli expressing wild-type acGFPL showed neither fluorescence nor visible coloration. Random mutagenesis generated green fluorescent mutants of acGFPL, with the strongest emitters found to contain an Glu(222)-->Gly (E222G) substitution, which removed the evolutionarily invariant Glu(222). Re-introduction of Glu(222) into the most fluorescent random mutant, named aceGFP, converted it into a colourless protein. This colourless aceGFP-G222E protein demonstrated a novel type of UV-induced photoconversion, from an immature non-fluorescent form into a green fluorescent form. Fluorescent aceGFP may be a useful biological tool, as it was able to be expressed in a number of mammalian cell lines. Furthermore, expression of a fusion protein of 'humanized' aceGFP and beta-actin produced a fluorescent pattern consistent with actin distribution in mammalian cells.

GFP-like proteins as ubiquitous metazoan superfamily: evolution of functional features and structural complexity.

Homologs of the green fluorescent protein (GFP), including the recently described GFP-like domains of certain extracellular matrix proteins in Bilaterian organisms, are remarkably similar at the protein structure level, yet they often perform totally unrelated functions, thereby warranting recognition as a superfamily. Here we describe diverse GFP-like proteins from previously undersampled and completely new sources, including hydromedusae and planktonic Copepoda. In hydromedusae, yellow and nonfluorescent purple proteins were found in addition to greens. Notably, the new yellow protein seems to follow exactly the same structural solution to achieving the yellow color of fluorescence as YFP, an engineered yellow-emitting mutant variant of GFP. The addition of these new sequences made it possible to resolve deep-level phylogenetic relationships within the superfamily. Fluorescence (most likely green) must have already existed in the common ancestor of Cnidaria and Bilateria, and therefore GFP-like proteins may be responsible for fluorescence and/or coloration in virtually any animal. At least 15 color diversification events can be inferred following the maximum parsimony principle in Cnidaria. Origination of red fluorescence and nonfluorescent purple-blue colors on several independent occasions provides a remarkable example of convergent evolution of complex features at the molecular level.