Allophycocyanin 1 as a near-infrared fluorescent tracer: isolation, characterization, chemical modification, and use in a homogeneous fluorescence resonance energy transfer system

Enhanced growth and metabolite production from a novel strain of Porphyridium sp

Microalgae are capable of generating numerous metabolites that possess notable biological activities and hold substantial promise for various industrial applications. Nevertheless, the taxonomic diversity of these photosynthetic microorganisms has not received thorough investigation. Using the 18S rRNA encoding gene, a recently discovered strain originating from the Tunisian coast (the governorate of Mahdia) was identified as a member of the Porphyridium genus. The growth response as well as the metabolite accumulation of Porphyridium sp. to different culture media (Pm, F/2, and Hemerick) was investigated over a period of 52 days. The highest biomass production was recorded with Pm medium (2 × 107 cell/mL). The apparent growth rates (µ) and the doubling time (Dt) were about 0.081 day-1 and 12.34 days, respectively. The highest chlorophyll a (0.678 ± 0.005 pg/cell), total carotenoids (0.18 ± 0.003 pg/cell), phycoerythrin (3.88 ± 0.003 pg/cell), and proteins (14.58 ± 0.35 pg/cell) contents were observed with F/2 medium. Cultivating Porphyridium sp. in both F/2 and Hemerick media yielded similar levels of starch accumulation. The Hemerick medium has proven to be the most suitable for the production of lipids (2.23% DW) and exopolysaccharides (5.41 ± 0.56 pg/cell).

Estimation of the relative contributions to the electronic energy transfer rates based on Förster theory: The case of C-phycocyanin chromophores

In the present study, we provide a reformulation of the theory originally proposed by Förster which allows for simple and convenient formulas useful to estimate the relative contributions of transition dipole moments of a donor and acceptor (chemical factors), their orientation factors (intermolecular structural factors), intermolecular center-to-center distances (intermolecular structural factors), spectral overlaps of absorption and emission spectra (photophysical factors), and refractive index (material factor) to the excitation energy transfer (EET) rate constant. To benchmark their validity, we focused on the EET occurring in C-phycocyanin (C-PC) chromophores. To this aim, we resorted to quantum chemistry calculations to get optimized molecular structures of the C-PC chromophores within the density functional theory (DFT) framework. The absorption and emission spectra, as well as transition dipole moments, were computed by using the time-dependent DFT (TDDFT). Our method was applied to several types of C-PCs showing that the EET rates are determined by an interplay of their specific physical, chemical, and geometrical features. These results show that our formulas can become a useful tool for a reliable estimation of the relative contributions of the factors regulating the EET transfer rate.

Metabolic engineering of Escherichia coli for efficient biosynthesis of fluorescent phycobiliprotein

Background

Phycobiliproteins (PBPs) are light-harvesting protein found in cyanobacteria, red algae and the cryptomonads. They have been widely used as fluorescent labels in cytometry and immunofluorescence analysis. A number of PBPs has been produced in metabolically engineered Escherichia coli. However, the recombinant PBPs are incompletely chromophorylated, and the underlying mechanisms are not clear.

Results and discussion

In this work, a pathway for SLA-PEB [a fusion protein of streptavidin and allophycocyanin that covalently binds phycoerythrobilin (PEB)] biosynthesis in E. coli was constructed using a single-expression plasmid strategy. Compared with a previous E. coli strain transformed with dual plasmids, the E. coli strain transformed with a single plasmid showed increased plasmid stability and produced SLA-PEB with a higher chromophorylation ratio. To achieve full chromophorylation of SLA-PEB, directed evolution was employed to improve the catalytic performance of lyase CpcS. In addition, the catalytic abilities of heme oxygenases from different cyanobacteria were investigated based on biliverdin IXα and PEB accumulation. Upregulation of the heme biosynthetic pathway genes was also carried out to increase heme availability and PEB biosynthesis in E. coli. Fed-batch fermentation was conducted for the strain V5ALD, which produced recombinant SLA-PEB with a chromophorylation ratio of 96.7%.

Conclusion

In addition to reporting the highest chromophorylation ratio of recombinant PBPs to date, this work demonstrated strategies for improving the chromophorylation of recombinant protein, especially biliprotein with heme, or its derivatives as a prosthetic group.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1100-6) contains supplementary material, which is available to authorized users.

Combinational biosynthesis and characterization of fusion proteins with tandem repeats of allophycocyanin holo-α subunits, and their application as bright fluorescent labels for immunofluorescence assay

Fusion protein of streptavidin and allophycocyanin holo-α subunit (ApcA) is fluorescent and is able to bind biotin. This fusion protein (SLA) can be used as fluorescent label in immunofluorescence assay. In this study, one or two repeats of ApcA were fused to the protein SLA, with the aim to improve its brightness. The fusion proteins SLA2 (two repeats of ApcA) and SLA3 (three repeats of ApcA), together with lyase (cpcS) and phycoerythrobilin synthesizing enzymes (Ho1 and PebS), were co-expressed in Escherichia coli. These fusion proteins were purified by affinity chromatography. While SLA2 and SLA3 shared similar absorbance spectra, fluorescence spectra and biotin-binding activities with SLA, their brightness were much higher than that of SLA. When used as fluorescent labels in immunofluorescence assay, SLA2 and SLA3 showed higher detection sensitivity than SLA. These results suggested that SLA2 and SLA3 were the preferable fluorescent labels in immunofluorescence assays.

Bacterial Phytochromes, Cyanobacteriochromes and Allophycocyanins as a Source of Near-Infrared Fluorescent Probes

Bacterial photoreceptors absorb light energy and transform it into intracellular signals that regulate metabolism. Bacterial phytochrome photoreceptors (BphPs), some cyanobacteriochromes (CBCRs) and allophycocyanins (APCs) possess the near-infrared (NIR) absorbance spectra that make them promising molecular templates to design NIR fluorescent proteins (FPs) and biosensors for studies in mammalian cells and whole animals. Here, we review structures, photochemical properties and molecular functions of several families of bacterial photoreceptors. We next analyze molecular evolution approaches to develop NIR FPs and biosensors. We then discuss phenotypes of current BphP-based NIR FPs and compare them with FPs derived from CBCRs and APCs. Lastly, we overview imaging applications of NIR FPs in live cells and in vivo. Our review provides guidelines for selection of existing NIR FPs, as well as engineering approaches to develop NIR FPs from the novel natural templates such as CBCRs.

Improving production of thermostable and fluorescent holo-β-allophycocyanin by metabolically engineered Escherichia coli using response surface methodology

A stable fluorescent holo-β-allophycocyanin (holo-ApcB) was produced by metabolically engineered Escherichia coli. The E. coli cells harbored two plasmids for expression of five genes that were involved in the holo-ApcB production. Response surface methodology was employed to investigate the individual and interactive effects of four variables, i.e., initial pH of culture medium, IPTG concentration, post-induction temperature, and induction start time, on holo-ApcB production by E. coli. The experimental results showed that the IPTG concentration, postinduction temperature, and induction start time had significant individual effects on holo-ApcB production. A significant interactive effect was also found between the initial pH of culture and induction start time. The maximum holo-ApcB production of 45.3 mg/L was predicted under the following optimized culture conditions: a postinduction temperature of 28.4°C, initial pH of culture of 7.3, IPTG concentration of 1.1 mM, and postinduction time of 66 min. Holo-ApcB production under the optimized culture conditions increased 5.8-fold, compared with that under the nonoptimized conditions. Response surface methodology proved to be a valuable tool for optimization of holo-ApcB production by metabolically engineered E. coli.

G protein-coupled receptor signaling analysis using homogenous time-resolved Förster resonance energy transfer (HTRF®) technology

Studying multidimensional signaling of G protein-coupled receptors (GPCRs) in search of new and better treatments requires flexible, reliable and sensitive assays in high throughput screening (HTS) formats. Today, more than half of the detection techniques used in HTS are based on fluorescence, because of the high sensitivity and rich signal, but quenching, optical interferences and light scattering are serious drawbacks. In the 1990s the HTRF® (Cisbio Bioassays, Codolet, France) technology based on Förster resonance energy transfer (FRET) in a time-resolved homogeneous format was developed. This improved technology diminished the traditional drawbacks. The optimized protocol described here based on HTRF® technology was used to study the activation and signaling pathways of the calcium-sensing receptor, CaSR, a GPCR responsible for maintaining calcium homeostasis. Stimulation of the CaSR by agonists activated several pathways, which were detected by measuring accumulation of the second messengers D-myo-inositol 1-phosphate (IP1) and cyclic adenosine 3',5'-monophosphate (cAMP), and by measuring the phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1/2). Here we show how an optimized HTRF® platform with numerous advantages compared to previous assays provides a substantial and robust mode of investigating GPCR signaling. It is furthermore discussed how these assays can be optimized and miniaturized to meet HTS requirements and for screening compound libraries.

Luminescent lanthanide cryptates: from the bench to the bedside

The design and application of luminescent lanthanide cryptates for sensing biological interactions is highlighted through the review of the work performed in our laboratory and with academic collaborations. The path from the initial applications probing biochemical interaction in vitro to "state-of-the-art" cellular assays toward clinical applications using homogeneous time-resolved fluorescence technology is described. An overview of the luminescent lanthanide macrocyclic compounds developed at Cisbio in the recent past is given with an emphasis on specific constraints required by specific applications. Recent assays for drug-discovery and diagnostic purposes using both antibody-based and suicide-enzyme-based technology are illustrated. New perspectives in the field of molecular medicine and time-resolved microscopy are discussed.

Biosynthesis of a stable allophycocyanin beta subunit in metabolically engineered Escherichia coli

Allophycocyanin (APC) is widely used as a fluorescent tag for fluorescence detection techniques. In this study, the apcB gene from a thermophilic cyanobacterium strain was cloned and ligated into an expression vector to construct a pathway for the biosynthesis of an allophycocyanin beta subunit (holo-apcBT) in Escherichia coli. Isopropyl β-d-1-thiogalactopyranoside induction successfully reconstituted holo-apcBT in E. coli. The recombinant holo-apcB showed spectroscopic properties similar to native APC. The stability and spectroscopic properties of this protein were then compared with another recombinant allophycocyanin beta subunit (holo-apcBM) whose apcB gene was cloned from mesophilic cyanobacterium. At high temperatures and during the course of storage, holo-apcBT was significantly more stable than holo-apcBM. In addition, holo-apcBT had an unexpectedly higher extinction coefficient and fluorescence quantum yield than holo-apcBM, suggesting that holo-apcBT would be a promising fluorescent tag and serve as a substitute for native APC.

Direct measurement of thermal stability of expressed CCR5 and stabilization by small molecule ligands

The inherent instability of heptahelical G protein-coupled receptors (GPCRs) during purification and reconstitution is a primary impediment to biophysical studies and to obtaining high-resolution crystal structures. New approaches to stabilizing receptors during purification and screening reconstitution procedures are needed. Here we report the development of a novel homogeneous time-resolved fluorescence assay (HTRF) to quantify properly folded CC-chemokine receptor 5 (CCR5). The assay permits high-throughput thermal stability measurements of femtomole quantities of CCR5 in detergent and in engineered nanoscale apolipoprotein-bound bilayer (NABB) particles. We show that recombinantly expressed CCR5 can be incorporated into NABB particles in high yield, resulting in greater thermal stability compared with that of CCR5 in a detergent solution. We also demonstrate that binding of CCR5 to the HIV-1 cellular entry inhibitors maraviroc, AD101, CMPD 167, and vicriviroc dramatically increases receptor stability. The HTRF assay technology reported here is applicable to other membrane proteins and could greatly facilitate structural studies of GPCRs.

Investigation of saturation and photobleaching of allophycocyanin by single-molecule recrossing events

Phycobiliprotein fluorescent labels are playing an increasingly important role in bioanalysis. They are also being used more and more frequently as light-harvesting materials for energy research. It is therefore critical to study the working conditions of these fluorescent dyes. Allophycocyanin (APC) belongs to a group of phycobiliproteins and features red excitation and emission, making it both a useful fluorophore and light-harvesting material. Saturation irradiance and photobleaching of APC were studied by single-molecule detection in this work. The mean fluorescence intensity at different laser powers was calculated from extracted single-molecule fluorescence peaks. By interpolating the figure of the mean fluorescence intensity as a function of excitation power, the experimental saturation irradiance can be extracted. By comparing the experimental with the calculated saturation irradiance, it can be demonstrated that the triplet state for APC was formed at higher excitation irradiance. The technique of molecular recrossing events was applied to investigate the photobleaching of APC. Normalized recrossing events confirmed that photobleaching occurred at high excitation power. This work provided the optimizing experimental conditions for APC both as a fluorophore and as a light-harvesting molecule.

Biochemical Detection of cGMP From Past to Present: An Overview

Cyclic guanosine monophosphate (cGMP), generated via the guanylate cyclase (GC)-catalyzed conversion from GTP, is unequivocally recognized as crucial second messenger, intimately involved in the regulation of a broad range of physiological processes such as long term potentiation, blood pressure regulation, or platelet aggregation (for review: Hobbs 2000). Since its first identification in rat urine by Ashman and co-workers (1963), various approaches have been conceived and established to quantify cGMP in biological samples, or to detect cGMP as the reaction product of enzymatic assays, allowing the determination of kinetic parameters. These approaches have evolved from laborious handling of small numbers of samples with average sensitivity to highly developed biochemical detection assays allowing the processing of very large numbers of samples. The present article focuses upon the history of biochemical cGMP detection from the pioneering work of the early years to the actual state-of-the-art approaches for the detection of this important biological messenger.

Functional adhesiveness of the CX3CL1 chemokine requires its aggregation. Role of the transmembrane domain

In its native form, the chemokine CX3CL1 is a firmly adhesive molecule promoting leukocyte adhesion and migration and hence involved, along with its unique receptor CX3CR1, in various inflammatory processes. Here we investigated the role of molecular aggregation in the CX3CL1 adhesiveness. Assays of bioluminescence resonance energy transfer (BRET) and homogeneous time-resolved fluorescence (HTRF) in transfected cell lines and in primary cells showed specific signals indicative of CX3CL1 clustering. Truncation experiments showed that the transmembrane domain played a central role in this aggregation. A chimera with mutations of the 12 central transmembrane domain residues had significantly reduced BRET signals and characteristics of a non-clustering molecule. This mutant was weakly adhesive according to flow and dual pipette adhesion assays and was less glycosylated than CX3CL1, although, as we demonstrated, loss of glycosylation did not affect the CX3CL1 adhesive potency. We postulate that cell surfaces express CX3CL1 as a constitutive oligomer and that this oligomerization is essential for its adhesive potency. Inhibition of CX3CL1 self-assembly could limit the recruitment of CX3CR1-positive cells and may be a new pathway for anti-inflammatory therapies.

Metal-Enhanced Fluorescence of Phycobiliproteins from Heterogeneous Plasmonic Nanostructures

We report here the use of plasmonic metal nanostructures in the form of silver island films (SiFs) to enhance the fluorescence emission of five different phycobiliproteins. Our findings clearly show that the phycobiliproteins display up to a 9-fold increase in fluorescence emission intensity, with a maximum 7-fold decrease in lifetime when they are assembled as a monolayer above SiFs, as compared to a monolayer assembled on the surface of amine-terminated glass slides of the control sample. The study was also repeated with a thin liquid layer of the phycobiliproteins sandwiched between two glass substrates (and a SiFs and a glass substrate) clamped together. Similarly, the results show a maximum 10-fold increase in fluorescence emission intensity coupled with a 2-fold decrease in lifetime of the phycobiliproteins in the SiF-glass setup as compared to the glass control sample, implying that near-field enhancement of phycobiliprotein emission can be attained both with and without chemical linkage of the proteins to the SiFs. Hence, our results clearly show that metal-enhanced fluorescence (MEF) can potentially be employed to increase the sensitivity and detection limit of the plethora of bioassays that employ phycobiliproteins as fluorescence labels, such as in fluoro-immunoassays where the assay can be tethered on the surface of SiFs, and also in flow cytometry where analytes in the liquid phase could potentially flow through channels coated with SiFs without actually being attached to the silver.

Lanthanides to quantum dots resonance energy transfer in time-resolved fluoro-immunoassays and luminescence microscopy

A time-resolved fluoro-immunoassay (TR-FIA) format is presented based on resonance energy transfer from visible emitting lanthanide complexes of europium and terbium, as energy donors, to semiconductor CdSe/ZnS core/shell nanocrystals (quantum dots, QD), as energy acceptors. The spatial proximity of the donor-acceptor pairs is obtained through the biological recognition process of biotin, coated at the surface of the dots (Biot-QD), and streptavidin labeled with the lanthanide markers (Ln-strep). The energy transfer phenomenon is evident from simultaneous lanthanide emission quenching and QD emission sensitization with a 1000-fold increase of the QD luminescence decay time reaching the hundred mus regime. Delayed emission detection allows for quantification of the recognition process and demonstrated a nearly quantitative association of the biotins to streptavidin with sensitivity limits reaching 1.2 pM of QD. Spectral characterization permits calculation of the energy transfer parameters. Extremely large Förster radii (R(0)) values were obtained for Tb (104 A) and Eu (96 A) as a result of the relevant spectral overlap of donor emission and acceptor absorption. Special attention was paid to interactions with the varying constituents of the buffer for sensitivity and transfer efficiency optimization. The energy transfer phenomenon was also monitored by time-resolved luminescence microscopy experiments. At elevated concentration (>10(-)(5) M), Tb-strep precipitated in the form of pellets with long-lived green luminescence, whereas addition of Biot-QD led to red emitting pellets, with long excited-state decay times. The Ln-QD donor-acceptor hybrids appear as highly sensitive analytical tools both for TR-FIA and time-resolved luminescence microscopy experiments.

d-myo-Inositol 1-phosphate as a surrogate of d-myo-inositol 1,4,5-tris phosphate to monitor G protein-coupled receptor activation

Phospholipase C beta (PLC-beta)-coupled G protein-coupled receptor (GPCR) activities traditionally are assessed by measuring Ca2+ triggered by D-myo-inositol 1,4,5-trisphosphate (IP3), a PLC-beta hydrolysis product, or by measuring the production of inositol phosphate using cumbersome radioactive assays. A specific detection of IP3 production was also established using IP3 binding proteins. The short lifetime of IP3 makes this detection very challenging in measuring GPCR responses. Indeed, this IP3 rapidly enters the metabolic inositol phosphate cascade. It has been known for decades that lithium chloride (LiCl) leads to D-myo-inositol 1-phosphate accumulation on GPCR activation by inhibiting inositol monophosphatase, the final enzyme of the IP3 metabolic cascade. We show here that IP1 can be used as a surrogate of IP3 to monitor GPCR activation. We developed a novel homogeneous time-resolved fluorescence (HTRF) assay that correlates perfectly with existing methods and is easily amenable to high-throughput screening. The IP-One assay was validated on various GPCR models. It has the advantage over the traditional Ca2+ assay of allowing the measurement of inverse agonist activity as well as the analysis of PLC-beta activity in any nontransfected primary cultures. Finally, the high assay specificity for D-myo-inositol 1 monophosphate (IP1(1)) opens new possibilities in developing selective assays to study the functional roles of the various isoforms of inositol phosphates.

Time-resolved Forster resonance energy transfer assays for the binding of nucleotide and protein substrates to p38alpha protein kinase

We have developed assays for the binding of nucleotide and protein substrates to p38alpha protein kinase based on time-resolved Forster resonance energy transfer. p38alpha was biotinylated by addition of a sequence that targets biotin to a single lysine when coexpressed with biotin ligase in Escherichia coli, allowing formation of a complex between a streptavidin "LANCE" europium chelate conjugate and p38alpha. When this reagent was combined with M39AF, a p38 inhibitor containing a fluorescent moiety whose excitation wavelengths match the emission wavelengths of the europium chelate, a change in ratio of light emitted at 665 nm/615 nm is detected. Less than 100pM complex was detected with a signal/background ratio of >30-fold. The complex exhibits slow, tight binding kinetics where the apparent K(d) decreases with a relaxation time of 21 min at 125 pM biotin-p38alpha. Preincubating inhibitors or ATP with biotin-p38alpha and adding M39AF as a competitor yielded IC(50)s consistent with those measured by enzyme assay for the activated form of biotin-p38alpha. The same technique was also used to measure affinity of inhibitors for the unphosphorylated and catalytically inactive form of biotin-p38alpha. To measure affinity of p38alpha for its protein substrate MK2, we incubated biotin-p38alpha with a glutathione S-transferase MK2 fusion protein. Detection of the complex after incubation with streptavidin-allophycocyanin and a LANCE-conjugated anti-GST allowed measurement of affinity of MK2 for biotin-p38alpha and detection of 0.5 nM p38alpha.MK2 complex with signal/background ratio >5-fold. Competition with unbiotinylated p38alpha yielded an IC(50) value of 5 nM. Activation of either p38alpha or MK2 had no effect on the measured K(d). M39AF was found to bind in a ternary complex with p38alpha.MK2 with lower affinity than that observed in the binary complex with p38alpha alone.

Two novel phycoerythrin-associated linker proteins in the marine cyanobacterium Synechococcus sp. strain WH8102

The recent availability of the whole genome of Synechococcus sp. strain WH8102 allows us to have a global view of the complex structure of the phycobilisomes of this marine picocyanobacterium. Genomic analyses revealed several new characteristics of these phycobilisomes, consisting of an allophycocyanin core and rods made of one type of phycocyanin and two types of phycoerythrins (I and II). Although the allophycocyanin appears to be similar to that found commonly in freshwater cyanobacteria, the phycocyanin is simpler since it possesses only one complete set of alpha and beta subunits and two rod-core linkers (CpcG1 and CpcG2). It is therefore probably made of a single hexameric disk per rod. In contrast, we have found two novel putative phycoerythrin-associated linker polypeptides that appear to be specific for marine Synechococcus spp. The first one (SYNW2000) is unusually long (548 residues) and apparently results from the fusion of a paralog of MpeC, a phycoerythrin II linker, and of CpeD, a phycoerythrin-I linker. The second one (SYNW1989) has a more classical size (300 residues) and is also an MpeC paralog. A biochemical analysis revealed that, like MpeC, these two novel linkers were both chromophorylated with phycourobilin. Our data suggest that they are both associated (partly or totally) with phycoerythrin II, and we propose to name SYNW2000 and SYNW1989 MpeD and MpeE, respectively. We further show that acclimation of phycobilisomes to high light leads to a dramatic reduction of MpeC, whereas the two novel linkers are not significantly affected. Models for the organization of the rods are proposed.

Cell surface detection of membrane protein interaction with homogeneous time-resolved fluorescence resonance energy transfer technology

Direct or indirect interactions between membrane proteins at the cell surface play a central role in numerous cell processes, including possible synergistic effects between different types of receptors. Here we describe a method and tools to analyze membrane protein-protein interaction at the surface of living cells. This technology is based on the use of specific antibodies directed against each partner and labeled either with europium cryptate or with Alexa Fluor 647. This allows the measurement of a fluorescence resonance energy transfer (FRET) signal in a time-resolved manner if both antibodies are in close proximity. This approach is here validated using the heterodimeric gamma-aminobutyrate B receptor as a model. We show that after washing out the unbound antibodies, the time-resolved FRET signal can be measured together with the expression level of both partners via the quantification of the donor and the acceptor fluorophores bound to the cells. Thanks to the high sensitivity of this method and to the low concentration of antibodies required, we show that the signal can also be measured directly after the incubation period without washing out the unbound antibody (homogeneous time-resolved FRET). As such, this method is highly sensitive, reproducible, and compatible with the development of high-throughput screening protocols.

Measuring human beta-secretase (BACE1) activity using homogeneous time-resolved fluorescence

The human beta-secretase enzyme, BACE1, mediates a critical step in the production of A beta(40) and A beta(42) peptides which are responsible for the severe neuronal cell death and insoluble amyloid plaques of Alzheimer's disease (AD). Several lines of evidence suggest that potent BACE1 inhibitors represent an attractive A beta-lowering strategy for AD. We designed a simple homogeneous time-resolved fluorescence (HTRF) assay which utilizes the fluorescence resonance energy transfer (FRET) pair europium and allophycocyanin for measuring BACE1 enzymatic activity in a high-throughput manner. Robust FRET was observed when an 18-amino-acid APP Swedish-synthetic peptide that was N-terminally labeled with europium cryptate and C-terminally biotinylated was incubated with streptavidin-coupled cross-linked allophycocyanin (SA-XL665). Purified BACE1 enzyme caused a time- and concentration-dependent linear change in FRET at low nanomolar enzyme concentrations. This assay was used to compare the autoprocessed "mature" BACE1 enzyme (sautoBACe1) and the soluble proBACE1 for activity and inhibition by selected peptidic BACE inhibitors. sautoBACE1 displayed only a modest increase in activity compared to sproBACE1 and this activity was uninhibited by the BACE1 prodomain peptide. Interestingly, the BACE1 prodomain peptide was able to partially inhibit sproBACE1 activity. IC(50s) for a P10-P4' statine BACE1 inhibitor, OM99-2, and OM-003 determined using the HTRF assay were in good agreement with those reported in the literature. The primary advantages of the HTRF-formatted BACE1 protease assay include appropriate reflection of native BACE1 activity, high sensitivity, low variability, and intrinsic quench correction afforded by ratiometric measurements made between EuK and SA-XL665 fluorophores.

Homogeneous time-resolved fluorescence assay for identifying p53 interactions with its protein partners, directly in a cellular extract

The p53 protein is a tumor suppressor that protects the organism against malignant consequences of DNA damage. Interaction of p53 with numerous cellular or viral proteins regulates its functional activity either positively or negatively. An approach leading to identification of such protein interactions directly in a cell extract could be of help in the development of screening assays to search for drugs acting on p53 in its cellular environment, either by disrupting its association with inhibitory proteins or by increasing its affinity for activating proteins. We show that the homogeneous time-resolved fluorescence (HTRF) assay based on the time-resolved amplified cryptate emission (TRACE) technology allows identification of such an interaction by simply adding a mixture of two labeled monoclonal antibodies, directly in a cellular extract. We validate this assay by studying p53/SV40-LTAg interactions. The antibodies directed against genuine p53 and SV40-LTAg epitopes were labeled with europium cryptate (donor) and XL665, a crosslinked allophycocyanin (acceptor), respectively. We demonstrated that a nonradiative energy transfer occurs between labeled antibodies only when p53 interacts with SV40-LTag, which opens up the possibility of extending this approach to other p53 partners to search for drugs that restore p53 tumor-suppressor activity.

Time resolved amplification of cryptate emission: a versatile technology to trace biomolecular interactions

Fluorescence resonance energy transfer (FRET) in association with a time-resolved fluorescence mode of detection was used to design a new homogeneous technology suitable to monitor biomolecular interactions. A lanthanide cryptate characterised by a long lived fluorescence emission was used as donor and a cross-linked allophycocyanine was used as acceptor. This new donor/acceptor pair displayed an exceptionally large Forster radius of 9 nm. This allowed to build up a set of labelling strategies to probe the interactions between biomolecules with an emphasis on fully indirect cassette formats particularly suitable for high throughput screening applications. Herein we describe the basics of the technology, review the latest applications to the study of molecular interactions involved in cells and new oligonucleotides based assays.