Thiol-reactive luminescent chelates of terbium and europium

The binding of a c-MYC promoter G-quadruplex to neurotransmitters: An analysis of G-quadruplex stabilization using DNA melting, fluorescence spectroscopy, surface-enhanced Raman scattering and molecular docking

The interactions of several neurotransmitter and neural hormone molecules with the c-MYC G-quadruplex DNA sequence were analyzed using a combination of spectroscopic and computational techniques. The interactions between indole, catecholamine, and amino acid neurotransmitters and DNA sequences could potentially add to the understanding of the role of G-quadruplex structures play in various diseases. Also, the interaction of the DNA sequence derived from the nuclear hypersensitivity element (NHE) III1 region of c-MYC oncogene (Pu22), 5'-TGAGGGTGGGTAGGGTGGGTAA-3', has added significance in that these molecules may promote or inhibit the formation of G-quadruplex DNA which could lead to the development of promising drugs for anticancer therapy. The results showed that these molecules did not disrupt G-quadruplex formation even in the absence of quadruplex-stabilizing cations. There was also evidence of concentration-dependent binding and high binding affinities based on the Stern-Volmer model, and thermodynamically favorable interactions in the form of hydrogen-bonding and interactions involving the π system of the aromatic neurotransmitters.

LRET Determination of Molecular Distances during pH Gating of the Mammalian Inward Rectifier Kir1.1b

Gating of the mammalian inward rectifier Kir1.1 at the helix bundle crossing (HBC) by intracellular pH is believed to be mediated by conformational changes in the C-terminal domain (CTD). However, the exact motion of the CTD during Kir gating remains controversial. Crystal structures and single-molecule fluorescence resonance energy transfer of KirBac channels have implied a rigid body rotation and/or a contraction of the CTD as possible triggers for opening of the HBC gate. In our study, we used lanthanide-based resonance energy transfer on single-Cys dimeric constructs of the mammalian renal inward rectifier, Kir1.1b, incorporated into anionic liposomes plus PIP₂, to determine unambiguous, state-dependent distances between paired Cys residues on diagonally opposite subunits. Functionality and pH dependence of our proteoliposome channels were verified in separate electrophysiological experiments. The lanthanide-based resonance energy transfer distances measured in closed (pH 6) and open (pH 8) conditions indicated neither expansion nor contraction of the CTD during gating, whereas the HBC gate widened by 8.8 ± 4 Å, from 6.3 ± 2 to 15.1 ± 6 Å, during opening. These results are consistent with a Kir gating model in which rigid body rotation of the large CTD around the permeation axis is correlated with opening of the HBC hydrophobic gate, allowing permeation of a 7 Å hydrated K ion.

Substrate-induced conformational changes in the nucleotide-binding domains of lipid bilayer-associated P-glycoprotein during ATP hydrolysis

P-glycoprotein (Pgp) is an efflux pump important in multidrug resistance of cancer cells and in determining drug pharmacokinetics. Pgp is a prototype ATP-binding cassette transporter with two nucleotide-binding domains (NBDs) that bind and hydrolyze ATP. Conformational changes at the NBDs (the Pgp engines) lead to changes across Pgp transmembrane domains that result in substrate translocation. According to current alternating access models (substrate-binding pocket accessible only to one side of the membrane at a time), binding of ATP promotes NBD dimerization, resulting in external accessibility of the drug-binding site (outward-facing, closed NBD conformation), and ATP hydrolysis leads to dissociation of the NBDs with the subsequent return of the accessibility of the binding site to the cytoplasmic side (inward-facing, open NBD conformation). However, previous work has not investigated these events under near-physiological conditions in a lipid bilayer and in the presence of transport substrate. Here, we used luminescence resonance energy transfer (LRET) to measure the distances between the two Pgp NBDs. Pgp was labeled with LRET probes, reconstituted in lipid nanodiscs, and the distance between the NBDs was measured at 37 °C. In the presence of verapamil, a substrate that activates ATP hydrolysis, the NBDs of Pgp reconstituted in nanodiscs were never far apart during the hydrolysis cycle, and we never observed the NBD-NBD distances of tens of Å that have previously been reported. However, we found two main conformations that coexist in a dynamic equilibrium under all conditions studied. Our observations highlight the importance of performing studies of efflux pumps under near-physiological conditions, in a lipid bilayer, at 37 °C, and during substrate-stimulated hydrolysis.

Luminescence resonance energy transfer spectroscopy of ATP-binding cassette proteins

The ATP-binding cassette (ABC) superfamily includes regulatory and transport proteins. Most human ABC exporters pump substrates out of cells using energy from ATP hydrolysis. Although major advances have been made toward understanding the molecular mechanism of ABC exporters, there are still many issues unresolved. During the last few years, luminescence resonance energy transfer has been used to detect conformational changes in real time, with atomic resolution, in isolated ABC nucleotide binding domains (NBDs) and full-length ABC exporters. NBDs are particularly interesting because they provide the power stroke for substrate transport. Luminescence resonance energy transfer (LRET) is a spectroscopic technique that can provide dynamic information with atomic-resolution of protein conformational changes under physiological conditions. Using LRET, it has been shown that NBD dimerization, a critical step in ABC proteins catalytic cycle, requires binding of ATP to two nucleotide binding sites. However, hydrolysis at just one of the sites can drive dissociation of the NBD dimer. It was also found that the NBDs of the bacterial ABC exporter MsbA reconstituted in a lipid bilayer membrane and studied at 37°C never separate as much as suggested by crystal structures. This observation stresses the importance of performing structural/functional studies of ABC exporters under physiologic conditions. This article is part of a Special Issue entitled: Beyond the Structure-Function Horizon of Membrane Proteins edited by Ute Hellmich, Rupak Doshi and Benjamin McIlwain.

Luminescence resonance energy transfer to study conformational changes in membrane proteins expressed in mammalian cells

Luminescence Resonance Energy Transfer, or LRET, is a powerful technique used to measure distances between two sites in proteins within the distance range of 10-100 Å. By measuring the distances under various ligated conditions, conformational changes of the protein can be easily assessed. With LRET, a lanthanide, most often chelated terbium, is used as the donor fluorophore, affording advantages such as a longer donor-only emission lifetime, the flexibility to use multiple acceptor fluorophores, and the opportunity to detect sensitized acceptor emission as an easy way to measure energy transfer without the risk of also detecting donor-only signal. Here, we describe a method to use LRET on membrane proteins expressed and assayed on the surface of intact mammalian cells. We introduce a protease cleavage site between the LRET fluorophore pair. After obtaining the original LRET signal, cleavage at that site removes the specific LRET signal from the protein of interest allowing us to quantitatively subtract the background signal that remains after cleavage. This method allows for more physiologically relevant measurements to be made without the need for purification of protein.

Direct and selective tagging of cysteine residues in peptides and proteins with 4-nitropyridyl lanthanide complexes

A cysteine-selective tagging method in water is reported, based on rapid displacement of a pyridyl nitro-substituent in simple pyridines and lanthanide complexes. The conjugation reaction creates a short link between the tag and peptide, holding the peptide closer to the Ln(3+) ion and with reduced flexibility compared to existing methods.

New quinolone-based thiol-reactive lanthanide luminescent probes

Luminescent lanthanide ion complexes are distinguished by unique light emitting properties that enable both highly sensitive detection of lanthanide labels attached to biomolecules and contrast imaging of various micro objects (cells, nanoparticles, etc.). Previously, we synthesized amine-reactive cs124-based luminescent lanthanide chelates with improved brightness and metal retention. Here we report the synthesis of new thiol-reactive derivatives of the same compounds including bromoacetamido-, and maleimido- forms of cs124 and cs124CF₃ fluorophores. Maleimido-compounds displayed exceptional reactivity instantaneously coupling to thiols at physiological conditions at micromolar probes concentrations. Surprisingly, they displayed strong quenching by adjacent maleimido-group, which was completely eliminated after reaction with thiols, thereby enabling their simple detection by monitoring the light emission of the reaction mixture. This reaction can be used for hyper-sensitive determination of biologically important sulphydryl compounds (e.g. glutathione, co-enzyme A, etc.) in time-resolved mode.

Association/dissociation of the nucleotide-binding domains of the ATP-binding cassette protein MsbA measured during continuous hydrolysis

In ATP-binding cassette proteins, the two nucleotide-binding domains (NBDs) work as dimers to bind and hydrolyze ATP, but the molecular mechanism of nucleotide hydrolysis is controversial. It is still unresolved whether hydrolysis leads to dissociation of the ATP-induced dimers or partial opening of the dimers such that the NBDs remain in contact during the hydrolysis cycle. We studied the bacterial lipid flippase MsbA by luminescence resonance energy transfer (LRET). The LRET signal between optical probes reacted with single-cysteine mutants was employed to follow NBD association/dissociation in real time. The intermonomer distances calculated from LRET data indicate that the NBDs separate completely following ATP hydrolysis, even in the presence of mm MgATP, and that the dissociation occurs following each hydrolysis cycle. The results support association/dissociation, as opposed to constant contact models, for the mode of operation of ATP-binding cassette proteins.

Simple no-chromatography procedure for amine-reactive Eu3+ luminescent chelates optimal for bioconjugation

Lanthanide ions luminescence has long life time enabling highly sensitive detection in time-gated mode. The synthesis of reactive lanthanide probes for covalent labeling of the objects of interest is cumbersome task due to the large size of the probes, complex multi-step procedures and the presence of sensitive groups, which often prevents introduction of reactive cross-linking functions optimal for conjugation. We suggest simple synthetic protocol for luminescent europium chelates based on serendipitous reaction yielding acylating compounds, whose reactivity is comparable to that of commonly used N-hydroxysuccinimide (NHS) esters. The probes react with proteins at pH 7.0 within several minutes at ambient temperature displaying high coupling efficiency. The resulting conjugates survive electrophoretic separation under denaturing conditions, which makes the labels useful in proteomic studies that rely on high detection sensitivity.

A limited 4 Å radial displacement of the S4-S5 linker is sufficient for internal gate closing in Kv channels

Voltage-gated ion channels are responsible for the generation of action potentials in our nervous system. Conformational rearrangements in their voltage sensor domains in response to changes of the membrane potential control pore opening and thus ion conduction. Crystal structures of the open channel in combination with a wealth of biophysical data and molecular dynamics simulations led to a consensus on the voltage sensor movement. However, the coupling between voltage sensor movement and pore opening, the electromechanical coupling, occurs at the cytosolic face of the channel, from where no structural information is available yet. In particular, the question how far the cytosolic pore gate has to close to prevent ion conduction remains controversial. In cells, spectroscopic methods are hindered because labeling of internal sites remains difficult, whereas liposomes or detergent solutions containing purified ion channels lack voltage control. Here, to overcome these problems, we controlled the state of the channel by varying the lipid environment. This way, we directly measured the position of the S4-S5 linker in both the open and the closed state of a prokaryotic Kv channel (KvAP) in a lipid environment using Lanthanide-based resonance energy transfer. We were able to reconstruct the movement of the covalent link between the voltage sensor and the pore domain and used this information as restraints for molecular dynamics simulations of the closed state structure. We found that a small decrease of the pore radius of about 3-4 Å is sufficient to prevent ion permeation through the pore.

Direct detection of adenosine in undiluted serum using a luminescent aptamer sensor attached to a terbium complex

Aptamers, single-stranded nucleic acids that can selectively bind to various target molecules, have been widely used for constructing biosensors. A major challenge in this field, however, is direct sensing of analytes in complex biological media such as undiluted serum. While progress has been made in developing an inhomogeneous assay by using a preseparation step to wash away the interferences within serum, a facile strategy for direct detection of targets in homogeneous unprocessed serum is highly desired. We herein report a turn-on luminescent aptamer biosensor for the direct detection of adenosine in undiluted and unprocessed serum, by taking advantage of a terbium chelate complex with long luminescence lifetime to achieve time-resolved detection. The sensor exhibits a detection limit of 60 μM adenosine while marinating excellent selectivity that is comparable to those in buffer. The approach demonstrated here can be applied for direct detection and quantification of a broad range of analytes in biological media by using other aptamers.

Dissociation of ATP-binding cassette nucleotide-binding domain dimers into monomers during the hydrolysis cycle

ATP-binding cassette (ABC) proteins have two nucleotide-binding domains (NBDs) that work as dimers to bind and hydrolyze ATP, but the molecular mechanism of nucleotide hydrolysis is controversial. In particular, it is still unresolved whether hydrolysis leads to dissociation of the ATP-induced dimers or opening of the dimers, with the NBDs remaining in contact during the hydrolysis cycle. We studied a prototypical ABC NBD, the Methanococcus jannaschii MJ0796, using spectroscopic techniques. We show that fluorescence from a tryptophan positioned at the dimer interface and luminescence resonance energy transfer between probes reacted with single-cysteine mutants can be used to follow NBD association/dissociation in real time. The intermonomer distances calculated from luminescence resonance energy transfer data indicate that the NBDs separate completely following ATP hydrolysis, instead of opening. The results support ABC protein NBD association/dissociation, as opposed to constant-contact models.

Luminescent trimethoprim-polyaminocarboxylate lanthanide complex conjugates for selective protein labeling and time-resolved bioassays

Labeling proteins with long-lifetime emitting lanthanide (III) chelate reporters enables sensitive, time-resolved luminescence bioaffinity assays. Heterodimers of trimethoprim (TMP) covalently linked to various cs124-sensitized, polyaminocarboxylate chelates stably retain lanthanide ions and exhibit quantum yields of europium emission up to 20% in water. A time-resolved, luminescence resonance energy transfer (LRET) assay showed that TMP-polyaminocarboxylates bind to Escherichia coli dihydrofolate reductase (eDHFR) fusion proteins with nanomolar affinity in purified solutions and in bacterial lysates. The ability to selectively impart terbium or europium luminescence to fusion proteins in complex physiological mixtures bypasses the need for specific antibodies and simplifies sample preparation.

Lanthanide-based time-resolved luminescence immunoassays

The sensitive and specific detection of analytes such as proteins in biological samples is critical for a variety of applications, for example disease diagnosis. In immunoassays a signal in response to the concentration of analyte present is generated by use of antibodies labeled with radioisotopes, luminophores, or enzymes. All immunoassays suffer to some extent from the problem of the background signal observed in the absence of analyte, which limits the sensitivity and dynamic range that can be achieved. This is especially the case for homogeneous immunoassays and surface measurements on tissue sections and membranes, which typically have a high background because of sample autofluorescence. One way of minimizing background in immunoassays involves the use of lanthanide chelate labels. Luminescent lanthanide complexes have exceedingly long-lived luminescence in comparison with conventional fluorophores, enabling the short-lived background interferences to be removed via time-gated acquisition and delivering greater assay sensitivity and a broader dynamic range. This review highlights the potential of using lanthanide luminescence to design sensitive and specific immunoassays. Techniques for labeling biomolecules with lanthanide chelate tags are discussed, with aspects of chelate design. Microtitre plate-based heterogeneous and homogeneous assays are reviewed and compared in terms of sensitivity, dynamic range, and convenience. The great potential of surface-based time-resolved imaging techniques for biomolecules on gels, membranes, and tissue sections using lanthanide tracers in proteomics applications is also emphasized.

Luminescent probes for ultrasensitive detection of nucleic acids

Novel amino-reactive derivatives of lanthanide-based luminescent labels of enhanced brightness and metal retention were synthesized and used for the detection of cDNA oligonucleotides by molecular beacons. Time-resolved acquisition of the luminescent signal that occurs upon hybridization of the probe to the target enabled the avoidance of short-lived background fluorescence, markedly enhancing the sensitivity of detection, which was less than 1 pM. This value is about 50 to 100 times more sensitive than the level achieved with conventional fluorescence-based molecular beacons, and is 10 to 60 times more sensitive than previously reported for other lanthanide-based hybridization probes. These novel luminescent labels should significantly enhance the sensitivity of all type of nucleic acid hybridization probes, and could dramatically improve the detection limit of other biopolymers and small compounds that are used in a variety of biological applications.

Paramagnetic labelling of proteins and oligonucleotides for NMR

Paramagnetic effects offer a rich source of long-range structural restraints. Here we review current methods for site-specific tagging of proteins and oligonucleotides with paramagnetic molecules. The paramagnetic tags include nitroxide radicals and metal chelators. Particular emphasis is placed on tags suitable for site-specific and rigid attachment of lanthanide ions to macromolecules.

Luminescent terbium protein labels for time-resolved microscopy and screening

Brilliance of terbium: Heterodimeric conjugates of trimethoprim covalently linked to sensitized terbium chelates bind to Escherichia coli dihydrofolate reductase fusion proteins with nanomolar affinity (see picture). Terbium luminescence enables sensitive and time-resolved detection of labeled proteins in vitro and on the surface of living mammalian cells.

A new quinoline sensitizer-centered lanthanide chelate and its use for protein labling on Ni-NTA beads for TR LRET assays

A quinoline sensitizer-centered lanthanide chelate system of novel design for TR-LRET was prepared; it exhibited high labelling efficiency with a his-tagged protein (ERalpha-LBD) on the Ni-NTA beads, using a mixed metal chelate protocol, and it functioned well in TR-LRET protein-protein interaction assays.

Extent of voltage sensor movement during gating of shaker K+ channels

Voltage-driven activation of Kv channels results from conformational changes of four voltage sensor domains (VSDs) that surround the K(+) selective pore domain. How the VSD helices rearrange during gating is an area of active research. Luminescence resonance energy transfer (LRET) is a powerful spectroscopic ruler uniquely suitable for addressing the conformational trajectory of these helices. Using a geometric analysis of numerous LRET measurements, we were able to estimate LRET probe positions relative to existing structural models. The experimental movement of helix S4 does not support a large 15-20 A transmembrane "paddle-type" movement or a near-zero A vertical "transporter-type" model. Rather, our measurements demonstrate a moderate S4 displacement of 10 +/- 5 A, with a vertical component of 5 +/- 2 A. The S3 segment moves 2 +/- 1 A in the opposite direction and is therefore not moving as an S3-S4 rigid body.

Transient inter-cellular polymeric linker

Three-dimensional (3D) tissue-engineered constructs with bio-mimicry cell-cell and cell-matrix interactions are useful in regenerative medicine. In cell-dense and matrix-poor tissues of the internal organs, cells support one another via cell-cell interactions, supplemented by small amount of the extra-cellular matrices (ECM) secreted by the cells. Here we connect HepG2 cells directly but transiently with inter-cellular polymeric linker to facilitate cell-cell interaction and aggregation. The linker consists of a non-toxic low molecular-weight polyethyleneimine (PEI) backbone conjugated with multiple hydrazide groups that can aggregate cells within 30 min by reacting with the aldehyde handles on the chemically modified cell-surface glycoproteins. The cells in the cellular aggregates proliferated; and maintained the cortical actin distribution of the 3D cell morphology while non-aggregated cells died over 7 days of suspension culture. The aggregates lost distinguishable cell-cell boundaries within 3 days; and the ECM fibers became visible around cells from day 3 onwards while the inter-cellular polymeric linker disappeared from the cell surfaces over time. The transient inter-cellular polymeric linker can be useful for forming 3D cellular and tissue constructs without bulk biomaterials or extensive network of engineered ECM for various applications.

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.

Change in permeant size selectivity by phosphorylation of connexin 43 gap-junctional hemichannels by PKC

Gap-junctional channels, permeable to large hydrophilic solutes of up to M(r) approximately 1,000, are responsible for cell-to-cell communication. Phosphorylation of connexin 43 (Cx43) by PKC abolishes the permeability of gap-junctional channels and hemichannels to large hydrophilic solutes, but not to small inorganic ions. Here, we report on a methodology to produce purified hemichannels of controlled subunit composition and apply it to the generation of hemichannels with variable number of PKC-phosphorylated subunits. The subunit composition was determined by luminescence resonance energy transfer. We show that all Cx43 subunits in the hemichannel hexamer have to be phosphorylated to abolish sucrose (M(r) 342) permeability. We also show that the hemichannel pores with all subunits phosphorylated by PKC have a sizable diameter, allowing for permeation of the small hydrophilic solute ethyleneglycol (M(r) 62). These results indicate that phosphorylation of Cx43 by PKC alters the hemichannel size selectivity and explain why PKC activity affects dye transfer between cells without consistent effects on electrical communication.

Near IR-emitting DNA-probes exploiting stepwise energy transfer processes

The synthesis and characterisation of two new cyclen-based near IR-emitting lanthanide complexes is reported; the lanthanides are sensitised by rhodamine, which in turn is excited by energy transfer from a coumarin 2 moiety. The three lumophores function as an energy transfer cascade spanning the UV-visible-near IR region of the spectrum, resulting in large Stokes shifts. Double stranded DNA selectively switches one of the two energy transfer processes off, enabling luminescent DNA-sensing in the near IR region. The regioselective di-alkylation of the cyclen scaffold is explained with the help of DFT calculations.

Synthesis of luminescent heterometallic bis-lanthanide complexes via selective, sequential metallation

A modular synthetic method for the differential incorporation of two lanthanide ions into a single molecular scaffold is reported; the mixed bimetallic Tb/Eu complex displays an interesting solvent polarity-dependent ratiometric luminescence.

Lanthanide complex-based fluorescence label for time-resolved fluorescence bioassay

Different from organic fluorescence dyes, fluorescent lanthanide complexes have the fluorescence properties of long fluorescence lifetime, large Stokes shift and sharp emission profile, which makes them favorable be used as the fluorescent labeling reagents for microsecond time-resolved fluorescence bioassay. Lanthanide complex-based fluorescence labels have been successfully used for highly sensitive time-resolved fluorescence immunoassay, DNA hybridization assay, cell activity assay, and bio-imaging microscopy assay. Since the technique allows easy distinction of the specific fluorescence signal of the long-lived label from short-lived background noises associated with biological samples, scattering lights (Tyndall, Rayleigh and Raman scatterings) and the optical components (cuvettes, filters and lenses), the sensitivity of fluorescence bioassay has been remarkably improved. This paper summarized the recent developments of lanthanide complex-based fluorescence labels and their applications in time-resolved fluorescence bioassays mainly based on the authors' researches and relative publications.

G-protein-coupled receptors in drug discovery: nanosizing using cell-free technologies and molecular biology approaches

Signal transduction by G-protein-coupled receptors (GPCRs) underpins a multitude of physiological processes. Ligand recognition by the receptor leads to activation of a generic molecular switch involving heterotrimeric G-proteins and guanine nucleotides. Signal transduction has been studied extensively with both cell-based systems and assays comprising isolated signaling components. Interest and commercial investment in GPCRs in areas such as drug targets, orphan receptors, high throughput screening, biosensors, and so on will focus greater attention on assay development to allow for miniaturization, ultra-high throughput and, eventually, microarray/biochip assay formats. Although cell-based assays are adequate for many GPCRs, it is likely that these formats will limit the development of higher density GPCR assay platforms mandatory for other applications. Stable, robust, cell-free signaling assemblies comprising receptor and appropriate molecular switching components will form the basis of future GPCR assay platforms adaptable for such applications as microarrays. The authors review current cell-free GPCR assay technologies and molecular biological approaches for construction of novel, functional GPCR assays.

Use of luminescence resonance energy transfer to measure distances in the AE1 anion exchange protein dimer

To understand how red blood cell and other proteins carry out their functions, it is necessary not only to have high-resolution crystal structures, but also to have methods that can measure changes in position of parts of the protein on the scale of Angstroms. The method of luminescence resonance energy transfer (LRET) has considerable advantages for this purpose, particularly for proteins, such as the AE1 anion exchange protein in the red cell, that are homodimers. We have applied this method, using a terbium maleimide chelate (TbM) as donor and fluorescein maleimide (FM) as acceptor, to measure the distance between the C201 residues in adjacent dimerized cytoplasmic domains of AE1 (cdAE1). The distance measured by LRET (40.8 A) corresponds closely with that calculated from the crystal structure of the cdAE1, indicating that the method can provide useful information for testing hypotheses concerning motions in this and other blood cell proteins.

Fluorescence lifetime-resolved imaging: measuring lifetimes in an image

We have given an overview of what one can gain by lifetime-resolved imaging and reviewed the major issues concerning lifetime-resolved measurements and FLI instrumentation. Instead of giving diverse selected examples, we have discussed the underlying basic pathways of deexcitation available to the molecules in the excited state. It is by traversing these pathways that compete kinetically with the fluorescence pathway of deactivation--and therefore affect the measured fluorescence lifetime--that we gain the information that lifetime-resolved fluorescence provides. It is hoped that being aware of the diversity, of pathways available to an excited fluorophore will facilitate potential users to recognize the value of FLI measurements and inspire innovative experiments using lifetime-resolved imaging. FLI gives us the ability within a fluorescence image of measuring and quantifying dynamic events taking place in the immediate surroundings of fluorophores as well as locating the fluorescent components within the image. Just as measurements in cuvettes, lifetime-resolved imaging extends considerably the potential information that can be derived from a fluorescence experiment. Our purpose has been to arouse an appreciation for the broad application of fluorescence lifetime-resolved measurements in imaging. We have given only general design characteristics of the instrumentation and discussed the characteristics that distinguish imaging from the single channel lifetime-resolved measurements. We have not provided details of the instrumentation or the presented many examples. These are available in the literature, and given in the references, and they are continually and rapidly growing.

An actin-dependent conformational change in myosin

Conformational changes within myosin lead to its movement relative to an actin filament. Several crystal structures exist for myosin bound to various nucleotides, but none with bound actin. Therefore, the effect of actin on the structure of myosin is poorly understood. Here we show that the swing of smooth muscle myosin lever arm requires both ADP and actin. This is the first direct observation that a conformation of myosin is dependent on actin. Conformational changes within myosin were monitored using fluorescence resonance energy transfer techniques. A cysteine-reactive probe is site-specifically labeled on a 'cysteine-light' myosin variant, in which the native reactive cysteines were removed and a cysteine engineered at a desired position. Using this construct, we show that the actin-dependent ADP swing causes an 18 A change in distance between a probe on the 25/50 kDa loop on the catalytic domain and a probe on the regulatory light chain, corresponding to a 23 degrees swing of the light-chain domain.

Principles and biophysical applications of lanthanide-based probes

Using luminescent lanthanides, instead of conventional fluorophores, as donor molecules in resonance energy transfer measurements offers many technical advantages and opens up a wide range of new applications. Advantages include farther measurable distances ( approximately 100 A) with greater accuracy, insensitivity to incomplete labeling, and the ability to use generic relatively large labels, when necessary. Applications highlighted include the study of ion channels in living cells, protein-protein interaction in cells, DNA-protein complexes, and high-throughput screening assays to measure peptide dimerization associated with DNA transcription factors and ligand-receptor interactions.

Engineering a terbium-binding site into an integral membrane protein for luminescence energy transfer

Luminescence resonance energy transfer with a lanthanide like Tb(3+) as donor is a useful technique for estimating intra- and intermolecular distances in macromolecules. However, the technique usually requires the use of a bulky chelator with a flexible linker attached to a Cys residue to bind Tb(3+) and, for intramolecular studies, an acceptor fluorophor attached to another Cys residue in the same protein. Here, an engineered EF- hand motif is incorporated into the central cytoplasmic loop of the lactose permease of Escherichia coli generating a high-affinity site for Tb(3+) (K(Tb)(3+) approximately 4.5 microM) or Gd(3+) (K(Gd)(3+) approximately 2.3 microM). By exciting a Trp residue in the coordination sequence, Tb(3+) bound to the EF-hand motif is sensitized specifically, and the efficiency of energy transfer to strategically placed Cys residues labeled with fluorophors is measured. In this study, we use the technique to measure distance from the EF-hand in the central cytoplasmic loop of lactose permease to positions 179 or 169 at the center or periplasmic end of helix VI, respectively. The average calculated distances of approximately 23 A (position 179) and approximately 33 A (position 169) observed with three different fluorophors as acceptors agree well with the geometry of a slightly tilted alpha-helix. The approach should be of general use for studying static and dynamic aspects of polytopic membrane protein structure and function.

Sensitization of lanthanides by nonnatural amino acids

The sensitization of Eu(III) and Tb(III) by ethylenediaminetetraaceticacid (EDTA)-derivatized tryptophan (Trp), 7-azatryptophan (7AW) and 5-hydroxytryptophan (5HW) has been examined. These Trp analogs were utilized in the present study because they can be incorporated into proteins in place of native Trp residues and because they absorb strongly beyond 305 nm (where Trp absorbance goes to zero), allowing selective excitation of such species in the presence of other Trp-containing proteins. All three indole derivatives were able to sensitize Tb(III) luminescence, with the relative sensitization being in the order Trp > 5HW > 7AW. On the other hand, only the 7AW-EDTA complex was able to sensitize Eu(III) luminescence, likely owing to a better spectral overlap between 7AW emission and Eu(III) absorbance. The sensitized emission of Tb(III) and Eu(II) displayed the expected long emission lifetimes at 545 nm [for Tb(III)] and 617 nm [for Eu(III)], indicating that long-lifetime lanthanide emission could be produced using nonnatural amino-acid donors. Thus, 7AW- and 5HW-sensitized lanthanide emissions should prove to be useful in biophysical studies, such as the use of fluorescence energy transfer to probe biomolecular interactions in vivo.

Multi-Photon Sensitized Excitation of Near Infrared Emitting Lanthanides

Near infrared (NIR) multi-photon excitation of the NIR-emitting lanthanides neodymium (Nd³⁺) and ytterbium (Yb³⁺) sensitized by a fluorescein-linked chelator was demonstrated. Because tissues display minimal absorbance near the excitation wavelength of 800 nm, and because the lanthanides display long decay times, these results suggest the use of Nd³⁺ and Yb³⁺ as luminescent probes in tissues with multi-photon excitation.

Luminescence resonance energy transfer analysis of RNA polymerase complexes

Resonance energy transfer allows measurement of atomic-scale distances under a variety of solution conditions. Luminescence resonance energy transfer (LRET) is a variant of energy transfer measurement in which lanthanide chelates are used as the probes. The unusual properties of lanthanide emission, in particular their long microsecond-scale lifetimes, offer several advantages for energy transfer measurements with biological samples. One of the unique features of LRET is the ability to measure energy transfer under conditions where severe heterogeneity of labeled macromolecules exists. This feature of LRET is the special emphasis of this article. We describe here LRET methodology with a particular attention to using sensitized acceptor emission to determine efficiency of energy transfer. Although we employed this technique in the characterization of Escherichia coli RNA polymerase complexes it is readily compatible with the study of essentially any protein.