A fluorescent biosensor reveals conformational changes in human immunoglobulin E Fc: implications for mechanisms of receptor binding, inhibition, and allergen recognition

IgE binding to its high affinity receptor FcεRI on mast cells and basophils is a key step in the mechanism of allergic disease and a target for therapeutic intervention. Early indications that IgE adopts a bent structure in solution have been confirmed by recent x-ray crystallographic studies of IgEFc, which further showed that the bend, contrary to expectation, is enhanced in the crystal structure of the complex with receptor. To investigate the structure of IgEFc and its conformational changes that accompany receptor binding in solution, we created a Förster resonance energy transfer (FRET) biosensor using biologically encoded fluorescent proteins fused to the N- and C-terminal IgEFc domains (Cε2 and Cε4, respectively) together with the theoretical basis for quantitating its behavior. This revealed not only that the IgEFc exists in a bent conformation in solution but also that the bend is indeed enhanced upon FcεRI binding. No change in the degree of bending was seen upon binding to the B cell receptor for IgE, CD23 (FcεRII), but in contrast, binding of the anti-IgE therapeutic antibody omalizumab decreases the extent of the bend, implying a conformational change that opposes FcεRI engagement. HomoFRET measurements further revealed that the (Cε2)(2) and (Cε4)(2) domain pairs behave as rigid units flanking the conformational change in the Cε3 domains. Finally, modeling of the accessible conformations of the two Fab arms in FcεRI-bound IgE revealed a mutual exclusion not seen in IgG and Fab orientations relative to the membrane that may predispose receptor-bound IgE to cross-linking by allergens.

Conformation and dynamics of a rhodamine probe attached at two sites on a protein: implications for molecular structure determination in situ

Replica exchange molecular dynamics (REMD) calculations were used to determine the conformation and dynamics of bifunctional rhodamine probes attached to pairs of cysteines in three model systems: (a) a polyalanine helix, (b) the isolated C helix (residues 53-66) of troponin C, and (c) the C helix of the N-terminal region (residues 1-90) of troponin C (sNTnC). In each case, and for both diastereoisomers of each probe-protein complex, the hydrophobic face of the probe is close to the protein surface, and its carboxylate group is highly solvated. The visible-range fluorescence dipole of the probe is approximately parallel to the line joining the two cysteine residues, as assumed in previous in situ fluorescence polarization studies. The independent rotational motion of the probe with respect to the protein on the nanosecond time scale is highly restricted, in agreement with data from fluorescence polarization and NMR relaxation studies. The detailed interaction of the probe with the protein surface depends on steric factors, electrostatic and hydrophobic interactions, hydrogen bonds, and hydration effects. The interaction is markedly different between diastereoisomers, and multiple preferred conformations exist for a single diasteroisomer. These results show that the combination of the hydrophobic xanthylium moiety of bifunctional rhodamine with the carboxylate substitution in its pendant phenyl ring causes the probe to be immobilized on the protein surface, while the two-site cysteine attachment defines the orientation of its fluorescence dipole. These features allow the orientation of protein components to be accurately determined in situ by polarized fluorescence measurements from bifunctional rhodamine probes.

Toward protein structure in situ: comparison of two bifunctional rhodamine adducts of troponin C

As part of a program to develop methods for determining protein structure in situ, sTnC was labeled with a bifunctional rhodamine (BR or BSR), cross-linking residues 56 and 63 of its C-helix. NMR spectroscopy of the N-terminal domain of BSR-labeled sTnC in complex with Ca(2+) and the troponin I switch peptide (residues 115-131) showed that BSR labeling does not significantly affect the secondary structure of the protein or its dynamics in solution. BR-labeling was previously shown to have no effect on the solution structure of this complex. Isometric force generation in isolated demembranated fibers from rabbit psoas muscle into which BR- or BSR-labeled sTnC had been exchanged showed reduced Ca(2+)-sensitivity, and this effect was larger with the BSR label. The orientation of rhodamine dipoles with respect to the fiber axis was determined by polarized fluorescence. The mean orientations of the BR and BSR dipoles were almost identical in relaxed muscle, suggesting that both probes accurately report the orientation of the C-helix to which they are attached. The BSR dipole had smaller orientational dispersion, consistent with less flexible linkers between the rhodamine dipole and cysteine-reactive groups.

Bifunctional rhodamine probes of Myosin regulatory light chain orientation in relaxed skeletal muscle fibers

The orientation of the regulatory light chain (RLC) region of the myosin heads in relaxed skinned fibers from rabbit psoas muscle was investigated by polarized fluorescence from bifunctional rhodamine (BR) probes cross-linking pairs of cysteine residues introduced into the RLC. Pure 1:1 BR-RLC complexes were exchanged into single muscle fibers in EDTA rigor solution for 30 min at 30 degrees C; approximately 60% of the native RLC was removed and stoichiometrically replaced by BR-RLC, and >85% of the BR-RLC was located in the sarcomeric A-bands. The second- and fourth-rank order parameters of the orientation distributions of BR dipoles linking RLC cysteine pairs 100-108, 100-113, 108-113, and 104-115 were calculated from polarized fluorescence intensities, and used to determine the smoothest RLC orientation distribution-the maximum entropy distribution-consistent with the polarized fluorescence data. Maximum entropy distributions in relaxed muscle were relatively broad. At the peak of the distribution, the "lever" axis, linking Cys707 and Lys843 of the myosin heavy chain, was at 70-80 degrees to the fiber axis, and the "hook" helix (Pro830-Lys843) was almost coplanar with the fiber and lever axes. The temperature and ionic strength of the relaxing solution had small but reproducible effects on the orientation of the RLC region.

Identifiability analysis of models for reversible intermolecular two-state excited-state processes coupled with species-dependent rotational diffusion monitored by time-resolved fluorescence depolarization

A deterministic identifiability analysis of the kinetic model for a reversible intermolecular two-state excited-state process with species-dependent rotational diffusion described by Brownian reorientation is presented. The cases of both spherically and cylindrically symmetric rotors, with no change in the principal axes of rotation on interconversion in the latter case, are specifically considered. The identifiability analysis is carried out in terms of compartmental modeling based on the S(t) identical with I( parallel)(t)+2I( perpendicular)(t) and D(t) identical with I( parallel)(t)-I( perpendicular)(t) functions, where I( parallel)(t) and I( perpendicular)(t) are the delta-response functions for fluorescence, polarized, respectively, parallel and perpendicular to the electric vector of linearly polarized excitation. It is shown that, from polarized time-resolved fluorescence data collected at two concentrations of coreactant and three appropriately chosen emission wavelengths, (a) a unique set of rate constants for the overall excited-state process is always obtained by making use of polarized measurements and (b) the rotational diffusion constants and geometrical factors associated with the different anisotropy decay components can be uniquely determined and assigned to each species. The geometrical factors are determined by the absorption and emission transitions in the two rotating species. For spherical rotors, these factors depend directly on the relative orientations of the transition moments, while for cylindrically symmetric rotors they depend on the orientations with respect to each other and to the symmetry axis.

Location and properties of the taxol binding center in microtubules: a picosecond laser study with fluorescent taxoids

The interaction of two bioactive, fluorescent analogues of the anticancer drug Taxol, Flutax1 [7-O-[N-(fluorescein-4'-carbonyl)-L-alanyl]taxol] and Flutax2 [7-O-[N-(2,7-difluorofluorescein-4'-carbonyl)-L-alanyl]taxol], with microtubules in solution has been studied with picosecond laser methods. As shown here, although a mixture of the fluorescein mono- and dianion species of Flutax1 is present in solution, the bound taxoid contains only the dianion form of the dye. This indicates strong electrostatic interactions at the microtubule lattice with the appending dye, most likely with charged residues of the M-loop of the beta-tubulin subunit. Moreover, analysis of the dynamic depolarization of microtubule-bound Flutax at low binding site occupancy was consistent with a protein active center with significant conformational flexibility. On the other hand, for microtubules fully saturated with the taxoid, a new, additional depolarizing process was observed, with relaxation times of 14 ns (Flutax1) and 8 ns (Flutax2), which is due to Förster resonance energy homotransfer (FREHT) between neighboring dye molecules. Application of a detailed analysis of FREHT-induced depolarization in a circular array of dye molecules presented here yielded a separation between nearest-neighbor Flutax moieties of 40 +/- 5 A, for microtubules made up of between 12 and 14 protofilaments, a value that is only compatible with the Taxol binding site being located at the inner wall of the microtubule. The internal position of the drug molecular target as measured here is also consistent with other spectroscopic observations and confirms existing predictions based on microtubule structures modeled from high-resolution, electron density maps of alphabeta-tubulin.

Polarized fluorescence depletion reports orientation distribution and rotational dynamics of muscle cross-bridges

The method of polarized fluorescence depletion (PFD) has been applied to enhance the resolution of orientational distributions and dynamics obtained from fluorescence polarization (FP) experiments on ordered systems, particularly in muscle fibers. Previous FP data from single fluorescent probes were limited to the 2(nd)- and 4(th)-rank order parameters, <P(2)(cos beta)> and <P(4)(cos beta)>, of the probe angular distribution (beta) relative to the fiber axis and <P(2d)>, a coefficient describing the extent of rapid probe motions. We applied intense 12-micros polarized photoselection pulses to transiently populate the triplet state of rhodamine probes and measured the polarization of the ground-state depletion using a weak interrogation beam. PFD provides dynamic information describing the extent of motions on the time scale between the fluorescence lifetime (e.g., 4 ns) and the duration of the photoselection pulse and it potentially supplies information about the probe angular distribution corresponding to order parameters above rank 4. Gizzard myosin regulatory light chain (RLC) was labeled with the 6-isomer of iodoacetamidotetramethylrhodamine and exchanged into rabbit psoas muscle fibers. In active contraction, dynamic motions of the RLC on the PFD time scale were intermediate between those observed in relaxation and rigor. The results indicate that previously observed disorder of the light chain region in contraction can be ascribed principally to dynamic motions on the microsecond time scale.

Orientation changes of the myosin light chain domain during filament sliding in active and rigor muscle

Structural changes in myosin power many types of cell motility including muscle contraction. Tilting of the myosin light chain domain (LCD) seems to be the final step in transducing the energy of ATP hydrolysis, amplifying small structural changes near the ATP binding site into nanometer-scale motions of the filaments. Here we used polarized fluorescence measurements from bifunctional rhodamine probes attached at known orientations in the LCD to describe the distribution of orientations of the LCD in active contraction and rigor. We applied rapid length steps to perturb the orientations of the population of myosin heads that are attached to actin, and thereby characterized the motions of these force-bearing myosin heads. During active contraction, this population is a small fraction of the total. When the filaments slide in the shortening direction in active contraction, the long axis of LCD tilts towards its nucleotide-free orientation with no significant twisting around this axis. In contrast, filament sliding in rigor produces coordinated tilting and twisting motions.

Dynamic measurement of myosin light-chain-domain tilt and twist in muscle contraction

A new method is described for measuring motions of protein domains in their native environment on the physiological timescale. Pairs of cysteines are introduced into the domain at sites chosen from its static structure and are crosslinked by a bifunctional rhodamine. Domain orientation in a reconstituted macromolecular complex is determined by combining fluorescence polarization data from a small number of such labelled cysteine pairs. This approach bridges the gap between in vitro studies of protein structure and cellular studies of protein function and is used here to measure the tilt and twist of the myosin light-chain domain with respect to actin filaments in single muscle cells. The results reveal the structural basis for the lever-arm action of the light-chain domain of the myosin motor during force generation in muscle.

Model-independent analysis of the orientation of fluorescent probes with restricted mobility in muscle fibers

The orientation of proteins in ordered biological samples can be investigated using steady-state polarized fluorescence from probes conjugated to the protein. A general limitation of this approach is that the probes typically exhibit rapid orientational motion ("wobble") with respect to the protein backbone. Here we present a method for characterizing the extent of this wobble and for removing its effects from the available information about the static orientational distribution of the probes. The analysis depends on four assumptions: 1) the probe wobble is fast compared with the nanosecond time scale of its excited-state decay; 2) the orientational distributions of the absorption and emission transition dipole moments are cylindrically symmetrical about a common axis c fixed in the protein; 3) protein motions are negligible during the excited-state decay; 4) the distribution of c is cylindrically symmetrical about the director of the experimental sample. In a muscle fiber, the director is the fiber axis, F. All of the information on the orientational order of the probe that is available from measurements of linearly polarized fluorescence is contained in five independent polarized fluorescence intensities measured with excitation and emission polarizers parallel or perpendicular to F and with the propagation axis of the detected fluorescence parallel or perpendicular to that of the excitation. The analysis then yields the average second-rank and fourth-rank order parameters (<P2> and <P4>) of the angular distribution of c relative to F, and <P2a> and <P2e>, the average second-rank order parameters of the angular distribution for wobble of the absorption and emission transition dipole moments relative to c. The method can also be applied to other cylindrically ordered systems such as oriented lipid bilayer membranes and to processes slower than fluorescence that may be observed using longer-lived optically excited states.

Major histocompatibility complex class I protein conformation altered by transmembrane potential changes

The nature of charge distributions in membrane-bound macromolecular structures renders them susceptible to interaction with transmembrane potential fields. As a result, conformational changes in such species may be expected to occur when this potential is altered. We have detected reversible conformational change in the major histocompatibility complex (MHC) class I antigen in the plasma membrane of human JY cells, as monitored by flow-cytometric resonance energy-transfer, upon reduction of the transmembrane potential (depolarization). This change increased the intramolecular energy-transfer efficiency between fluorescent donor- and acceptor-labeled monoclonal antibodies directed, respectively, to epitopes on the light (beta 2-microglobulin) and the heavy chains of the MHC class I antigen. Repolarization of the depolarized samples restored the energy-transfer efficiency to the original values measured before depolarization. Depolarization caused similar relative changes in fluorescence resonance energy-transfer efficiency when Fab fragments were used for labeling MHC class I complex, suggesting that the observed phenomenon is not restricted to whole monoclonal antibodies.

Role of interleukin-3 in the regulation of intracellular K+ homeostasis in cultured murine haemopoietic cells

Comparative fluorimetric, flow cytofluorimetric and fluorescence ratiometric determinations of intracellular K+ concentrations in murine haemopoietic cells (FDCP-Mix clone A4) cultured in the presence and absence of the specific growth factor Interleukin-3 were carried out with the fluorescent potassium-binding benzofuran-isophthalate acetoxymethyl ester probe. Cell suspensions kept in the absence of Interleukin-3 for 5 hours exhibited lower fluorescence intensities and smaller fluorescence ratios than their growth-factor-replete counterparts, an effect found to be reversible by readdition of the growth factor. It is concluded that Interleukin-3 deprivation of these cells leads to loss of intracellular K+. It is tentatively suggested that this deprivation-induced K+ loss might be associated with an early event in apoptotic cell death.

Transmembrane potentials in cells: a diS-C3(3) assay for relative potentials as an indicator of real changes

The mechanism by which the fluorescent cationic dye diS-C3(3) reports on cellular transmembrane potential has been investigated in murine haemopoietic cells. Due to the large molar absorbance of diS-C3(3) and its high quantum yield of fluorescence in cells, this dye can be used at very low labelling concentrations (5 x 10(-8) to 2 x 10(-7) M). In contrast to the quenching of fluorescence observed for the most commonly used voltage-sensitive dyes of the carbocyanine class, the fluorescence intensity of diS-C3(3) increases when the dye accumulates in the cells. The method of synchronous emission spectroscopy was used to resolve the intracellular and extracellular components of the diS-C3(3) fluorescence of suspensions of labelled cells. In comparing changes in these signals consequent on changes in transmembrane potential induced by varying the extracellular concentration of potassium ions in the presence of valinomycin, the logarithm of the ratio of intensities of these two components, as predicted theoretically, was found to be a good linear measure of transmembrane potential under these conditions. The dye was also demonstrated to be suitable for flow-cytofluorimetric analysis, the logarithm of the mean population signal similarly being found to provide a good linear measure of the transmembrane potential. The conditions under which such linearity may be expected with respect to possible effects due to changes in the capacity for binding of the dye to proteins and various cytosolic structures are delineated and their validity with respect to the possibly contentious role of mitochondria in such measurements examined in particular. The use of the method in indicating changes in the transmembrane potential and/or changes in the transport numbers of the major ions determining transmembrane potential between different physiological states, the possible extension to determinations of absolute differences in potential between different cell states without calibration or comparison with potassium-ion potentials, and the conditions for validity and limitations of these partially complementary measurements, are discussed.

Interpretation of fluorescence photobleaching recovery experiments on oriented cell membranes

Both quantitative and qualitative aspects of the interpretation of fluorescence photobleaching recovery experiments as typically practised to obtain information on lateral diffusion processes in cell membranes are called into question in view of the polarized nature of the laser light sources routinely employed. Protocols which will eliminate the effects elicited under these conditions by any concurrent slow rotational diffusion are delineated.

Transverse location of the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene in model lipid bilayer membrane systems by resonance excitation energy transfer

A fluorescent phospholipid derivative, the fluoresceinthiocarbamyl adduct of a natural phosphatidylethanolamine, has been synthesized and incorporated into sonicated single-bilayer vesicles of egg lecithin and dipalmitoyllecithin. The surface location of this probe has been confirmed by using extrinsic fluorescence quenching studies together with steady-state emission anisotropy measurements. Electronic excitation energy transfer between 1,6-diphenyl-1,3,5-hexatriene incorporated within the hydrophobic core of the bilayer and the novel derivative has been investigated to estimate the depth within the bilayer at which the former is located. Efficiencies have been measured for two different phospholipids, egg lecithin and dipalmitoyllecithin, in the latter case both above and below the phospholipid phase transition, with and without added cholesterol. The observed dependence of the transfer efficiency on the acceptor concentration was compared with that calculated according to Förster theory applied to random two-dimensional distributions of donor and acceptor molecules in parallel planes for various interplanar separations, taking into account orientational effects. The Förster R0 of about 45 A for this donor-acceptor pair is particularly well suited to such studies since it is of the order of the width of the bilayer. The experiments showed that energy-transfer spectroscopy can provide useful quantitative information as to the transverse location of diphenylhexatriene in homogeneous phospholipid bilayers and may also reflect lateral partitioning of donor or of both donor and acceptor into different phases in systems exhibiting phase separations.

Rotational diffusion of calcium-dependent adenosine-5'-triphosphatase in sarcoplasmic reticulum: a detailed study

The Ca2+-Mg2+ adenosine-5'-triphosphatase (ATPase) in sarcoplasmic reticulum has been covalently labeled with the phosphorescent triplet probe erythrosinyl 5-isothiocyanate. The rotational diffusion of the protein in the membrane at 25 degrees C was examined by measuring the time dependence of the phosphorescence emission anisotropy. Detailed analysis of both the total emission S(t) = Iv(t) + 2IH(t) and anisotropy R(t) = [Iv(t) - IH(t)]/[Iv(t) + 2IH(t)] curves shows the presence of multiple components. The latter is incompatible with a simple model of protein movement. The experimental data are consistent with a model in which the sum of four exponential components defines the phosphorescence decay. The anisotropy decay corresponds to a model in which the phosphor itself or a small phosphor-bearing segment reorients on a sub-microsecond time scale about an axis attached to a larger segment, which in turn reorients on a time scale of a few microseconds about an axis fixed in the frame of the ATPase. A fraction of the protein molecules rotate on a time scale of 100-200 microseconds about the normal to the bilayer, while the rest are rotationally stationary, at least on a sub-millisecond time scale.

The orientational freedom of molecular probes. The orientation factor in intramolecular energy transfer

The measurement of the efficiency of Förster long-range resonance energy transfer between donor (D) and acceptor (A) luminophores attached to the same macromolecular substrate can be used to estimate the D-A separation, R. If the D and A transition dipoles sample all orientations with respect to the substrate (the isotropic condition) in a time short compared with the transfer time (the dynamic averaging condition), the average orientation factor less than K2 greater than is 2/3. If the isotropic condition is not satisfied but the dynamic averaging condition is, upper and lower bounds for less than K2 greater than, and thus R, may be obtained from observed D and A depolarizations, and these limits may be further narrowed if the transfer depolarization is also known. This paper offers experimental protocols for obtaining this reorientational information and presents contour plots of less than K2 greater than min and less than K2 greater than max as functions of generally observable depolarizations. This permits an uncertainty to be assigned to the determined value of R. The details of the D and A reoreintational process need not be known, but the orientational distributions are assumed to have at least approximate axial symmetry with respect to a stationary substrate. Average depolarization factors are derived for various orientational distribution functions that demonstrate the effects of various mechanisms for reorientation of the luminophores. It is shown that in general the static averaging regime does not lend itself to determinations of R.

Rotational relaxation of the "microviscosity" probe diphenylhexatriene in paraffin oil and egg lecithin vesicles

The rotational relaxation of the widely used "microviscosity" probe, 1,6-diphenyl-1,3,5-hexatriene, was examined by the technique of nanosecond time-resolved fluorescence depolarization. The decays of the emission anisotropy were determined at five temperatures in the range 3-31 degrees both in a reference paraffin oil and in sonicated egg lecithin vesicles. These decays were complex in both media. Marked qualitative as well as quantitative differences were observed in the rotational behavior of the probe in the complex bilayer medium as opposed to the homogeneous reference solvent. The results are discussed in relation to the structure of the hydrophobic bilayer membrane interior and the concept of its "microviscosity".

Nanosecond time-dependent fluorescence depolarization of diphenylhexatriene in dimyristoyllecithin vesicles and the determination of "microviscosity"

The nanosecond time dependence of the fluorescence depolarization of 1,6-diphenyl-1,3,5-hexatriene in L-alpha-dimyristoyllecithin vesicles was determined at temperatures above and below the midpoint of the gel-liquid crystalline transition. In neither case could the decay of the total fluorescent emission or the decay of the emission anisotropy be described adequately in terms of single exponential decay laws. At the lower temperature, the emission anisotropy did not approach zero in the time window available for measurement, a finding which may indicate that the range over which rotation of the probe can freely occur is restricted. The results are discussed in relation to the concept of microviscosity of bilayer membranes.

The interaction of the polyene antibiotic lucensomycin with cholesterol in erythrocyte membranes and in model systems. III. Characterization of spectral parameters

The variations of optical density and fluorescence of lucensomycin are good indices of the binding of this polyenic antibiotic to membranes. The former parameter reflects more generally the binding to any site present in the membrane, while the latter is more specific for binding to cholesterol. The chromophore of the lucensomycin-cholesterol complex has a relatively long lifetime, is almost immobile in the membrane, and is not accessible to water-soluble fluorescence-quenching agents. The stoichiometry, evaluated fluorometrically, corresponds to about two cholesterol molecules per polyene. In colloidal cholesterol suspensions, the extent of binding as a function of free polyene concentration is described by rectangular hyperbolae, the dissociation constant being, however, dependent on the sterol concentration. In erythrocyte membranes, on the other hand, and even more markedly in model systems containing appropriate solvents, the combination between lucensomycin and the sterol sites is described by sigmoid titration curves, indicative of cooperative effects, and probably due to solvation of cholesterol.

Intramolecular energy transfer and molecular conformation

A discussion of the range of applicability of Forster long-range energy transfer in the determination of macromolecular dimensions and conformational dynamics is given. Emphasis is laid on the effect of restrictions in the orientational freedom of donor and acceptor and on the importance of the orientational averaging regime. The usefulness and limitations of polarized emission measurements in this regard are discussed.

Isolation and spectral characterization of phycobiliproteins

Several phycobiliproteins were prepared chromatographically pure and their absorption, fluorescence-emission, fluorescence-excitation and fluorescence-excitation polarization spectra determined. Changes in these spectra with ionic strength of the aqueous medium and chromoprotein concentration were interpreted in terms of interchromophore energy transfer and protein subunit equilibria. The complexity of the polarization spectra confirms the presence of different types of chromophore, designated sensitizing (;s') and fluorescing (;f'), in a single protein.