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Ries, J., Schwarze, S., Johnson, C.M., Neuweiler, H.: Microsecond folding and domain motions of a spider silk protein structural switch. Journal of the American Chemical Society. null (2014).
Web spiders rapidly assemble protein monomers, so-called spidroins, into extraordinarily tough silk fibers. The process involves the pH-triggered self-association of the spidroin N-terminal domain (NTD), which contains a structural switch connecting spidroins to super-molecules. Single-molecule spectroscopy can detect conformational heterogeneity that is hidden to conventional methods, but motions of the NTD are beyond the resolution limit. Here, we engineered probes for 1-nm conformational changes based on the phenomenon of fluorescence quenching by photoinduced electron transfer into the isolated NTD of a spidroin from the nursery web spider Euprosthenops australis. Correlation analysis of single-molecule fluorescence fluctuations uncovered site-dependent nano-to-microsecond movement of secondary and tertiary structure. Kinetic amplitudes were most pronounced for helices that are part of the association interface and where structural studies show large displacements between monomeric and dimeric conformations. A single tryptophan at the center of the five-helix bundle toggled conformations in ~100 µs and in a pH-dependent manner. Equilibrium denaturation and temperature-jump relaxation experiments revealed cooperative and ultrafast folding in only 60 µs. We deduced a free-energy surface that exhibits native-state ruggedness with apparently similar barrier heights to folding and native motions. Observed equilibrium dynamics within the domain suggest a conformational selection mechanism in the rapid association of spidroins through their NTDs during silk synthesis by web spiders.
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Sauer, M., Neuweiler, H.: PET-FCS: Probing Rapid Structural Fluctuations of Proteins and Nucleic Acids by Single-Molecule Fluorescence Quenching. In: Engelborghs, Y. und Visser, A.J.W.G. (hrsg.) Methods Mol Biol - Fluorescence Spectroscopy and Microscopy. S. 597-615. Humana Press (2014).
Quenching of organic fluorophores by aromatic amino acids and DNA nucleotides with expelled electron donating properties allows the study of conformational dynamics of biomolecules. Efficient fluorescence quenching via photoinduced electron transfer (PET) requires van der Waals contact and can be used as reporter for structural fluctuations at the 1-nm scale in proteins, peptides, and nucleic acids. The combination of PET with fluorescence correlation spectroscopy (FCS) establishes a powerful method (PET-FCS) to study equilibrium dynamics at the single-molecule level on time scales from nano- to milliseconds. We delineate the fundamentals of PET-based fluorescence quenching, reporter engineering, instrumental and experimental design, and provide examples.
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Jensen, M.H., Sukumaran, M., Johnson, C.M., Greger, I.H., Neuweiler, H.: Intrinsic Motions in the N-Terminal Domain of an Ionotropic Glutamate Receptor Detected by Fluorescence Correlation Spectroscopy. Journal of Molecular Biology. 414, 96 - 105 (2011).
Ionotropic glutamate receptors (iGluRs) mediate excitatory neurotransmission in the central nervous system and play key roles in brain development and disease. iGluRs have two distinct extracellular domains, but the functional role of the distal N-terminal domain (NTD) is poorly understood. Crystal structures of the NTD from some non-N-methyl-d-aspartate (NMDA) iGluRs are consistent with a rigid body that facilitates receptor assembly but suggest an additional dynamic role that could modulate signaling. Here, we moved beyond spatial and temporal limitations of conventional protein single-molecule spectroscopy by employing correlation analysis of extrinsic oxazine fluorescence fluctuations. We observed nanosecond (ns)-to-microsecond (μs) motions of loop segments and helices within a region of an AMPA-type iGluR NTD, which has been identified previously to be structurally variable. Our data reveal that the AMPA receptor NTD undergoes rapid conformational fluctuations, suggesting an inherent allosteric capacity for this domain in addition to its established assembly function.
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Teufel, D.P., Johnson, C.M., Lum, J.K., Neuweiler, H.: Backbone-Driven Collapse in Unfolded Protein Chains. Journal of Molecular Biology. 409, 250 - 262 (2011).
Collapse of unfolded protein chains is an early event in folding. It affects structural properties of intrinsically disordered proteins, which take a considerable fraction of the human proteome. Collapse is generally believed to be driven by hydrophobic forces imposed by the presence of nonpolar amino acid side chains. Contributions from backbone hydrogen bonds to protein folding and stability, however, are controversial. To date, the experimental dissection of side-chain and backbone contributions has not yet been achieved because both types of interactions are integral parts of protein structure. Here, we realized this goal by applying mutagenesis and chemical modification on a set of disordered peptides and proteins. We measured the protein dimensions and kinetics of intra-chain diffusion of modified polypeptides at the level of individual molecules using fluorescence correlation spectroscopy, thereby avoiding artifacts commonly caused by aggregation of unfolded protein material in bulk. We found no contributions from side chains to collapse but, instead, identified backbone interactions as a source sufficient to form globules of native-like dimensions. The presence of backbone hydrogen bonds decreased polypeptide water solubility dramatically and accelerated the nanosecond kinetics of loop closure, in agreement with recent predictions from computer simulation. The presence of side chains, instead, slowed loop closure and modulated the dimensions of intrinsically disordered domains. It appeared that the transient formation of backbone interactions facilitates the diffusive search for productive conformations at the early stage of folding and within intrinsically disordered proteins.
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Neuweiler, H., Banachewicz, W., Fersht, A.R.: Kinetics of chain motions within a protein-folding intermediate. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA. 107, 22106-22110 (2010).
Small proteins can fold remarkably rapidly, even in mu s. What limits their rate of folding? The Engrailed homeodomain is a particularly well-characterized example, which folds ultrafast via an intermediate, I, of solved structure. It is a puzzle that the helix2-turn-helix3 motif of the 3-helix bundle forms in approximately 2 mu s, but the final docking of preformed helix1 in I requires approximately 20 mu s. Simulation and structural data suggest that nonnative interactions may slow down helix docking. Here we report the direct measurement of chain motions in I by using photoinduced electron transfer fluorescence-quenching correlation spectroscopy (PET-FCS). We use a mutant that traps I at physiological ionic strength but refolds at higher ionic strength. A single Trp in helix3 quenches the fluorescence of an extrinsic label on contact with it. We placed the label along the sequence to probe segmental chain motions. At high ionic strength, we found two relaxations for all probed positions on the 2- and 20-mu s time scale, corresponding to the known folding processes, and a 200-ns phase attributable to loop closure kinetics in the unfolded state. At low ionic strength, we found only the 2-mu s and 200-ns phase for labels in the helix2-turn-helix3 motif of I, because the native state is not significantly populated. But for labels in helix1 we observed an additional approximately 10-mu s phase showing that it was moving slowly, with a rate constant similar to that for overall folding under native conditions. Folding was rate-limited by chain motions on a rough energy surface where nonnative interactions constrain motion.
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Daidone, I., Neuweiler, H., Doose, S., Sauer, M., Smith, J.C.: Hydrogen-Bond Driven Loop-Closure Kinetics in Unfolded Polypeptide Chains. PLOS COMPUTATIONAL BIOLOGY. 6, (2010).
Characterization of the length dependence of end-to-end loop-closure kinetics in unfolded polypeptide chains provides an understanding of early steps in protein folding. Here, loop-closure in poly-glycine-serine peptides is investigated by combining single-molecule fluorescence spectroscopy with molecular dynamics simulation. For chains containing more than 10 peptide bonds loop-closing rate constants on the 20-100 nanosecond time range exhibit a power-law length dependence. However, this scaling breaks down for shorter peptides, which exhibit slower kinetics arising from a perturbation induced by the dye reporter system used in the experimental setup. The loop-closure kinetics in the longer peptides is found to be determined by the formation of intra-peptide hydrogen bonds and transient beta-sheet structure, that accelerate the search for contacts among residues distant in sequence relative to the case of a polypeptide chain in which hydrogen bonds cannot form. Hydrogen-bond-driven polypeptide-chain collapse in unfolded peptides under physiological conditions found here is not only consistent with hierarchical models of protein folding, that highlights the importance of secondary structure formation early in the folding process, but is also shown to speed up the search for productive folding events.
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Arbely, E., Rutherford, T.J., Neuweiler, H., Sharpe, T.D., Ferguson, N., Fersht, A.R.: Carboxyl pK(a) Values and Acid Denaturation of BBL. JOURNAL OF MOLECULAR BIOLOGY. 403, 313-327 (2010).
The protein BBL undergoes structural transitions and acid denaturation between pH 1.2 and 8.0. Using NMR spectroscopy, we measured the pK(a) values of all the carboxylic residues in this pH range. We employed C-13 direct-detection two-dimensional IPAP (in-phase antiphase) CACO NMR spectroscopy to monitor the ionization state of different carboxylic groups and demonstrated its advantages over other NMR techniques in measuring pKa values of carboxylic residues. The two residues Glu161 and Asp162 had significantly lowered pKa values, showing that these residues are involved in a network of stabilizing electrostatic interactions, as is His166. The other carboxylates had unperturbed values. The pH dependence of the free energy of denaturation was described quantitatively by the ionizations of those three residues of perturbed pKa, and, using thermodynamic cycles, we could calculate their pK(a)s in the native and denatured states as well as the equilibrium constants for denaturation of the different protonation states. We also measured C-13(alpha) chemical shifts of individual residues as a function of pH. These shifts sense structural transitions rather than ionizations, and they titrated with pH consistent with the change in equilibrium constant for denaturation. Kinetic measurements of the folding of BBL E161Q indicated that, at pH 7, the stabilizing interactions with Glu161 are formed mainly in the transition state. We also found that local interactions still exist in the acid-denatured state of BBL, which attenuate somewhat the flexibility of the acid-denatured state. (C) 2010 Elsevier Ltd. All rights reserved.
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Arbely, E., Neuweiler, H., Sharpe, T.D., Johnson, C.M., Fersht, A.R.: The human peripheral subunit-binding domain folds rapidly while overcoming repulsive Coulomb forces. PROTEIN SCIENCE. 19, 1704-1713 (2010).
Peripheral subunit binding domains (PSBDs) are integral parts of large multienzyme complexes involved in carbohydrate metabolism. PSBDs facilitate shuttling of prosthetic groups between different catalytic subunits. Their protein surface is characterized by a high density of positive charges required for binding to subunits within the complex. Here, we investigated folding thermodynamics and kinetics of the human PSBD (HSBD) using circular dichroism and tryptophan fluorescence experiments. HSBD was only marginally stable under physiological solvent conditions but folded within microseconds via a barrier-limited apparent two-state transition, analogous to its bacterial homologues. The high positive surface-charge density of HSBD leads to repulsive Coulomb forces that modulate protein stability and folding kinetics, and appear to even induce native-state movement. The electrostatic strain was alleviated at high solution-ionic-strength by Debye-Huckel screening. Differences in ionic-strength dependent characteristics among PSBD homologues could be explained by differences in their surface charge distributions. The findings highlight the trade-off between protein function and stability during protein evolution.
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Doose, S., Neuweiler, H., Sauer, M.: Fluorescence Quenching by Photoinduced Electron Transfer: A Reporter for Conformational Dynamics of Macromolecules. CHEMPHYSCHEM. 10, 1389-1398 (2009).
Photoinduced electron transfer (PET) between organic fluorophores and suitable electron donating moieties, for example, the amino acid tryptophan or the nucleobase guanine, can quench fluorescence upon van der Waals contact and thus report-on molecular contact. PET-quenching has been used as reporter for monitoring conformational dynamics in polypeptides, proteins, and oligonucleotides. Whereas dynamic quenching transiently influences quantum yield and fluorescence life-time of the fluorophore, static quenching in pi-stacked complexes efficiently suppresses fluorescence emission over time scales longer than the fluorescence lifetime. Static quenching therefore provides sufficient contrast to be observed at the single-molecule level. Here, we review complex formation and static quenching of different fluorophores by various molecular compounds, discuss applications as reporter system for macromolecular dynamics, and give illustrating examples.
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Neuweiler, H., Sharpe, T.D., Rutherford, T.J., Johnson, C.M., Allen, M.D., Ferguson, N., Fersht, A.R.: The Folding Mechanism of BBL: Plasticity of Transition-State Structure Observed within an Ultrafast Folding Protein Family. JOURNAL OF MOLECULAR BIOLOGY. 390, 1060-1073 (2009).
Studies on members of protein families with similar structures but divergent sequences provide insights into the effects of sequence composition on the mechanism of folding. Members of the peripheral subunit-binding domain (PSBD) family fold ultrafast and approach the smallest size for cooperatively folding proteins. Phi-Value analysis of the PSBDs E3BD and POB reveals folding via nucleation-condensation through structurally very similar, polarized transition states. Here, we present a Phi-value analysis of the family member BBL and found that it also folds by a nucleation-condensation mechanism. The mean Phi values of BBL, E3BD, and POB were near identical, indicating similar fractions of non-covalent interactions being formed in the transition state. Despite the overall conservation of folding mechanism in this protein family, however, the pattern of Phi values determined for BBL revealed a larger dispersion of the folding nucleus across the entire structure, and the transition state was less polarized. The observed plasticity of transition-state structure can be rationalized by the different helix-forming propensities of PSBD sequences. The very strong helix propensity in the first helix of BBL, relative to E3BD and POB, appears to recruit more structure formation in that helix in the transition state at the expense of weaker interactions in the second helix. Differences in sequence composition can modulate transition-state structure of even the smallest natural protein domains. (C) 2009 Elsevier Ltd. All rights reserved.
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Huang, F., Ying, L., Neuweiler, H., Fersht, A.R.: Reply to Campos et al.: Direct observation versus ambiguous kinetics and thermodynamics. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA. 106, E140 (2009).
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Neuweiler, H., Johnson, C.M., Fersht, A.R.: Direct observation of ultrafast folding and denatured state dynamics in single protein molecules. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA. 106, 18569-18574 (2009).
Single-molecule fluorescence resonance energy transfer (smFRET) experiments are extremely useful in studying protein folding but are generally limited to time scales of greater than approximate to 100 mu s and distances greater than approximate to 2 nm. We used single-molecule fluorescence quenching by photoinduced electron transfer, detecting short-range events, in combination with fluorescence correlation spectroscopy (PET-FCS) to investigate folding dynamics of the small binding domain BBL with nanosecond time resolution. The kinetics of folding appeared as a 10-mu s decay in the autocorrelation function, resulting from stochastic fluctuations between denatured and native conformations of individual molecules. The observed rate constants were probe independent and in excellent agreement with values derived from conventional temperature-jump (T-jump) measurements. A submicrosecond relaxation was detected in PET-FCS data that reported on the kinetics of intrachain contact formation within the thermally denatured state. We engineered a mutant of BBL that was denatured under the reaction conditions that favored folding of the parent wild type (''D-phys''). D-phys had the same kinetic signature as the thermally denatured state and revealed segmental diffusion with a time constant of intrachain contact formation of 500 ns. This time constant was more than 10 times faster than folding and in the range estimated to be the ``speed limit'' of folding. D-phys exhibited significant deviations from a random coil. The solvent viscosity and temperature dependence of intrachain diffusion showed that chain motions were slaved by the presence of intramolecular interactions. PET-FCS in combination with protein engineering is a powerful approach to study the early events and mechanism of ultrafast protein folding.
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Neuweiler, H., Sharpe, T.D., Johnson, C.M., Teufel, D.P., Ferguson, N., Fersht, A.R.: Downhill versus Barrier-Limited Folding of BBL 2: Mechanistic Insights from Kinetics of Folding Monitored by Independent Tryptophan Probes. JOURNAL OF MOLECULAR BIOLOGY. 387, 975-985 (2009).
Barrier-free downhill folding has been proposed for the peripheral subunit-binding domain BBL. To date, ultrafast kinetic experiments on BBL, which are crucial for a mechanistic understanding of folding, have been hampered by the lack of good intrinsic spectroscopic probes. Here, we present a detailed kinetic characterization of three single-point tryptophan mutants of BBL that have suitable fluorescence properties for following microsecond and nanosecond folding kinetics using temperature jump fluorescence spectroscopy. Experiments were performed at pH 7, which is optimal for stability and minimizes complications that arise from the presence of ail alternative native-state conformation of BBL at lower pH. We examined the dependence of rate and equilibrium constants on concentration of denaturant and found that they follow well-established laws allowing kinetic transients to be related to events in folding and compared with equilibrium data. Logarithms of rate constants versus denaturant concentration yielded plots (chevrons) that are characteristic of barrier-limited folding for all mutants investigated, including a truncated sequence that was previously used in the proposal of downhill folding. The thermodynamic quantities calculated from the rate constants were in excellent agreement with those directly determined from equilibrium denaturation based on empirical two-state equations. We found that sequence truncation of BBL as used in studies proposing downhill folding leads to a large loss in helical content and protein stability, which were exacerbated at the low pH used in those studies. The kinetics and equilibria of folding of BBL fit to conventional barrier-limited kinetics. (C) 2009 Elsevier Ltd. All rights reserved.
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Schuettpelz, M., Schoening, J.C., Doose, S., Neuweiler, H., Peters, E., Staiger, D., Sauer, M.: Changes in conformational dynamics of mRNA upon AtGRP7 binding studied by fluorescence correlation spectroscopy. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. 130, 9507-9513 (2008).
The clock-regulated RNA recognition motif (RRM)-containing protein AtGRP7 (Arabidopsis thaliana glycine-rich RNA-binding protein) influences the amplitude of its transcript oscillation at the post-transcriptional level. This autoregulation relies on AtGRP7 binding to its own pre-mRNA. The sequence and structural requirements for this interaction are unknown at present. In this work, we used photoinduced electron transfer fluorescence correlation spectroscopy (PET-FCS) as a novel technique to study the role of target RNA secondary structure and conformational dynamics during the recognition and binding process. Conformational dynamics of single-stranded (ss) oligonucleotides were studied in aqueous solution with single-molecule sensitivity and high temporal resolution by monitoring fluorescence quenching of the oxazine fluorophore MR121 by guanosine residues. Comparative analysis of translational diffusion constants revealed that both ssRNA and ssDNA bind to AtGRP7 with similar dissociation constants on the order of 10(-7) M and that a minimal binding sequence 5'-UUC UGG-3' is needed for recognition by AtGRP7. PET-FCS experiments demonstrated that conformational flexibility of short, single-stranded, MR121-labeled oligonucleotides is reduced upon AtGRP7 binding. In contrast to many other RRM proteins, AtGRP7 binds to ssRNA preferentially if the RNA is fully stretched and not embedded within a stable secondary structure. The results suggest that AtGRP7 binding leads to a conformational rearrangement in the mRNA, arresting the flexible target sequence in an extended structure of reduced flexibility that may have consequences for further post-transcriptional processing of the mRNA.
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Doose, S., Neuweiler, H., Barsch, H., Sauer, M.: Probing polyproline structure and dynamics by photoinduced electron transfer provides evidence for deviations from a regular polyproline type II helix. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA. 104, 17400-17405 (2007).
Polyprolines are well known for adopting a regular polyproline type II helix in aqueous solution, rendering them a popular standard as molecular ruler in structural molecular biology. However, single-molecule spectroscopy studies based on Forster resonance energy transfer (FRET) have revealed deviations of experimentally observed end-to-end distances of polyprolines from theoretical predictions, and it was proposed that the discrepancy resulted from dynamic flexibility of the polyproline helix. Here, we probe end-to-end distances and conformational dynamics Of Poly-L-prolines with 1-10 residues using fluorescence quenching by photoinduced-electron transfer (PET). A single fluorophore and a tryptophan residue, introduced at the termini of polyproline peptides, serve as sensitive probes for distance changes on the subnanometer length scale. Using a combination of ensemble fluorescence and fluorescence correlation spectroscopy, we demonstrate that polyproline samples exhibit static structural heterogeneity with subpopulations of distinct end-to-end distances that do not interconvert on time scales from nano- to milliseconds. By observing prolyl isomerization through changes in PET quenching interactions, we provide experimental evidence that the observed heterogeneity can be explained by interspersed cis isomers. Computer simulations elucidate the influence of trans/cis isomerization on polyproline structures in terms of end-to-end distance and provide a structural justification for the experimentally observed effects. Our results demonstrate that structural heterogeneity inherent in polyprolines, which to date are commonly applied as a molecular ruler, disqualifies them as appropriate tool for an accurate determination of absolute distances at a molecular scale.
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Neuweiler, H., Lollmann, M., Doose, S., Sauer, M.: Dynamics of unfolded polypeptide chains in crowded environment studied by fluorescence correlation spectroscopy. JOURNAL OF MOLECULAR BIOLOGY. 365, 856-869 (2007).
Proteins have evolved to fold and function within a cellular environment that is characterized by high macromolecular content. The earliest step of protein folding represents intrachain contact formation of amino acid residues within an unfolded polypeptide chain. It has been proposed that macromolecular crowding can have significant effects on rates and equilibria of biomolecular processes. However, the kinetic consequences on intrachain diffusion of polypeptides, have not been tested experimentally, yet. Here, we demonstrate that selective fluorescence quenching of the oxazine fluorophore MR121 by the amino acid tryptophan (Trp) in combination with fast fluorescence correlation spectroscopy (FCS) can be used to monitor end-to-end contact formation rates of unfolded polypeptide chains. MR121 and Trp were incorporated at the terminal ends of polypeptides. consisting of repetitive units of glycine (G) and serine (S) residues. End-to-end contact formation and dissociation result in ``off'' and ``on'' switching of MR121 fluorescence and underlying kinetics can be revealed in FCS experiments with nanosecond time resolution. We revisit previous experimental studies concerning the dependence of end-to-end contact formation rates on polypeptide chain length, showing that kinetics can be described by Gaussian chain theory. We further investigate effects of solvent viscosity and temperature on contact formation rates demonstrating that intrachain diffusion represents a purely diffusive, entropy-controlled process. Finally, we study the influence of macromolecular crowding on polypepticle chain dynamics. The data presented demonstrate that intrachain diffusion is fast in spite of hindered diffusion caused by repulsive interactions with macromolecules. Findings can be explained by effects of excluded volume reducing chain entropy and therefore accelerating the loop search process. Our results suggest that within a cellular environment the early formation of structural elements in k unfolded proteins can still proceed quite efficiently in spite of hindered L diffusion caused by high macromolecular content. (c) 2006 Elsevier Ltd. All rights reserved.
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Agaylan, A., Binder, D., Sauer, M., Neuweiler, H., Meyer, O., Kiesewetter, H., Salama, A.: A highly sensitive particle agglutination assay for the detection of P53 autoantibodies in patients with lung cancer. Cancer. 110, 2502--2506 (2007).
Abstract BACKGROUND.Numerous assays have been described for the detection of p53 autoantibodies. These assays are highly specific with low sensitivity. In this report, the authors describe a highly sensitive and simple particle agglutination immunoassay using superparamagnetic particles for capturing p5 autoantibodies, p53 protein, and p53 protein-antibody complexes from large volumes of serum samples (2 mL).METHODS.Superparamagnetic particles were coated with different peptides spanning the entire p53 protein. These particles were incubated with serum samples from healthy blood donors (n = 180), from patients without malignancies (n = 27), and from patients with various forms of lung cancer (n = 166). The particles were washed and placed into the reaction chamber of a gel card. After centrifugation, agglutination results were read visually. Positive reactions were defined by a layer of particles on top of the gel or agglutinated particles dispersed through the gel matrix.RESULTS.Depending on the peptide used, p53 autoantibodies were detected in from 17.5% to 35% of the investigated patients with lung cancer. By using a commercially available enzyme-linked immunoadsorbent assay (ELISA) kit, p53 autoantibodies were detected in only 3% of those patients. P53 protein and p53 protein-antibody complexes were not detected in patients with lung cancer (n = 20).CONCLUSIONS.The newly developed assay was easy to perform and had sensitivity superior to that of the currently available p53 ELISAs. Cancer 2007. © 2007 American Cancer Society.
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Schüttpelz, M., Müller, C., Neuweiler, H., Sauer, M.: UV Fluorescence Lifetime Imaging Microscopy: A Label-Free Method for Detection and Quantification of Protein Interactions. Analytical Chemistry. 78, 663-669 (2006).
Due to the ability to detect multiple parameters simultaneously, protein microarrays have found widespread applications from basic biological research to diagnosis of diseases. Generally, readout of protein microarrays is performed by fluorescence detection using either dye-labeled detector antibodies or direct labeling of the target proteins. We developed a method for the label-free detection and quantification of proteins based on time-gated, wide-field, camera-based UV fluorescence lifetime imaging microscopy to gain lifetime information from each pixel of a sensitive CCD camera. The method relies on differences in the native fluorescence lifetime of proteins and takes advantage of binding-induced lifetime changes for the unequivocal detection and quantification of target proteins. Since fitting of the fluorescence decay for every pixel in an image using a classical exponential decay model is time-consuming and unstable at very low fluorescence intensities, we used a new, very robust and fast alternative method to generate UV fluorescence lifetime images by calculating the average lifetime of the decay for each pixel in the image stack using a model-free average decay time algorithm.To validate the method, we demonstrate the detection and quantification of p53 antibodies, a tumor marker in cancer diagnosis. Using tryptophan-containing capture peptides, we achieved a detection sensitivity for monoclonal antibodies down to the picomolar concentration range. The obtained affinity constant, Ka, of (1.4 ± 0.6) × 109 M-1, represents a typical value for antigen/antibody binding and is in agreement with values determined by traditional binding assays
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Kim, J., Doose, S., Neuweiler, H., Sauer, M.: The initial step of DNA hairpin folding: a kinetic analysis using fluorescence correlation spectroscopy. NUCLEIC ACIDS RESEARCH. 34, 2516-2527 (2006).
Conformational fluctuations of single-stranded DNA (ssDNA) oligonucleotides were studied in aqueous solution by monitoring contact-induced fluorescence quenching of the oxazine fluorophore MR121 by intrinsic guanosine residues (dG). We applied fluorescence correlation spectroscopy as well as steady-state and time-resolved fluorescence spectroscopy to analyze kinetics of DNA hairpin folding. We first characterized the reporter system by investigating bimolecular quenching interactions between MR121 and guanosine monophosphate in aqueous solution estimating rate constants, efficiency and stability for formation of quenched complexes. We then studied the kinetics of complex formation between MR121 and dG residues site-specifically incorporated in DNA hairpins. To uncover the initial steps of DNA hairpin folding we investigated complex formation in ssDNA carrying one or two complementary base pairs (dC-dG pairs) that could hybridize to form a short stem. Our data show that incorporation of a single dC-dG pair leads to non-exponential decays for opening and closing kinetics and reduces rate constants by one to two orders of magnitude. We found positive activation enthalpies independent of the number of dC-dG pairs. These results imply that the rate limiting step of DNA hairpin folding is not determined by loop dynamics, or by mismatches in the stem, but rather by interactions between stem and loop nucleotides.
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Scheffler, S., Sauer, M., Neuweiler, H.: Monitoring antibody binding events in homogeneous solution by single-molecule fluorescence spectroscopy. ZEITSCHRIFT FUR PHYSIKALISCHE CHEMIE-INTERNATIONAL JOURNAL OF RESEARCH IN PHYSICAL CHEMISTRY & CHEMICAL PHYSICS. 219, 665-678 (2005).
We demonstrate the potential of modern confocal fluorescence microscopy in combination with quenched peptide-based fluorescence probes for sensitive detection of p53 antibodies directly in homogeneous solution. Single tryptophan (Trp) residues in the sequences of short, synthetic peptide epitopes of the human p53 protein efficiently quench the fluorescence of an oxazine fluorophore attached to the amino terminal ends of the peptides. The fluorescence quenching mechanism is thought to be a photoinduced electron transfer reaction from Trp to the dye enabled by the formation of intramolecular complexes between dye and Trp. Specific recognition of the epitope by the antibody confines the conformational flexibility of the peptide. Consequently, complex formation between dye and Trp is abolished and fluorescence is recovered. Using fluorescence correlation spectroscopy (FCS), antibody binding can be monitored observing simultaneously two parameters: the diffusional mobility of the peptide as well as the quenching amplitude induced by the conformational flexibility of the peptide change significantly upon antibody binding. Furthermore, we demonstrate that the strong fluorescence increase upon binding can also be used to directly detect p53 autoantibodies from human blood serum samples in fluorescence intensity time traces. Our data demonstrate that new refined single-molecule fluorescence techniques in combination with quenched peptide epitopes open new possibilities for the reliable detection of antibody binding events in homogeneous solution.
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Neuweiler, H., Doose, S., Sauer, M.: A microscopic view of miniprotein folding: Enhanced folding efficiency through formation of an intermediate. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA. 102, 16650-16655 (2005).
The role of polypeptide collapse and formation of intermediates in protein folding is still under debate. Miniproteins, small globular peptide structures, serve as ideal model systems to study the basic principles that govern folding. Experimental investigations of folding dynamics of such small systems, however, turn out to be challenging, because requirements for high temporal and spatial resolution have to be met simultaneously. Here, we demonstrate how selective quenching of an extrinsic fluorescent label by the amino acid tryptophan (Trp) can be used to probe folding dynamics of Trp-cage (TC), the smallest protein known to date. Using fluorescence correlation spectroscopy, we monitor folding transitions as well as conformational flexibility in the denatured state of the 20-residue protein under thermodynamic equilibrium conditions with nanosecond time resolution. Besides microsecond folding kinetics, we reveal hierarchical folding of TC, hidden to previous experimental studies. We show that specific collapse of the peptide to a molten globule-like intermediate enhances folding efficiency considerably. A single point mutation destabilizes the intermediate, switching the protein to two-state folding behavior and slowing down the folding process. Our results underscore the importance of preformed structure in the denatured state for folding of even the smallest globular structures. A unique method emerges for monitoring conformational dynamics and ultrafast folding events of polypeptides at the nanometer scale.
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Neuweiler, H., Sauer, M.: Exploring life by single-molecule fluorescence spectroscopy. ANALYTICAL CHEMISTRY. 77, 178A-185A (2005).
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Doose, S., Neuweiler, H., Sauer, M.: A close look at fluorescence quenching of organic dyes by tryptophan. CHEMPHYSCHEM. 6, 2277-2285 (2005).
Understanding fluorescence quenching processes of organic dyes by biomolecular compounds is of fundamental importance for in-vitro and in-vivo fluorescence studies. It has been reported that the excited singlet state of some oxazine and rhodamine derivatives is efficiently and almost exclusively quenched by the amino acid tryptophan (Trp) and the DNA base guanine via photoinduced electron transfer (PET). We present a detailed analysis of the quenching interactions between the oxazine dye MR121 and Trp in aqueous buffer. Steady-state and time-resolved fluorescence spectroscopy, together with fluorescence correlation spectroscopy (FCS), reveal three contributing quenching mechanisms: 1) diffusion-limited dynamic quenching with a bimolecular quenching rate constant k(d) of 4.0 x 10(9) s(-1) M-1, 2) static quenching with a bimolecular association constant K-s of 61 M-1, and 3) a sphere-of-action contribution to static quenching described by an exponential factor with a quenching constant lambda of 22 M-1. The latter two are characterized as nonfluorescent complexes, formed with approximate to 30% efficiency upon encounter, that are stable for tens of nanoseconds. The measured binding energy of 2030 kJmol(-1) is consistent with previous estimates from molecular dynamics simulations that proposed stacked complexes due to hydrophobic forces. We further evaluate the influence of glycerol and denaturant (guanidine hydrochloride) on the formation and stability of quenched complexes. Comparative measurements performed with two other dyes, ATTO 655 and Rhodamine 6G show similar results and thus demonstrate the general applicability of utilizing PET between organic dyes and Trp for the study of conformational dynamics of biopolymers on sub-nanometer length and nanosecond time-scales.
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Neuweiler, H., Sauer, M.: Using photoinduced charge transfer reactions to study conformational dynamics of biopolymers at the single-molecule level. CURRENT PHARMACEUTICAL BIOTECHNOLOGY. 5, 285-298 (2004).
This mini-review describes how single-molecule sensitive fluorescence resonance energy transfer (FRET) and photoinduced electron transfer (PET) reactions can be successfully applied to monitor conformational dynamics in biopolymers. Single-pair FRET experiments are ideally suited to study conformational dynamics occurring on the nanometer scale, e.g. during protein folding or unfolding. In contrast, conformational dynamics with functional significance, for example occurring in enzymes at work, often appear on much smaller spatial scales of up to several Angstroms. Our results demonstrate that selective PET-reactions between fluorophores and amino acids or DNA nucleotides represent a versatile tool to measure small-scale conformational dynamics in biopolymers on a wide range of time scales, extending from nanoseconds to seconds, at the single-molecule level. That is, the monitoring of conformational dynamics of biopolymers with temporal resolutions comparable to those within reach using new techniques of molecular dynamic simulations. Furthermore, we demonstrate that the strong distance dependence of charge separation reactions on the sub-nanometer scale can be used to develop conformationally flexible PET-biosensors. These sensors enable the detection of specific target molecules in the sub-picomolar range and allow one to follow their molecular binding dynamics with temporal resolution.
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Neuweiler, H., Schulz, A., Bohmer, M., Enderlein, J., Sauer, M.: Measurement of submicrosecond intramolecular contact formation in peptides at the single-molecule level. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. 125, 5324-5330 (2003).
We describe a single-molecule-sensitive method to determine the rate of contact formation and dissociation between tryptophan and an oxazine derivative (MR121) on the basis of measurements of the photon distance distribution. Two short peptides (15 and 20 amino acids) derived from the transactivation domain of the human oncoprotein p53 were investigated. With the fluorophore attached at the N-terminal end of the flexible peptides, fluorescence of the dye is efficiently quenched upon contact formation with a tryptophan residue. The mechanism responsible for the efficient fluorescence quenching observed in the complexes is assumed to be a photoinduced electron-transfer reaction occurring predominantly at van der Waals contact. Fluorescence fluctuations caused by intramolecular contact formation and dissociation were recorded using confocal fluorescence microscopy with two avalanche photodiodes and the time-correlated single-photon-counting technique, enabling a temporal resolution of 1.2 ns. Peptides containing a tryptophan residue at positions 9 and 8, respectively, show contact formation with rate constants of 1/120 and 1/152 ns(-1), respectively. Whereas the rate constants of contact formation most likely directly report on biopolymer chain mobility, the dissociation rate constants of 1/267 and 1/742 ns(-1), respectively, are significantly smaller and reflect strong hydrophobic interactions between the dye and tryptophan. Fluorescence experiments on point-mutated peptides where tryptophan is exchanged by phenylalanine show no fluorescence quenching.
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Vaiana, A.C., Neuweiler, H., Schulz, A., Wolfrum, J., Sauer, M., Smith, J.C.: Fluorescence quenching of dyes by tryptophan: Interactions at atomic detail from combination of experiment and computer simulation. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. 125, 14564-14572 (2003).
Fluorescence spectroscopy and molecular dynamics (MD) simulation are combined to characterize the interaction of two organic fluorescent dyes, rhodamine 6G (R6G) and an oxazine derivative (MR121), with the amino acid tryptophan in aqueous solution. Steady-state and time-resolved fluorescence quenching experiments reveal the formation of essentially nonfluorescent ground-state dye/Trp complexes. The MD simulations are used to elucidate the molecular interaction geometries involved. The MD-derived probability distribution of the distance r between the centers of geometry of the dye and quencher ring systems, P(r), extends to higher distances for R6G than for MR121 due to population in the R6G/Trp system of fluorescent interaction geometries between Trp and the phenyl ring and ester group of the dye. The consequence of this is the experimental finding that under the conditions used in the simulations about 25% of the R6G dye is fluorescent in comparison with 10% of the MR121. Combining the above findings allows determination of the ``quenching distance'', r*, above which no quenching occurs. r* is found to be very similar (similar to5.5 Angstrom) for both dye/Trp systems, corresponding to close to van der Waals contact. Both experimental dynamic Stern-Volmer analysis and the MD trajectories demonstrate that the main determinant of the fluorescence intensity is static quenching. The approach presented is likely to be useful in the structural interpretation of data obtained from fluorescent conjugates commonly used for monitoring the binding and dynamics of biomolecular systems.
