Ligand binding of a ribosome-displayed protein detected in solution at the single molecule level by fluorescence correlation spectroscopy

Interaction of a single-chain antibody fragment (scFv) with its cognate antigen while still attached to the ribosome was studied by fluorescence correlation spectroscopy (FCS). In experiments with purified scFv, FCS was capable of resolving the difference in diffusion time between free and antibody-bound labelled antigen. Ribosome-displayed antibody fragments generated by in vitro translation, in which neither the protein nor the mRNA leaves the ribosome owing to the absence of a stop codon and stabilizing buffer conditions, could be shown to specifically bind the antigen. The antibody-antigen interaction was specific, as shown by inhibition or displacement with unlabelled antigen and by control experiments with a non-cognate antibody fragment.     

Fluorescence correlation spectroscopy as a tool to investigate single molecule probe dynamics in thin polymer films

Fluorescence correlation experiments were performed on rhodamine 6G in PDMS spin-coated films on glass surfaces. With polarised excitation, ensemble bleaching of the dye and single molecule intensity fluctuations were observed. From the statistics of single molecule intensity data taken at different positions in the film, correlation functions were calculated. Two modes of motion with exponential decay shapes and correlation times of tau(c) = 0.15 s and tau(c) = 0.7 s could be detected. Potential origins of intensity fluctuations are lateral diffusion, rotational diffusion or intramolecular fluctuations of dyes involving spectral diffusion or photoinduced processes. From the experimental results, lateral diffusion can be ruled out as a motional mode. Single molecule fluctuations are assigned to changes of the molecular configuration of the dyes, which are rigidly bound to the glass. To assess the environmental influence on such molecular motions, the bulk viscosity of the PDMS was varied over two orders of magnitude, leading to changes of tau(c) of the slow mode by a factor of four. This result proves the sensitivity of the single molecule fluctuations to the molecular scale dynamics of the surrounding polymer matrix and makes the correlation time a measure of the local environment of dye probes.     

Fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is a powerful technique to measure concentrations, mobilities, and interactions of fluorescent biomolecules. It can be applied to various biological systems such as simple homogeneous solutions, cells, artificial, or cellular membranes and whole organisms. Here, we introduce the basic principle of FCS, discuss its application to biological questions as well as its limitations and challenges, present an overview of novel technical developments to overcome those challenges, and conclude with speculations about the future applications of fluorescence fluctuation spectroscopy.     

Efficient unfolding pattern recognition in single molecule force spectroscopy data

Background

Single-molecule force spectroscopy (SMFS) is a technique that measures the force necessary to unfold a protein. SMFS experiments generate Force-Distance (F-D) curves. A statistical analysis of a set of F-D curves reveals different unfolding pathways. Information on protein structure, conformation, functional states, and inter- and intra-molecular interactions can be derived.

Results

In the present work, we propose a pattern recognition algorithm and apply our algorithm to datasets from SMFS experiments on the membrane protein bacterioRhodopsin (bR). We discuss the unfolding pathways found in bR, which are characterised by main peaks and side peaks. A main peak is the result of the pairwise unfolding of the transmembrane helices. In contrast, a side peak is an unfolding event in the alpha-helix or other secondary structural element. The algorithm is capable of detecting side peaks along with main peaks. Therefore, we can detect the individual unfolding pathway as the sequence of events labeled with their occurrences and co-occurrences special to bR's unfolding pathway. We find that side peaks do not co-occur with one another in curves as frequently as main peaks do, which may imply a synergistic effect occurring between helices. While main peaks co-occur as pairs in at least 50% of curves, the side peaks co-occur with one another in less than 10% of curves. Moreover, the algorithm runtime scales well as the dataset size increases.

Conclusions

Our algorithm satisfies the requirements of an automated methodology that combines high accuracy with efficiency in analyzing SMFS datasets. The algorithm tackles the force spectroscopy analysis bottleneck leading to more consistent and reproducible results.     

Spectroscopic characterization of Venus at the single molecule level

Venus is a recently developed, fast maturating, yellow fluorescent protein that has been used as a probe for in vivo applications. In the present work the photophysical characteristics of Venus were analyzed spectroscopically at the bulk and single molecule level. Through time-resolved single molecule measurements we found that single molecules of Venus display pronounced fluctuations in fluorescence emission, with clear fluorescence on- and off-times. These fluorescence intermittencies were found to occupy a broad range of time scales, ranging from milliseconds to several seconds. Such long off-times can complicate the analysis of single molecule counting experiments or single-molecule FRET experiments.     

Visualizing helicases unwinding DNA at the single molecule level

DNA helicases are motor proteins that catalyze the unwinding of double-stranded DNA into single-stranded DNA using the free energy from ATP hydrolysis. Single molecule approaches enable us to address detailed mechanistic questions about how such enzymes move processively along DNA. Here, an optical method has been developed to follow the unwinding of multiple DNA molecules simultaneously in real time. This was achieved by measuring the accumulation of fluorescent single-stranded DNA-binding protein on the single-stranded DNA product of the helicase, using total internal reflection fluorescence microscopy. By immobilizing either the DNA or helicase, localized increase in fluorescence provides information about the rate of unwinding and the processivity of individual enzymes. In addition, it reveals details of the unwinding process, such as pauses and bursts of activity. The generic and versatile nature of the assay makes it applicable to a variety of DNA helicases and DNA templates. The method is an important addition to the single-molecule toolbox available for studying DNA processing enzymes.     

单分子层次上相互作用自动检测

任何生命过程都与分子间相互作用有关。这些相互作用决定了生物分子间的＂交流＂方式,组成了生物过程的基本语言。Müller教授研究组发展了一种全自动＂机器人＂（一种全自动原子力显微镜）,可以通过检测细胞上的＂分子机器＂分析分子间相互作用。为了实现这样的目标,该仪器需要将不同的空间尺度联系起来：宏观尺度的悬臂利用其微观尺度的针尖与纳米尺度的蛋白质相接触,进而在亚纳米的尺度上定位与检测分子间相互作用。这项技术能够帮助人们以亚纳米尺度的分辨率定位细胞内分子机器的相互作用位置,并且观察分子间相互作用如何驱动这些分子机器行使功能。在药物筛选研究领域,该技术可以被用来检测配体以及抑制剂与蛋白质结合的位点和强度,还可以检测受体的不同功能状态。     

Measuring antibody affinity and performing immunoassay at the single molecule level

Fluorescence correlation spectroscopy (FCS) enables direct observation of the translational diffusion of single fluorescent molecules in solution. When fluorescent hapten binds to antibody, analysis of FCS data yields the fractional amounts of free and bound hapten, allowing determination of the equilibrium binding constant. Equilibrium dissociation constants of anti-digoxin antibodies and corresponding fluorescein-labeled digoxigenin obtained by FCS and fluorescence polarization measurements are identical. It is also possible to follow a competitive displacement of the tracer from the antibody by unlabeled hapten using FCS in an immunoassay format. The fluorescence polarization immunoassay for vancomycin detection was used to test the FCS approach. Fitting of the FCS data for the molar fractions of free and bound fluorescein-labeled vancomycin yielded a calibration curve which could serve for determination of the vancomycin concentration in biological samples.     

Peptide translocation through the mesoscopic channel: binding kinetics at the single molecule level

Single channel electrophysiological studies have been carried out to elucidate the underlying interactions during the translocation of polypeptides through protein channels. For this we used OmpF from the outer cell membrane of E. coli and arginine-based peptides of different charges, lengths and covalently linked polyethylene glycol as a model system. In order to reveal the fast kinetics of peptide binding, we performed a temperature scan. Together with the voltage-dependent single-channel conductance, we quantify peptide binding and translocation.     

Fluorescence correlation spectroscopy and nonideal solutions

The information that may be obtained from a fluorescence correlation spectroscopic study of a nonideal solution is considered. If all of the macromolecules in a two-component solution are fluorescently labeled, the mutual diffusion coefficient will be measured. If only a few of the macromolecules in a solution are fluorescently labeled, the tracer diffusion coefficient will be obtained. Two nonideal systems that probably may usefully be studied with fluorescence correlation spectroscopy are proposed. The application of fluorescence correlation spectroscopy to studies of lateral diffusion in biological membranes is discussed; the form of the contribution to the fluorescence correlation spectrum of bulk motion within a membrane is noted.     

Fluorescence correlation spectroscopy of finite-sized particles

A theory is presented to study fluorescence correlation spectroscopy for particles with size comparable to the beam waist of the observation volume. Analytical correlation curves are derived for some experimentally interesting particle geometries. It is found that the finiteness of the particle generally decreases the value of the correlation amplitude and increases the correlation time compared to a point particle model. Furthermore, not only the size but also the distribution of fluorophores affects the shape of the correlation function. This is experimentally demonstrated with surface and internally labeled fluorescent spheres. In addition, experiments are performed on fluorescent spheres of different radii to validate the model by comparing the results to theoretical predictions.     

Sequence-specific fluorescent labeling of double-stranded DNA observed at the single molecule level

Fluorescent labeling of a short sequence of double-stranded DNA (dsDNA) was achieved by ligating a labeled dsDNA fragment to a stem–loop triplex forming oligonucleotide (TFO). After the TFO has wound around the target sequence by ligand-induced triple helix formation, its extremities hybridize to each other, leaving a dangling single-stranded sequence, which is then ligated to a fluorescent dsDNA fragment using T4 DNA ligase. A non-repeated 15 bp sequence present on lambda DNA was labeled and visualized by fluorescence microscopy after DNA combing. The label was found to be attached at a specific position located at 4.2 ± 0.5 kb from one end of the molecule, in agreement with the location of the target sequence for triple helix formation (4.4 kb from one end). In addition, an alternative combing process was noticed in which a DNA molecule becomes attached to the combing slide from the label rather than from one of its ends. The method described herein provides a new tool for the detection of very short sequences of dsDNA and offers various perspectives in the micromanipulation of single DNA molecules.     

Regulation of myosin V processivity by calcium at the single molecule level

Calcium can affect myosin V (myoV) function in at least two ways. The full-length molecule, which adopts a folded inhibited conformation in EGTA, becomes extended and active in the presence of calcium. Calcium also dissociates one or more calmodulin molecules from the extended neck. Here we investigated at the single molecule level how calcium regulates the processive run length of full-length myosin V (dFull) and a truncated dimeric construct (dHMM), which cannot adopt the folded conformation. The processivity of dFull and dHMM is tightly controlled by the calcium and calmodulin concentration, with shorter runs occurring at higher calcium concentration. The data indicate that a calcium-dependent dissociation of calmodulin from the neck region of myoV terminates its processive run. dFull showed unexpected processive movement in EGTA, suggesting that a small population of extended, active molecules are in equilibrium with the inhibited, folded form. Single turnover assays showed that the ATPase activity of the folded full-length molecule is inhibited by more than 50-fold compared with the extended molecule. The results imply that activation and termination of the processive runs of myoV can be accomplished by multiple mechanisms.     

Fluorescence correlation spectroscopy: past, present, future

In recent years fluorescence correlation spectroscopy (FCS) has become a routine method for determining diffusion coefficients, chemical rate constants, molecular concentrations, fluorescence brightness, triplet state lifetimes, and other molecular parameters. FCS measures the spatial and temporal correlation of individual molecules with themselves and so provides a bridge between classical ensemble and contemporary single-molecule measurements. It also provides information on concentration and molecular number fluctuations for nonlinear reaction systems that complement single-molecule measurements. Typically implemented on a fluorescence microscope, FCS samples femtoliter volumes and so is especially useful for characterizing small dynamic systems such as biological cells. In addition to its practical utility, however, FCS provides a window on mesoscopic systems in which fluctuations from steady states not only provide the basis for the measurement but also can have important consequences for the behavior and evolution of the system. For example, a new and potentially interesting field for FCS studies could be the study of nonequilibrium steady states, especially in living cells.     

Dynamic polymorphism of Ras observed by single molecule FRET is the basis for molecular recognition

Ras regulates signal transduction pathway function by dynamically interacting with various effectors. To understand the basis for Ras function, its conformational dynamics were measured in the absence and presence of effectors using single molecule fluorescence resonance energy transfer (FRET) between probes located on the Switch II region and GTP. The time trajectories of FRET efficiency from GTP-bound Ras showed that this conformation spontaneously varies among multiple states. Among them, a low FRET state was identified as an inactive state. The transition involving the inactive conformational state occurred in the time range of seconds. In contrast, fluctuation occurring most probably between multiple active high FRET conformational states lasted approximately 30 ms but converged to a specific conformational state upon binding to an effector. Thus, Ras conformation spontaneously fluctuates to readily interact with various effectors.     

Fluorescence correlation spectroscopy and Brownian rotational diffusion

A theroy relating rotational Brownian motion to the time autocorrelation function of the intensity of radiation from a fluorescent system composed of spherical rotors is presented. The calculation shows three relaxation times, two associated with the rotational diffusion, and the third associated with the natural decay of the fluorescence. The correlation function contains terms that relax independently of the fluorescence decay time, thus arbitrarily extending the time range over which rotational diffusion can be studied by fluorescence.     

Fluorescence correlation spectroscopy and quantitative cell biology

Fluorescence correlation spectroscopy (FCS) analyzes fluctuations in fluorescence within a small observation volume. Autocorrelation analysis of FCS fluctuation data can be used to measure concentrations, diffusion properties, and kinetic constants for individual fluorescent molecules. Photon count histogram analysis of fluorescence fluctuation data can be used to study oligomerization of individual fluorescent molecules. If the FCS observation volume is positioned inside a living cell, these parameters can be measured in vivo. FCS can provide the requisite quantitative data for analysis of molecular interaction networks underlying complex cell biological processes.