Single-molecule fluorescence microscopy of native macromolecular complexes

Gasdermin D cysteine residues synergistically control its palmitoylation-mediated membrane targeting and assembly

Gasdermin D (GSDMD) executes the cell death program of pyroptosis by assembling into oligomers that permeabilize the plasma membrane. Here, by single-molecule imaging, we elucidate the yet unclear mechanism of Gasdermin D pore assembly and the role of cysteine residues in GSDMD oligomerization. We show that GSDMD preassembles at the membrane into dimeric and trimeric building blocks that can either be inserted into the membrane, or further assemble into higher-order oligomers prior to insertion into the membrane. The GSDMD residues Cys39, Cys57, and Cys192 are the only relevant cysteines involved in GSDMD oligomerization. S-palmitoylation of Cys192, combined with the presence of negatively-charged lipids, controls GSDMD membrane targeting. Simultaneous Cys39/57/192-to-alanine (Ala) mutations, but not Ala mutations of Cys192 or the Cys39/57 pair individually, completely abolish GSDMD insertion into artificial membranes as well as into the plasma membrane. Finally, either Cys192 or the Cys39/Cys57 pair are sufficient to enable formation of GSDMD dimers/trimers, but they are all required for functional higher-order oligomer formation. Overall, our study unveils a cooperative role of Cys192 palmitoylation-mediated membrane binding and Cys39/57/192-mediated oligomerization in GSDMD pore assembly. This study supports a model in which Gasdermin D oligomerization relies on a two-step mechanism mediated by specific cysteine residues.

Electrochemical Monitoring of Real-Time Vesicle Dynamics Induced by Tau in a Confined Nanopipette

The microtubule-associated protein tau participates in neurotransmission regulation via its interaction with synaptic vesicles (SVs). The precise nature and mechanics of tau's engagement with SVs, especially regarding alterations in vesicle dynamics, remain a matter of discussion. We report an electrochemical method using a synapse-mimicking nanopipette to monitor vesicle dynamics induced by tau. A model vesicle of ~30 nm is confined within a lipid-modified nanopipette orifice with a comparable diameter to mimic the synaptic lipid environment. Both tau and phosphorylated tau (p-tau) present two-state dynamic behavior in this biomimetic system, showing typical ionic current oscillation, induced by lipid-tau interaction. The results indicate that p-tau has a stronger affinity to the lipid vesicles in the confined environment, blocking the vesicle movement to a higher degree. Taken together, this method bridges a gap for sensing synaptic vesicle dynamics in a confined lipid environment, mimicking vesicle movement near the synaptic membrane. These findings contribute to understanding how different types of tau protein regulate synaptic vesicle motility and to underlying its functional and pathological behaviours in disease.

Electroosmotic Sensing of Uncharged Peptides and Differentiating Their Phosphorylated States Using Nanopores

The correct characterization and identification of different kinds of proteins is crucial for the survival and development of living organisms, and proteomics research promotes the analysis and understanding of future genome functions. Nanopore technique has been proved to accurately identify individual nucleotides. However, accurate and rapid protein sequencing is difficult due to the variability of protein structures that contains more than 20 amino acids, and it remains very challenging especially for uncharged peptides as they can not be electrophoretically driven through the nanopore. Graphene nanopores have the advantages of high accuracy, sensitivity and low cost in identifying protein phosphorylation modifications. Here, by using all-atom molecular dynamics simulations, charged graphene nanopores are employed to electroosmotically capture and sense uncharged peptides. By further mimicking AFM manipulation of single molecules, it is also found that the uncharged peptides and their phosphorylated states could also be differentiated by both the ionic current and pulling force signals during their pulling processes through the nanopore with a slow and constant velocity. The results shows ability of using nanopores to detect and discriminate single amino acid and its phosphorylation, which is essential for the future low-cost and high-throughput sequencing of protein residues and their post-translational modifications.

Nanopore DNA sequencing technologies and their applications towards single-molecule proteomics

Sequencing of nucleic acids with nanopores has emerged as a powerful tool offering rapid readout, high accuracy, low cost and portability. This label-free method for sequencing at the single-molecule level is an achievement on its own. However, nanopores also show promise for the technologically even more challenging sequencing of polypeptides, something that could considerably benefit biological discovery, clinical diagnostics and homeland security, as current techniques lack portability and speed. Here we survey the biochemical innovations underpinning commercial and academic nanopore DNA/RNA sequencing techniques, and explore how these advances can fuel developments in future protein sequencing with nanopores.

TXNIP-mediated crosstalk between oxidative stress and glucose metabolism

Thioredoxin-interacting protein (TXNIP) has emerged as a key player in cancer and diabetes since it targets thioredoxin (TRX)-mediated redox regulation and glucose transporter (GLUT)-mediated metabolism. TXNIP consists of two arrestin (ARR, N-ARR and C-ARR) domains at its amino-terminus and two PPxY (PY) motifs and a di-leucine (LL) motif for endocytosis at its carboxyl-terminus. Here, we report that TXNIP shuffles between TRX and GLUTs to regulate homeostasis of intracellular oxidative stress and glucose metabolism. While TXNIP functions as a gatekeeper of TRX by default, it robustly interacted with class I GLUTs through its C-ARR domain upon increase of intracellular reactive oxygen species. This interaction prompted the surface expression downregulation and lysosomal degradation of GLUTs by its carboxyl-terminal LL endocytic signaling motif to attenuate glucose uptake. Consequently, TXNIP expression significantly limited glucose uptake, leading to the suppression of glycolysis, hexosamine biosynthesis, and the pentose phosphate pathway. Our findings establish a fundamental link between ROS and glucose metabolism through TXNIP and provide a promising target for the drug development against GLUT-related metabolic disorders.

Technical advances in high-speed atomic force microscopy

It has been 30 years since the outset of developing high-speed atomic force microscopy (HS-AFM), and 15 years have passed since its establishment in 2008. This advanced microscopy is capable of directly visualizing individual biological macromolecules in dynamic action and has been widely used to answer important questions that are inaccessible by other approaches. The number of publications on the bioapplications of HS-AFM has rapidly increased in recent years and has already exceeded 350. Although less visible than these biological studies, efforts have been made for further technical developments aimed at enhancing the fundamental performance of HS-AFM, such as imaging speed, low sample disturbance, and scan size, as well as expanding its functionalities, such as correlative microscopy, temperature control, buffer exchange, and sample manipulations. These techniques can expand the range of HS-AFM applications. After summarizing the key technologies underlying HS-AFM, this article focuses on recent technical advances and discusses next-generation HS-AFM.

Tuning Formation of Protein-DNA Coacervates by Sequence and Environment

The high concentration of nucleic acids and positively charged proteins in the cell nucleus provides many possibilities for complex coacervation. We consider a prototypical mixture of nucleic acids together with the polycationic C-terminus of histone H1 (CH1). Using a minimal coarse-grained model that captures the shape, flexibility, and charge distributions of the molecules, we find that coacervates are readily formed at physiological ionic strengths, in agreement with experiment, with a progressive increase in local ordering at low ionic strength. Variation of the positions of charged residues in the protein tunes phase separation: for well-mixed or only moderately blocky distributions of charge, there is a modest increase of local ordering with increasing blockiness that is also associated with an increased propensity to phase separate. This ordering is also associated with a slowdown of rotational and translational diffusion in the dense phase. However, for more extreme blockiness (and consequently higher local charge density), we see a qualitative change in the condensed phase to become a segregated structure with a dramatically increased ordering of the DNA. Naturally occurring proteins with these sequence properties, such as protamines in sperm cells, are found to be associated with very dense packing of nucleic acids.

Comparative Analysis of Antibodies and Heavily Glycosylated Macromolecular Immune Complexes by Size-Exclusion Chromatography Multi-Angle Light Scattering, Native Charge Detection Mass Spectrometry, and Mass Photometry

Qualitative and quantitative mass analysis of antibodies and related macromolecular immune complexes is a prerequisite for determining their identity, binding partners, stoichiometries, and affinities. A plethora of bioanalytical technologies exist to determine such characteristics, typically based on size, interaction with functionalized surfaces, light scattering, or direct mass measurements. While these methods are highly complementary, they also exhibit unique strengths and weaknesses. Here, we benchmark mass photometry (MP), a recently introduced technology for mass measurement, against native mass spectrometry (MS) and size exclusion chromatography multi-angle light scattering (SEC-MALS). We examine samples of variable complexity, namely, IgG4Δhinge dimerizing half-bodies, IgG-RGY hexamers, heterogeneously glycosylated IgG:sEGFR antibody-antigen complexes, and finally megadalton assemblies involved in complement activation. We thereby assess the ability to determine (1) binding affinities and stoichiometries, (2) accurate masses, for extensively glycosylated species, and (3) assembly pathways of large heterogeneous immune complexes. We find that MP provides a sensitive approach for characterizing antibodies and stable assemblies, with dissociation correction enabling us to expand the measurable affinity range. In terms of mass resolution and accuracy, native MS performs the best but is occasionally hampered by artifacts induced by electrospray ionization, and its resolving power diminishes when analyzing extensively glycosylated proteins. In the latter cases, MP performs well, but single-particle charge detection MS can also be useful in this respect, measuring masses of heterogeneous assemblies even more accurately. Both methods perform well compared to SEC-MALS, still being the most established method in biopharma. Together, our data highlight the complementarity of these approaches, each having its unique strengths and weaknesses.

The emerging landscape of single-molecule protein sequencing technologies

Single-cell profiling methods have had a profound impact on the understanding of cellular heterogeneity. While genomes and transcriptomes can be explored at the single-cell level, single-cell profiling of proteomes is not yet established. Here we describe new single-molecule protein sequencing and identification technologies alongside innovations in mass spectrometry that will eventually enable broad sequence coverage in single-cell profiling. These technologies will in turn facilitate biological discovery and open new avenues for ultrasensitive disease diagnostics.

AutoStepfinder: A fast and automated step detection method for single-molecule analysis

Single-molecule techniques allow the visualization of the molecular dynamics of nucleic acids and proteins with high spatiotemporal resolution. Valuable kinetic information of biomolecules can be obtained when the discrete states within single-molecule time trajectories are determined. Here, we present a fast, automated, and bias-free step detection method, AutoStepfinder, that determines steps in large datasets without requiring prior knowledge on the noise contributions and location of steps. The analysis is based on a series of partition events that minimize the difference between the data and the fit. A dual-pass strategy determines the optimal fit and allows AutoStepfinder to detect steps of a wide variety of sizes. We demonstrate step detection for a broad variety of experimental traces. The user-friendly interface and the automated detection of AutoStepfinder provides a robust analysis procedure that enables anyone without programming knowledge to generate step fits and informative plots in less than an hour.

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Fast, automated, and bias-free detection of steps within single-molecule trajectories

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Robust step detection without any prior knowledge on the data

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A dual-pass strategy for the detection of steps over a wide variety of scales

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A user-friendly interface for a simplified step fitting procedure

Single-molecule techniques have made it possible to track individual protein complexes in real time with a nanometer spatial resolution and a millisecond timescale. Accurate determination of the dynamic states within single-molecule time traces provides valuable kinetic information that underlie the function of biological macromolecules. Here, we present a new automated step detection method called AutoStepfinder, a versatile, robust, and easy-to-use algorithm that allows researchers to determine the kinetic states within single-molecule time trajectories without any bias.

Quantifying misfolded protein oligomers as drug targets and biomarkers in Alzheimer and Parkinson diseases

Protein misfolding and aggregation are characteristic of a wide range of neurodegenerative disorders, including Alzheimer and Parkinson diseases. A hallmark of these diseases is the aggregation of otherwise soluble and functional proteins into amyloid aggregates. Although for many decades such amyloid deposits have been thought to be responsible for disease progression, it is now increasingly recognized that the misfolded protein oligomers formed during aggregation are, instead, the main agents causing pathological processes. These oligomers are transient and heterogeneous, which makes it difficult to detect and quantify them, generating confusion about their exact role in disease. The lack of suitable methods to address these challenges has hampered efforts to investigate the molecular mechanisms of oligomer toxicity and to develop oligomer-based diagnostic and therapeutic tools to combat protein misfolding diseases. In this Review, we describe methods to quantify misfolded protein oligomers, with particular emphasis on diagnostic applications as disease biomarkers and on therapeutic applications as target biomarkers. The development of these methods is ongoing, and we discuss the challenges that remain to be addressed to establish measurement tools capable of overcoming existing limitations and to meet present needs.

Single-vesicle imaging and co-localization analysis for tetraspanin profiling of individual extracellular vesicles

Extracellular vesicles (EVs) are secreted nano-sized vesicles that contain cellular proteins, lipids, and nucleic acids. Although EVs are expected to be biologically diverse, current analyses cannot adequately characterize this diversity because most are ensemble methods that inevitably average out information from diverse EVs. Here we describe a single vesicle analysis, which directly visualizes marker expressions of individual EVs using a total internal-reflection microscopy and analyzes their co-localization to investigate EV subpopulations. The single-vesicle imaging and co-localization analysis successfully illustrated the diversity of EVs and revealed distinct patterns of tetraspanin expressions. Application of the analysis demonstrated similarities and dissimilarities between the EV fractions that had been acquired from different conventional EV isolation methods. The analysis method developed in this study will provide a new and reliable tool for investigating characteristics of single EVs, and the findings of the analysis might increase understanding of the characteristics of EVs.

A Toolbox for Site-Specific Labeling of RecQ Helicase With a Single Fluorophore Used in the Single-Molecule Assay

Fluorescently labeled proteins can improve the detection sensitivity and have been widely used in a variety of biological measurements. In single-molecule assays, site-specific labeling of proteins enables the visualization of molecular interactions, conformational changes in proteins, and enzymatic activity. In this study, based on a flexible linker in the Escherichia coli RecQ helicase, we established a scheme involving a combination of fluorophore labeling and sortase A ligation to allow site-specific labeling of the HRDC domain of RecQ with a single Cy5 fluorophore, without inletting extra fluorescent domain or peptide fragment. Using single-molecule fluorescence resonance energy transfer, we visualized that Cy5-labeled HRDC could directly interact with RecA domains and could bind to both the 3′ and 5′ ends of the overhang DNA dynamically in vitro for the first time. The present work not only reveals the functional mechanism of the HRDC domain, but also provides a feasible method for site-specific labeling of a domain with a single fluorophore used in single-molecule assays.

Quantifying Protein-Protein Interactions by Molecular Counting with Mass Photometry

Interactions between biomolecules control the processes of life in health and their malfunction in disease, making their characterization and quantification essential. Immobilization- and label-free analytical techniques are desirable because of their simplicity and minimal invasiveness, but they struggle with quantifying tight interactions. Here, we show that mass photometry can accurately count, distinguish by molecular mass, and thereby reveal the relative abundances of different unlabelled biomolecules and their complexes in mixtures at the single-molecule level. These measurements determine binding affinities over four orders of magnitude at equilibrium for both simple and complex stoichiometries within minutes, as well as the associated kinetics. These results introduce mass photometry as a rapid, simple and label-free method for studying sub-micromolar binding affinities, with potential for extension towards a universal approach for characterizing complex biomolecular interactions.

Fluorescence microscopy for visualizing single-molecule protein dynamics

BACKGROUND

Single-molecule fluorescence imaging (smFI) has evolved into a valuable method used in biophysical and biochemical studies as it can observe the real-time behavior of individual protein molecules, enabling understanding of their detailed dynamic features. smFI is also closely related to other state-of-the-art microscopic methods, optics, and nanomaterials in that smFI and these technologies have developed synergistically.

SCOPE OF REVIEW

This paper provides an overview of the recently developed single-molecule fluorescence microscopy methods, focusing on critical techniques employed in higher-precision measurements in vitro and fluorescent nanodiamond, an emerging promising fluorophore that will improve single-molecule fluorescence microscopy.

MAJOR CONCLUSIONS

smFI will continue to improve regarding the photostability of fluorophores and will develop via combination with other techniques based on nanofabrication, single-molecule manipulation, and so on.

GENERAL SIGNIFICANCE

Quantitative, high-resolution single-molecule studies will help establish an understanding of protein dynamics and complex biomolecular systems.

Facile single-molecule pull-down assay for analysis of endogenous proteins

The single-molecule pull-down (SiMPull) assay analyzes molecular complexes in physiological conditions from cell or tissue lysates. Currently the approach requires a lengthy sample preparation process, which has largely prevented the widespread adoption of this technique in bioanalysis. Here, we present a simplified SiMPull assay based upon dichlorodimethylsilane-Tween-20 passivation and F(ab) fragment labeling. Our passivation is a much shorter process than the standard polyethylene glycol passivation used in most single-molecule studies. The use of F(ab) fragments for indirect fluorescent labeling rather than divalent F(ab')₂ or whole IgG antibodies allows for the pre-incubation of the detection antibodies, reducing the sample preparation time for single-molecule immunoprecipitation samples. We examine the applicability of our approach to recombinant proteins and endogenous proteins from mammalian cell lysates.

Biophysical mechanisms underlying the membrane trafficking of synaptic adhesion molecules

Adhesion proteins play crucial roles at synapses, not only by providing a physical trans-synaptic linkage between axonal and dendritic membranes, but also by connecting to functional elements including the pre-synaptic neurotransmitter release machinery and post-synaptic receptors. To mediate these functions, adhesion proteins must be organized on the neuronal surface in a precise and controlled manner. Recent studies have started to describe the mobility, nanoscale organization, and turnover rate of key synaptic adhesion molecules including cadherins, neurexins, neuroligins, SynCAMs, and LRRTMs, and show that some of these proteins are highly mobile in the plasma membrane while others are confined at sub-synaptic compartments, providing evidence for different regulatory pathways. In this review article, we provide a biophysical view of the diffusional trapping of adhesion molecules at synapses, involving both extracellular and intracellular protein interactions. We review the methodology underlying these measurements, including biomimetic systems with purified adhesion proteins, means to perturb protein expression or function, single molecule imaging in cultured neurons, and analytical models to interpret the data. This article is part of the special issue entitled 'Mobility and trafficking of neuronal membrane proteins'.

Statistical Learning of Discrete States in Time Series

Time series obtained from time-dependent experiments contain rich information on kinetics and dynamics of the system under investigation. This work describes an unsupervised learning framework, along with the derivation of the necessary analytical expressions, for the analysis of Gaussian-distributed time series that exhibit discrete states. After the time series has been partitioned into segments in a model-free manner using the previously developed change-point (CP) method, this protocol starts with an agglomerative hierarchical clustering algorithm to classify the detected segments into possible states. The initial state clustering is further refined using an expectation-maximization (EM) procedure, and the number of states is determined by a Bayesian information criterion (BIC). Also introduced here is an achievement scalarization function, usually seen in artificial intelligence literature, for quantitatively assessing the performance of state determination. The statistical learning framework, which is comprised of three stages, detection of signal change, clustering, and number-of-state determination, was thoroughly characterized using simulated trajectories with random intensity segments that have no underlying kinetics, and its performance was critically evaluated. The application to experimental data is also demonstrated. The results suggested that this general framework, the implementation of which is based on firm theoretical foundations and does not require the imposition of any kinetics model, is powerful in determining the number of states, the parameters contained in each state, as well as the associated statistical significance.

Toward Single-Cell Single-Molecule Pull-Down

Single-molecule pull-down (SiMPull) can capture native protein complexes directly from cell lysates for analysis of complex composition and activities at the single-molecule level. Although SiMPull requires many fewer cells compared to conventional pull-down assays, all studies so far have been performed using lysates from many cells. In principle, extending SiMPull to the single-cell level will allow the investigation of cell-to-cell variations on the stoichiometry and activities of biomolecular complexes. We developed a protocol to lyse bacterial cells in situ and capture the released proteins on the imaging surface using antibodies. The use of lysozymes delayed the protein release until after the flow has ceased, and the use of a 10-μm spacer reduces the capture radius within which ∼70% of target proteins can be captured to below 30 μm. Proteins thus captured can be unambiguously assigned to the originating cell. The developed platform should be compatible with high-throughput protein analysis and protein-protein interaction analysis at the single-cell level through single-molecule imaging.

Roles of Gag-RNA interactions in HIV-1 virus assembly deciphered by single-molecule localization microscopy

During HIV-1 assembly, the retroviral structural protein Gag forms an immature capsid, containing thousands of Gag molecules, at the plasma membrane (PM). Interactions between Gag nucleocapsid (NC) and viral RNA (vRNA) are thought to drive assembly, but the exact roles of these interactions have remained poorly understood. Since previous studies have shown that Gag dimer- or trimer-forming mutants (Gag_ZiL) lacking an NC domain can form immature capsids independent of RNA binding, it is often hypothesized that vRNA drives Gag assembly by inducing Gag to form low-ordered multimers, but is dispensable for subsequent assembly. In this study, we examined the role of vRNA in HIV-1 assembly by characterizing the distribution and mobility of Gag and Gag NC mutants at the PM using photoactivated localization microscopy (PALM) and single-particle tracking PALM (spt-PALM). We showed that both Gag and Gag_ZiL assembly involve a similar basic assembly unit, as expected. Unexpectedly, the two proteins underwent different subsequent assembly pathways, with Gag cluster density increasing asymptotically, while Gag_ZiL cluster density increased linearly. Additionally, the directed movement of Gag, but not Gag_ZiL, was maintained at a constant speed, suggesting that the two proteins experience different external driving forces. Assembly was abolished when Gag was rendered monomeric by NC deletion. Collectively, these results suggest that, beyond inducing Gag to form low-ordered multimer basic assembly units, vRNA is essential in scaffolding and maintaining the stability of the subsequent assembly process. This finding should advance the current understanding of HIV-1 and, potentially, other retroviruses.

Sm-ChIPi: Single-Molecule Chromatin Immunoprecipitation Imaging

Epigenetic complexes regulate chromatin dynamics via binding to and assembling on chromatin. However, the mechanisms of chromatin binding and assembly of epigenetic complexes within cells remain incompletely understood, partly due to technical challenges. Here, we present a new approach termed single-molecule chromatin immunoprecipitation imaging (Sm-ChIPi) that enables to assess the cellular assembly stoichiometry of epigenetic complexes on chromatin. Sm-ChIPi was developed based on chromatin immunoprecipitation followed by single-molecule fluorescence microscopy imaging. In this method, an epigenetic complex subunit fused with a gene coding for a fluorescent protein is stably expressed in its corresponding knockout cells. Nucleosomes associated with epigenetic complexes are isolated from cells at native conditions and incubated with biotinylated antibodies. The resulting complexes are immobilized on a quartz slide that had been passivated and functionalized with NeutrAvidin. Image stacks are then acquired by using single-molecule TIRF microscopy. The individual spots imaged by TIRF microscopy represent single protein-nucleosome complexes. The number of copies of the protein complexes on a nucleosome is inferred from the fluorescence photobleaching measurements. Sm-ChIPi is a sensitive and direct method that can quantify the cellular assembly stoichiometry of epigenetic complexes on chromatin.

An engineered opsin monomer scrambles phospholipids

The G protein-coupled receptor opsin is a phospholipid scramblase that facilitates rapid transbilayer phospholipid exchange in liposomes. The mechanism by which opsin scrambles lipids is unknown. It has been proposed that lipid translocation may occur at protein-protein interfaces of opsin dimers. To test this possibility, we rationally engineered QUAD opsin by tryptophan substitution of four lipid-facing residues in transmembrane helix 4 (TM4) that is known to be important for dimerization. Atomistic molecular dynamics simulations of wild type and QUAD opsins combined with continuum modeling revealed that the tryptophan substitutions lower the energetically unfavorable residual hydrophobic mismatch between TM4 and the membrane, reducing the drive of QUAD opsin to dimerize. We purified thermostable wild type and QUAD opsins, with or without a SNAP tag for fluorescence labeling. Single molecule fluorescence measurements of purified SNAP-tagged constructs revealed that both proteins are monomers. Fluorescence-based activity assays indicated that QUAD opsin is a fully functional scramblase. However, unlike wild type opsin which dimerizes en route to insertion into phospholipid vesicles, QUAD opsin reconstitutes as a monomer. We conclude that an engineered opsin monomer can scramble phospholipids, and that the lipid-exposed face of TM4 is unlikely to contribute to transbilayer phospholipid exchange.

Endogenous Alpha-Synuclein Protein Analysis from Human Brain Tissues Using Single-Molecule Pull-Down Assay

Alpha-synuclein (α-SYN) is a central molecule in Parkinson's disease pathogenesis. Despite several studies, the molecular nature of endogenous α-SYN especially in human brain samples is still not well understood due to the lack of reliable methods and the limited amount of biospecimens. Here, we introduce α-SYN single-molecule pull-down (α-SYN SiMPull) assay combined with in vivo protein crosslinking to count individual α-SYN protein and assess its native oligomerization states from biological samples including human postmortem brains. This powerful single-molecule assay can be highly useful in diagnostic applications using various specimens for neurodegenerative diseases including Alzheimer's disease and Parkinson's disease.

Single-molecule force spectroscopy of protein-membrane interactions

Many biological processes rely on protein–membrane interactions in the presence of mechanical forces, yet high resolution methods to quantify such interactions are lacking. Here, we describe a single-molecule force spectroscopy approach to quantify membrane binding of C2 domains in Synaptotagmin-1 (Syt1) and Extended Synaptotagmin-2 (E-Syt2). Syts and E-Syts bind the plasma membrane via multiple C2 domains, bridging the plasma membrane with synaptic vesicles or endoplasmic reticulum to regulate membrane fusion or lipid exchange, respectively. In our approach, single proteins attached to membranes supported on silica beads are pulled by optical tweezers, allowing membrane binding and unbinding transitions to be measured with unprecedented spatiotemporal resolution. C2 domains from either protein resisted unbinding forces of 2–7 pN and had binding energies of 4–14 k_BT per C2 domain. Regulation by bilayer composition or Ca²⁺ recapitulated known properties of both proteins. The method can be widely applied to study protein–membrane interactions.

Oligomerization of the tetramerization domain of p53 probed by two- and three-color single-molecule FRET

We describe a method that combines two- and three-color single-molecule FRET spectroscopy with 2D FRET efficiency-lifetime analysis to probe the oligomerization process of intrinsically disordered proteins. This method is applied to the oligomerization of the tetramerization domain (TD) of the tumor suppressor protein p53. TD exists as a monomer at subnanomolar concentrations and forms a dimer and a tetramer at higher concentrations. Because the dissociation constants of the dimer and tetramer are very close, as we determine in this paper, it is not possible to characterize different oligomeric species by ensemble methods, especially the dimer that cannot be readily separated. However, by using single-molecule FRET spectroscopy that includes measurements of fluorescence lifetime and two- and three-color FRET efficiencies with corrections for submillisecond acceptor blinking, we show that it is possible to obtain structural information for individual oligomers at equilibrium and to determine the dimerization kinetics. From these analyses, we show that the monomer is intrinsically disordered and that the dimer conformation is very similar to that of the tetramer but the C terminus of the dimer is more flexible.

Pseudoknot Formation Seeds the Twister Ribozyme Cleavage Reaction Coordinate

The twister RNA is a recently discovered nucleolytic ribozyme that is present in both bacteria and eukarya. While its biological role remains unclear, crystal structure analyses and biochemical approaches have revealed critical features of its catalytic mechanism. Here, we set out to explore dynamic aspects of twister RNA folding along the cleavage reaction coordinate. To do so, we have employed both bulk and single-molecule fluorescence resonance energy transfer (FRET) methods to investigate a set of twister RNAs with labels strategically positioned at communicating segments. The data reveal that folding of the central pseudoknot (T1), the most crucial structural determinant to promote cleavage, exhibits reversible opening and closing dynamics at physiological Mg2+ concentration. Uncoupled folding, in which T1 formation precedes structuring for closing of stem P1, was confirmed using pre-steady-state three-color smFRET experiments initiated by Mg2+ injection. This finding suggests that the folding path of twister RNA requires proper orientation of the substrate prior to T1 closure such that the U5-A6 cleavage site becomes embraced to achieve its cleavage competent conformation. We also find that the cleaved 3'-fragment retains its compacted pseudoknot fold, despite the absence of the phylogenetically conserved stem P1, rationalizing the poor turnover efficiency of the twister ribozyme.