Real-time detection of human telomerase DNA synthesis by multiplexed single-molecule FRET.

Genomic stability in proliferating cells critically depends on telomere maintenance by telomerase reverse transcriptase. Here we report the development and proof-of-concept results of a single-molecule approach to monitor the catalytic activity of human telomerase in real time and with single-nucleotide resolution. Using zero-mode waveguides and multicolor FRET, we recorded the processive addition of multiple telomeric repeats to individual DNA primers. Unlike existing biophysical and biochemical tools, the novel approach enables the quantification of nucleotide-binding kinetics before nucleotide incorporation. Moreover, it provides a means to dissect the unique translocation dynamics that telomerase must undergo after synthesis of each hexameric DNA repeat. We observed an unexpectedly prolonged binding dwell time of dGTP in the enzyme active site at the start of each repeat synthesis cycle, suggesting that telomerase translocation is composed of multiple rate-contributing sub-steps that evade classical biochemical analysis.

The Processivity of Telomerase: Insights from Kinetic Simulations and Analyses.

Telomerases are moderately processive reverse transcriptases that use an integral RNA template to extend the 3' end of linear chromosomes. Processivity values, defined as the probability of extension rather than dissociation, range from about 0.7 to 0.99 at each step. Consequently, an average of tens to hundreds of nucleotides are incorporated before the single-stranded sDNA product dissociates. The RNA template includes a six nucleotide repeat, which must be reset in the active site via a series of translocation steps. Nucleotide addition associated with a translocation event shows a lower processivity (repeat addition processivity, RAP) than that at other positions (nucleotide addition processivity, NAP), giving rise to a characteristic strong band every 6th position when the product DNA is analyzed by gel electrophoresis. Here, we simulate basic reaction mechanisms and analyze the product concentrations using several standard procedures to show how the latter can give rise to systematic errors in the processivity estimate. Complete kinetic analysis of the time course of DNA product concentrations following a chase with excess unlabeled DNA primer (i.e., a pulse-chase experiment) provides the most rigorous approach. This analysis reveals that the higher product concentrations associated with RAP arise from a stalling of nucleotide incorporation reaction during translocation rather than an increased rate constant for the dissociation of DNA from the telomerase.

Reconstitution of an intact clock reveals mechanisms of circadian timekeeping.

Circadian clocks control gene expression to provide an internal representation of local time. We report reconstitution of a complete cyanobacterial circadian clock in vitro, including the central oscillator, signal transduction pathways, downstream transcription factor, and promoter DNA. The entire system oscillates autonomously and remains phase coherent for many days with a fluorescence-based readout that enables real-time observation of each component simultaneously without user intervention. We identified the molecular basis for loss of cycling in an arrhythmic mutant and explored fundamental mechanisms of timekeeping in the cyanobacterial clock. We find that SasA, a circadian sensor histidine kinase associated with clock output, engages directly with KaiB on the KaiC hexamer to regulate period and amplitude of the central oscillator. SasA uses structural mimicry to cooperatively recruit the rare, fold-switched conformation of KaiB to the KaiC hexamer to form the nighttime repressive complex and enhance rhythmicity of the oscillator, particularly under limiting concentrations of KaiB. Thus, the expanded in vitro clock reveals previously unknown mechanisms by which the circadian system of cyanobacteria maintains the pace and rhythmicity under variable protein concentrations.

Telomere DNA G-quadruplex folding within actively extending human telomerase.

Telomerase reverse transcribes short guanine (G)-rich DNA repeat sequences from its internal RNA template to maintain telomere length. G-rich telomere DNA repeats readily fold into G-quadruplex (GQ) structures in vitro, and the presence of GQ-prone sequences throughout the genome introduces challenges to replication in vivo. Using a combination of ensemble and single-molecule telomerase assays, we discovered that GQ folding of the nascent DNA product during processive addition of multiple telomere repeats modulates the kinetics of telomerase catalysis and dissociation. Telomerase reactions performed with telomere DNA primers of varying sequence or using GQ-stabilizing K+ versus GQ-destabilizing Li+ salts yielded changes in DNA product profiles consistent with formation of GQ structures within the telomerase-DNA complex. Addition of the telomerase processivity factor POT1-TPP1 altered the DNA product profile, but was not sufficient to recover full activity in the presence of Li+ cations. This result suggests GQ folding synergizes with POT1-TPP1 to support telomerase function. Single-molecule Förster resonance energy transfer experiments reveal complex DNA structural dynamics during real-time catalysis in the presence of K+ but not Li+, supporting the notion of nascent product folding within the active telomerase complex. To explain the observed distributions of telomere products, we globally fit telomerase time-series data to a kinetic model that converges to a set of rate constants describing each successive telomere repeat addition cycle. Our results highlight the potential influence of the intrinsic folding properties of telomere DNA during telomerase catalysis, and provide a detailed characterization of GQ modulation of polymerase function.

Peptide exchange on MHC-I by TAPBPR is driven by a negative allostery release cycle.

Chaperones TAPBPR and tapasin associate with class I major histocompatibility complexes (MHC-I) to promote optimization (editing) of peptide cargo. Here, we use solution NMR to investigate the mechanism of peptide exchange. We identify TAPBPR-induced conformational changes on conserved MHC-I molecular surfaces, consistent with our independently determined X-ray structure of the complex. Dynamics present in the empty MHC-I are stabilized by TAPBPR and become progressively dampened with increasing peptide occupancy. Incoming peptides are recognized according to the global stability of the final pMHC-I product and anneal in a native-like conformation to be edited by TAPBPR. Our results demonstrate an inverse relationship between MHC-I peptide occupancy and TAPBPR binding affinity, wherein the lifetime and structural features of transiently bound peptides control the regulation of a conformational switch located near the TAPBPR binding site, which triggers TAPBPR release. These results suggest a similar mechanism for the function of tapasin in the peptide-loading complex.

Addressable and unidirectional energy transfer along a DNA three-way junction programmed by pyrrole-imidazole polyamides.

We describe a photonic waveguide where FRET is routed uni-directionally along a double-stranded DNA track. The efficiency of FRET is modulated by the supramolecular control of fluorophores along double-stranded DNA using fluorophore-tethered Pyrrole-Imidazole polyamides (PAs). We show that uni-directional FRET is enhanced by the complete assembly of each of the constituent parts, resulting in the selective routing of light along simple DNA duplexes as well as a three-way junction (3WJ).

Halide and proton binding kinetics of yellow fluorescent protein variants.

A T203Y substitution in green fluorescent protein causes a red shift in emission to yield a class of mutants known as yellow fluorescent protein (YFP). Many of these YFP mutants bind halides with affinities in the millimolar range, which often results in the chromophore pK values being shifted into the physiological range. While such sensitivities may be exploited for halide and pH sensors, it is desirable to reduce such environmental sensitivities in other studies, such as in Förster resonance energy transfer probes to measure conformational changes within fusion proteins. Venus and Citrine are two such variants that have been developed with much reduced halide sensitivities. Here we compare the kinetics of halide binding, and the coupled protonation reaction, for several YFP variants and detect slow kinetics (dissociation rate constants in the range of 0.1-1 s(-1)), indicative of binding to an internal site, in all cases. The effective halide affinity for Venus and Citrine is much reduced compared with that of the original YFP 10C construct, primarily through a reduced association rate constant. Nuclear magnetic resonance studies of YFP 10C confirm halide binding occurs on a slow time scale (<4 s(-1)) and that perturbations in the chemical shift occur throughout the sequence and structure.

Mechanical unfolding of human telomere G-quadruplex DNA probed by integrated fluorescence and magnetic tweezers spectroscopy.

Single-molecule techniques facilitate analysis of mechanical transitions within nucleic acids and proteins. Here, we describe an integrated fluorescence and magnetic tweezers instrument that permits detection of nanometer-scale DNA structural rearrangements together with the application of a wide range of stretching forces to individual DNA molecules. We have analyzed the force-dependent equilibrium and rate constants for telomere DNA G-quadruplex (GQ) folding and unfolding, and have determined the location of the transition state barrier along the well-defined DNA-stretching reaction coordinate. Our results reveal the mechanical unfolding pathway of the telomere DNA GQ is characterized by a short distance (<1 nm) to the transition state for the unfolding reaction. This mechanical unfolding response reflects a critical contribution of long-range interactions to the global stability of the GQ fold, and suggests that telomere-associated proteins need only disrupt a few base pairs to destabilize GQ structures. Comparison of the GQ unfolded state with a single-stranded polyT DNA revealed the unfolded GQ exhibits a compacted non-native conformation reminiscent of the protein molten globule. We expect the capacity to interrogate macromolecular structural transitions with high spatial resolution under conditions of low forces will have broad application in analyses of nucleic acid and protein folding.

Asymmetric mode of Ca²âº-S100A4 interaction with nonmuscle myosin IIA generates nanomolar affinity required for filament remodeling.

Filament assembly of nonmuscle myosin IIA (NMIIA) is selectively regulated by the small Ca²âº-binding protein, S100A4, which causes enhanced cell migration and metastasis in certain cancers. Our NMR structure shows that an S100A4 dimer binds to a single myosin heavy chain in an asymmetrical configuration. NMIIA in the complex forms a continuous helix that stretches across the surface of S100A4 and engages the Ca²âº-dependent binding sites of each subunit in the dimer. Synergy between these sites leads to a very tight association (K(D) ∼1 nM) that is unique in the S100 family. Single-residue mutations that remove this synergy weaken binding and ameliorate the effects of S100A4 on NMIIA filament assembly and cell spreading in A431 human epithelial carcinoma cells. We propose a model for NMIIA filament disassembly by S100A4 in which initial binding to the unstructured NMIIA tail initiates unzipping of the coiled coil and disruption of filament packing.

Analysing the ATP turnover cycle of microtubule motors.

Proteins of the kinesin superfamily share a conserved motor domain, which both hydrolyses adenosine-5'-triphosphate (ATP) and binds microtubules. To determine the mechanism of action of a kinesin, it is necessary to relate the chemical cycle of ATP turnover to the mechanics of microtubule interaction. In this chapter, a number of methods are outlined by which the ATP turnover cycle of a kinesin can be analysed with a particular focus on the use of fluorescently labelled ATP and ADP analogues as a means of isolating individual steps in the cycle. By analysing the ATP turnover cycle of a kinesin, both in solution and in the presence of microtubules, the change in nucleotide state triggered upon microtubule binding can be determined. This provides information vital to understanding the coupling of the chemical and mechanical cycles that is integral to the action of members of the kinesin superfamily.

Mechanism of the Ca²+-dependent interaction between S100A4 and tail fragments of nonmuscle myosin heavy chain IIA.

The interaction between the calcium-binding protein S100A4 and the C-terminal fragments of nonmuscle myosin heavy chain IIA has been studied by equilibrium and kinetic methods. Using site-directed mutants, we conclude that Ca(2+) binds to the EF2 domain of S100A4 with micromolar affinity and that the K(d) value for Ca(2+) is reduced by several orders of magnitude in the presence of myosin target fragments. The reduction in K(d) results from a reduced dissociation rate constant (from 16 s(-1) to 0.3 s(-1) in the presence of coiled-coil fragments) and an increased association rate constant. Using peptide competition assays and NMR spectroscopy, we conclude that the minimal binding site on myosin heavy chain IIA corresponds to A1907-G1938; therefore, the site extends beyond the end of the coiled-coil region of myosin. Electron microscopy and turbidity assays were used to assess myosin fragment filament disassembly by S100A4. The latter assay demonstrated that S100A4 binds to the filaments and actively promotes disassembly rather than just binding to the myosin monomer and displacing the equilibrium. Quantitative modelling of these in vitro data suggests that S100A4 concentrations in the micromolar region could disassemble myosin filaments even at resting levels of cytoplasmic [Ca(2+)]. However, for Ca(2+) transients to be effective in further promoting dissociation, the elevated Ca(2+) signal must persist for tens of seconds. Fluorescence recovery after photobleaching of A431/SIP1 cells expressing green fluorescent protein-myosin IIA, immobilised on fibronectin micropatterns to control stress fibre location, yielded a recovery time constant of around 20 s, consistent with in vitro data.

Stoichiometry of a regulatory splicing complex revealed by single-molecule analyses.

Splicing is regulated by complex interactions of numerous RNA-binding proteins. The molecular mechanisms involved remain elusive, in large part because of ignorance regarding the numbers of proteins in regulatory complexes. Polypyrimidine tract-binding protein (PTB), which regulates tissue-specific splicing, represses exon 3 of alpha-tropomyosin through distant pyrimidine-rich tracts in the flanking introns. Current models for repression involve either PTB-mediated looping or the propagation of complexes between tracts. To test these models, we used single-molecule approaches to count the number of bound PTB molecules both by counting the number of bleaching steps of GFP molecules linked to PTB within complexes and by analysing their total emissions. Both approaches showed that five or six PTB molecules assemble. Given the domain structures, this suggests that the molecules occupy primarily multiple overlapping potential sites in the polypyrimidine tracts, excluding propagation models. As an alternative to direct looping, we propose that repression involves a multistep process in which PTB binding forms small local loops, creating a platform for recruitment of other proteins that bring these loops into close proximity.

Insights into the importance of hydrogen bonding in the gamma-phosphate binding pocket of myosin: structural and functional studies of serine 236.

The active site of myosin contains a group of highly conserved amino acid residues whose roles in nucleotide hydrolysis and energy transduction might appear to be obvious from the initial structural and kinetic analyses but become less clear on deeper investigation. One such residue is Ser236 (Dictyostelium discoideum myosin II numbering) which was proposed to be involved in a hydrogen transfer network during gamma-phosphate hydrolysis of ATP, which would imply a critical function in ATP hydrolysis and motility. The S236A mutant protein shows a comparatively small decrease in hydrolytic activity and motility, and thus this residue does not appear to be essential. To understand better the contribution of Ser236 to the function of myosin, structural and kinetic studies have been performed on the S236A mutant protein. The structures of the D. discoideum motor domain (S1dC) S236A mutant protein in complex with magnesium pyrophosphate, MgAMPPNP, and MgADP.vanadate have been determined. In contrast to the previous structure of wild-type S1dC, the S236A.MgAMPPNP complex crystallized in the closed state. Furthermore, transient-state kinetics showed a 4-fold reduction of the nucleotide release step, suggesting that the mutation stabilizes a closed active site. The structures show that a water molecule approximately adopts the location of the missing hydroxyl of Ser236 in the magnesium pyrophosphate and MgAMPPNP structures. This study suggests that the S236A mutant myosin proceeds via a different structural mechanism than wild-type myosin, where the alternate mechanism is able to maintain near normal transient-state kinetic values.

The photochemistry of fluorescent proteins: implications for their biological applications.

Green fluorescent protein from Aequorea victoria, its relatives and derivatives are ubiquitous in their use as biological probes. In this tutorial review, we discuss the photochemistry of this fascinating class of proteins and illustrate some of their advantages and drawbacks in a range of applications. In particular, we focus on the ionisation states of the chromophore and how they are affected by internal and external proton transfer. Light-induced reversible and irreversible events are discussed in terms of the underlying chromophore structure. These phenomena have an influence on the interpretation of FRET (Förster resonance energy transfer), FRAP (fluorescence recovery after photobleaching), as well as single molecule studies.

Probing complexes with single fluorophores: factors contributing to dispersion of FRET in DNA/RNA duplexes.

Single molecule fluorescent microscopy is a method for the analysis of the dynamics of biological macromolecules by detecting the fluorescence signal produced by fluorophores associated with the macromolecule. Two fluorophores located in a close proximity may result in Förster resonance energy transfer (FRET), which can be detected at the single molecule level and the efficiency of energy transfer calculated. In most cases, the experimentally observed distribution of FRET efficiency exhibits a significant width corresponding to 0.07-0.2 (on a scale of 0-1). Here, we present a general approach describing the analysis of experimental data for a DNA/RNA duplex. We have found that for a 15 bp duplex with Cy3 and Cy5 fluorophores attached to the opposite ends of the helix, the width of the energy transfer distribution is mainly determined by the photon shot noise and the orientation factor, whereas the variation of inter-dye distances plays a minor role.

Structural basis for the interaction between the cytoplasmic domain of the hyaluronate receptor layilin and the talin F3 subdomain.

Talin is a large cytoskeletal protein that is involved in coupling the integrin family of cell adhesion molecules to the actin cytoskeleton, colocalising with the integrins in focal adhesions (FAs). However, at the leading edge of motile cells, talin colocalises with the hyaluronan receptor layilin in what are thought to be transient adhesions, some of which subsequently mature into more stable FAs. During this maturation process, layilin is replaced with integrins, which are highly clustered in FAs, where localised production of PI(4,5)P(2) by type 1 phosphatidyl inositol phosphate kinase type 1gamma (PIPK1gamma) is thought to play a role in FA assembly. The talin FERM F3 subdomain binds both the integrin beta-subunit cytoplasmic domain and PIPK1gamma, and these interactions are understood in detail at the atomic level. The talin F3 domain also binds to short sequences in the layilin cytoplasmic domain, and here we report the structure of the talin/layilin complex, which shows that talin binds integrins, PIPK1gamma and layilin in similar although subtly different ways. Based on structure comparisons, we designed a set of talin F3 mutations that selectively affected the affinity of talin for its targets, as determined by stopped-flow fluorescence measurements. Such mutations will help to assess the importance of the interactions between talin and its various ligands in cell adhesion and migration.

Crystal structure of the Ca(2+)-form and Ca(2+)-binding kinetics of metastasis-associated protein, S100A4.

S100A4 takes part in control of tumour cell migration and contributes to metastatic spread in in vivo models. In the active dimeric Ca(2+)-bound state it interacts with multiple intracellular targets. Conversely, oligomeric forms of S100A4 are linked with the extracellular function of this protein. We report the 1.5A X-ray crystal structure of Ca(2+)-bound S100A4 and use it to identify the residues involved in target recognition and to derive a model of the oligomeric state. We applied stopped-flow analysis of tyrosine fluorescence to derive kinetics of S100A4 activation by Ca(2+) (k(on)=3.5 microM(-1)s(-1), k(off)=20s(-1)).

The mechanism of the reverse recovery step, phosphate release, and actin activation of Dictyostelium myosin II.

The rate-limiting step of the myosin basal ATPase (i.e. in absence of actin) is assumed to be a post-hydrolysis swinging of the lever arm (reverse recovery step), that limits the subsequent rapid product release steps. However, direct experimental evidence for this assignment is lacking. To investigate the binding and the release of ADP and phosphate independently from the lever arm motion, two single tryptophan-containing motor domains of Dictyostelium myosin II were used. The single tryptophans of the W129+ and W501+ constructs are located at the entrance of the nucleotide binding pocket and near the lever arm, respectively. Kinetic experiments show that the rate-limiting step in the basal ATPase cycle is indeed the reverse recovery step, which is a slow equilibrium step (k(forward) = 0.05 s(-1), k(reverse) = 0.15 s(-1)) that precedes the phosphate release step. Actin directly activates the reverse recovery step, which becomes practically irreversible in the actin-bound form, triggering the power stroke. Even at low actin concentrations the power stroke occurs in the actin-attached states despite the low actin affinity of myosin in the pre-power stroke conformation.