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The HIV capsid self-assembles a protective conical shell that simultaneously prevents host sensing whilst permitting the import of nucleotides to drive DNA synthesis. This is accomplished through the construction of dynamic, highly charged pores at the centre of each capsid multimer. The clustering of charges required for dNTP import is strongly destabilising and it is proposed that HIV uses the metabolite IP6 to coordinate the pore during assembly. Here we have investigated the role of inositol phosphates in coordinating a ring of positively charged lysine residues (K25) that forms at the base of the capsid pore. We show that whilst IP5, which can functionally replace IP6, engages an arginine ring (R18) at the top of the pore, the lysine ring simultaneously binds a second IP5 molecule. Dose dependent removal of K25 from the pore severely inhibits HIV infection and concomitantly prevents DNA synthesis. Cryo-tomography reveals that K25A virions have a severe assembly defect that inhibits the formation of mature capsid cones. Monitoring both the kinetics and morphology of capsids assembled in vitro reveals that while mutation K25A can still form tubes, the ability of IP6 to drive assembly of capsid cones has been lost. Finally, in single molecule TIRF microscopy experiments, capsid lattices in permeabilised K25 mutant virions are rapidly lost and cannot be stabilised by IP6. These results suggest that the coordination of IP6 by a second charged ring in mature hexamers drives the assembly of conical capsids capable of reverse transcription and infection.

HIV virion assembly begins with the construction of an immature lattice consisting of Gag hexamers. Upon virion release, protease-mediated Gag cleavage leads to a maturation event in which the immature lattice disassembles and the mature capsid assembles. The cellular metabolite inositiol hexakisphosphate (IP6) and maturation inhibitors (MIs) both bind and stabilize immature Gag hexamers, but whereas IP6 promotes virus maturation, MIs inhibit it. Here we show that HIV is evolutionarily constrained to maintain an immature lattice stability that ensures IP6 packaging without preventing maturation. Replication-deficient mutant viruses with reduced IP6 recruitment display increased infectivity upon treatment with the MI PF46396 (PF96) or the acquisition of second-site compensatory mutations. Both PF96 and second-site mutations stabilise the immature lattice and restore IP6 incorporation, suggesting that immature lattice stability and IP6 binding are interdependent. This IP6 dependence suggests that modifying MIs to compete with IP6 for Gag hexamer binding could substantially improve MI antiviral potency.

The HIV capsid is a multifunctional protein capsule that mediates the delivery of the viral genetic material into the nucleus of the target cell. Host cell proteins bind to a number of repeating binding sites on the capsid to regulate steps in the replication cycle. Here, we develop a fluorescence fluctuation spectroscopy method using self-assembled capsid particles as the bait to screen for fluorescence-labeled capsid-binding analytes (“prey” molecules) in solution. The assay capitalizes on the property of the HIV capsid as a multivalent interaction platform, facilitating high sensitivity detection of multiple prey molecules that have accumulated onto capsids as spikes in fluorescence intensity traces. By using a scanning stage, we reduced the measurement time to 10 s without compromising on sensitivity, providing a rapid binding assay for screening libraries of potential capsid interactors. The assay can also identify interfaces for host molecule binding by using capsids with defects in known interaction interfaces. Two-color coincidence detection using the fluorescent capsid as the bait further allows the quantification of binding levels and determination of binding affinities. Overall, the assay provides new tools for the discovery and characterization of molecules used by the HIV capsid to orchestrate infection. The measurement principle can be extended for the development of sensitive interaction assays, utilizing natural or synthetic multivalent scaffolds as analyte-binding platforms.

The Australasian Virology Society (AVS) aims to promote, support and advocate for the discipline of virology in the Australasian region. The society was incorporated in 2011 after 10 years operating as the Australian Virology Group (AVG) founded in 2001, coinciding with the inaugural biennial scientific meeting. AVS conferences aim to provide a forum for the dissemination of all aspects of virology, foster collaboration, and encourage participation by students and post-doctoral researchers. The tenth Australasian Virology Society (AVS10) scientific meeting was held on 2-5 December 2019 in Queenstown, New Zealand. This report highlights the latest research presented at the meeting, which included cutting-edge virology presented by our international plenary speakers Ana Fernandez-Sesma and Benjamin tenOever, and keynote Richard Kuhn. AVS10 honoured female pioneers in Australian virology, Lorena Brown and Barbara Coulson. We report outcomes from the AVS10 career development session on "Successfully transitioning from post-doc to lab head", winners of best presentation awards, and the AVS gender equity policy, initiated in 2013. Plans for the 2021 meeting are underway which will celebrate the 20th anniversary of AVS where it all began, in Fraser Island, Queensland, Australia.

© 2020 American Chemical Society. The human immunodeficiency virus (HIV) capsid is a cone-shaped capsule formed from the viral capsid protein (CA), which is arranged into a lattice of hexamers and pentamers. The capsid comprises multiple binding interfaces for the recruitment of host proteins and macromolecules used by the virus to establish infection. Here, we coassembled CA proteins engineered for pentamer cross-linking and fluorescence labeling, into spherical particles. The CA spheres, which resemble the pentamer-rich structure of the end caps of the native HIV capsid, were immobilized onto surfaces as biorecognition elements for fluorescence microscopy-based quantification of host protein binding. The capsid-binding host protein cyclophilin A (CypA) is bound to CA spheres with the same affinity as CA tubes but at a higher CypA/CA stoichiometry, suggesting that the level of recruitment of CypA to the HIV capsid is dependent on curvature.

Background: Efficient HIV-1 replication depends on interaction of the viral capsid with the host protein cyclophilin A (CypA). CypA, a peptidylprolyl isomerase, binds to an exposed loop in the viral CA protein via the enzyme's active site. Recent structural analysis of CypA in complex with CA tubes in conjunction with molecular dynamics simulations identified a secondary CA binding site on CypA that allows a bridging interaction with two hexameric subunits of the assembled CA lattice, leading to capsid stabilization (Liu et al. in Nat Commun 7:10714, 2016). Results: We performed mutational analysis of residues that have been proposed to mediate CA binding at the secondary binding site on CypA (A25, K27, P29 and K30) and tested the effects of the amino acid substitutions using interaction assays and HIV-1 infection assays in cells. The binding of recombinant CypA to self-assembled CA tubes or native HIV-1 capsids was measured in vitro using a quantitative fluorescence microscopy binding assay revealing that affinity and stoichiometry of CypA to the CA lattice was not affected by the substitutions. To test for functionality of the CypA secondary CA-binding site in HIV-1 infection, mutant CypA proteins were expressed in cells in which endogenous CypA was deleted, and the effects on HIV-1 infection were assayed. In normal HeLa-P4 cells, infection with HIV-1 bearing the A92E substitution in CA is inhibited by endogenous CypA and was inhibited to the same extent by expression of CypA mutants in CypA-null HeLa-P4 cells. Expression of the mutant CypA proteins in CypA-null Jurkat cells restored their permissiveness to infection by wild type HIV-1. Conclusions: The amino acid changes at A25, K27, P29 and K30 did not affect the affinity of CypA for the CA lattice and did not impair CypA function in infection assays suggesting that these residues are not part of a secondary CA binding site on CypA.

Tropomyosins (Tpm) determine the functional capacity of actin filaments in an isoform-specific manner. The primary isoform in cancer cells is Tpm3.1 and compounds that target Tpm3.1 show promising results as anti-cancer agents both in vivo and in vitro. We have determined the molecular mechanism of interaction of the lead compound ATM-3507 with Tpm3.1-containing actin filaments. When present during co-polymerization of Tpm3.1 with actin, 3H-ATM-3507 is incorporated into the filaments and saturates at approximately one molecule per Tpm3.1 dimer and with an apparent binding affinity of approximately 2 µM. In contrast, 3H-ATM-3507 is poorly incorporated into preformed Tpm3.1/actin co-polymers. CD spectroscopy and thermal melts using Tpm3.1 peptides containing the C-terminus, the N-terminus, and a combination of the two forming the overlap junction at the interface of adjacent Tpm3.1 dimers, show that ATM-3507 shifts the melting temperature of the C-terminus and the overlap junction, but not the N-terminus. Molecular dynamic simulation (MDS) analysis predicts that ATM-3507 integrates into the 4-helix coiled coil overlap junction and in doing so, likely changes the lateral movement of Tpm3.1 across the actin surface resulting in an alteration of filament interactions with actin binding proteins and myosin motors, consistent with the cellular impact of ATM-3507.

Many actin filaments in animal cells are co-polymers of actin and tropomyosin. In many cases, non-muscle myosin II associates with these co-polymers to establish a contractile network. However, the temporal relationship of these three proteins in the de novo assembly of actin filaments is not known. Intravital subcellular microscopy of secretory granule exocytosis allows the visualisation and quantification of the formation of an actin scaffold in real time, with the added advantage that it occurs in a living mammal under physiological conditions. We used this model system to investigate the de novo assembly of actin, tropomyosin Tpm3.1 (a short isoform of TPM3) and myosin IIA (the form of non-muscle myosin II with its heavy chain encoded by Myh9) on secretory granules in mouse salivary glands. Blocking actin polymerization with cytochalasin D revealed that Tpm3.1 assembly is dependent on actin assembly. We used time-lapse imaging to determine the timing of the appearance of the actin filament reporter LifeAct-RFP and of Tpm3.1-mNeonGreen on secretory granules in LifeAct-RFP transgenic, Tpm3.1- mNeonGreen and myosin IIA-GFP (GFP-tagged MYH9) knock-in mice. Our findings are consistent with the addition of tropomyosin to actin filaments shortly after the initiation of actin filament nucleation, followed by myosin IIA recruitment.

Single-molecule localization microscopy (SMLM) has created the opportunity of pushing fluorescence microscopy from being a biological imaging tool to a surface characterization and possibly even a quantitative analytical tool. The latter could be achieved by molecular counting using pointillist SMLM data sets. However, SMLM is especially sensitive to background fluorescent signals, which influences any subsequent analysis. Therefore, fabricating sensing surfaces that resist nonspecific adsorption of proteins, even after multiple modification steps, has become paramount. Herein is reported two different ways to modify surfaces: dichlorodimethylsilane-biotinylated bovine serum albumin-Tween-20 (DbT20) and poly-l-lysine grafted polyethylene glycol (PLL-PEG) mixed with biotinylated PLL-PEG (PLL-PEG/PEGbiotin). The results show that the ability to resist nonspecific adsorption of DbT20 surfaces deteriorates with an increase in the number of modification steps required after the addition of the DbT20, which limits the applicability of this surface for SMLM. As such, a new surface for SMLM that employs PLL-PEG/PEGbiotin was developed that exhibits ultralow amounts of nonspecific protein adsorption even after many modification steps. The utility of the surface was demonstrated for human influenza hemagglutinin-tagged mEos2, which was directly pulled down from cell lysates onto the PLL-PEG/PEGbiotin surface. The results strongly indicated that the PLL-PEG/PEGbiotin surface satisfies the criteria of SMLM imaging of a negligible background signal and negligible nonspecific adsorption.

Skeletal muscle tropomyosin (Tpm1.1) is an elongated, rod-shaped, alpha-helical coiled-coil protein that forms continuous head-to-tail polymers along both sides of the actin filament. In this study we use single molecule fluorescence TIRF microscopy combined with a microfluidic device and fluorescently labelled proteins to measure Tpm1.1 association to and dissociation from single actin filaments. Our experimental setup allows us to clearly resolve Tpm1.1 interactions on both sides of the filaments. Here we provide a semi-automated method for the extraction and quantification of kymograph data for individual actin filaments bound at different Tpm1.1 concentrations. We determine boundaries on the kymograph on each side of the actin filament, based on intensity thresholding, performing fine manual editing of the boundaries (if needed) and extracting user defined kinetic properties of the system. Using our analytical tools we can determine (i) nucleation point(s) and rates, (ii) elongation rates of Tpm1.1, (iii) identify meeting points after the saturation of filament, and when dissociation occurs, (iv) initiation point(s), (v) the final dissociation point(s), as well as (vi) dissociation rates. All of these measurements can be extracted from both sides of the filament, allowing for the determination of possible differences in behaviour on the two sides of the filament, and across concentrations. The robust and repeatable nature of the method allows quantitative, semi-automated analyses to be made of large studies of acto-tropomyosin interactions, as well as for other actin binding proteins or filamentous structures, opening the way for dissection of the dynamics underlying these interactions.

© 2017 Nicovich et al.This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Modern fluorescence microscopy requires software-controlled illumination sources with high power across a wide range of wavelengths. Diode lasers meet the power requirements and combining multiple units into a single fiber launch expands their capability across the required spectral range. We present the NicoLase, an open-source diode laser combiner, fiber launch, and software sequence controller for fluorescence microscopy and super-resolution microscopy applications. Two configurations are described, giving four or six output wavelengths and one or two single-mode fiber outputs, with all CAD files, machinist drawings, and controller source code openly available.

© 2017 The Authors Competing models exist in the literature for the relationship between mutant Huntingtin exon 1 (Httex1) inclusion formation and toxicity. In one, inclusions are adaptive by sequestering the proteotoxicity of soluble Httex1. In the other, inclusions compromise cellular activity as a result of proteome co-aggregation. Using a biosensor of Httex1 conformation in mammalian cell models, we discovered a mechanism that reconciles these competing models. Newly formed inclusions were composed of disordered Httex1 and ribonucleoproteins. As inclusions matured, Httex1 reconfigured into amyloid, and other glutamine-rich and prion domain-containing proteins were recruited. Soluble Httex1 caused a hyperpolarized mitochondrial membrane potential, increased reactive oxygen species, and promoted apoptosis. Inclusion formation triggered a collapsed mitochondrial potential, cellular quiescence, and deactivated apoptosis. We propose a revised model where sequestration of soluble Httex1 inclusions can remove the trigger for apoptosis but also co-aggregate other proteins, which curtails cellular metabolism and leads to a slow death by necrosis.

Protein aggregation is a hallmark of many neurodegenerative diseases, notably Alzheimer's and Parkinson's disease. Parkinson's disease is characterized by the presence of Lewy bodies, abnormal aggregates mainly composed of α-synuclein. Moreover, cases of familial Parkinson's disease have been linked to mutations in α-synuclein. In this study, we compared the behavior of wild-type (WT) α-synuclein and five of its pathological mutants (A30P, E46K, H50Q, G51D and A53T). To this end, single-molecule fluorescence detection was coupled to cell-free protein expression to measure precisely the oligomerization of proteins without purification, denaturation or labelling steps. In these conditions, we could detect the formation of oligomeric and pre-fibrillar species at very short time scale and low micromolar concentrations. The pathogenic mutants surprisingly segregated into two classes: one group forming large aggregates and fibrils while the other tending to form mostly oligomers. Strikingly, co-expression experiments reveal that members from the different groups do not generally interact with each other, both at the fibril and monomer levels. Together, this data paints a completely different picture of α-synuclein aggregation, with two possible pathways leading to the development of fibrils.

Tropomyosin (Tpm) is an α helical coiled-coil dimer that forms a co-polymer along the actin filament. Tpm is involved in the regulation of actin's interaction with binding proteins as well as stabilization of the actin filament and its assembly kinetics. Recent studies show that multiple Tpm isoforms also define the functional properties of distinct actin filament populations within a cell. Subtle structural variations within well conserved Tpm isoforms are the key to their functional specificity. Therefore, we purified and characterized a comprehensive set of 8 Tpm isoforms (Tpm1.1, Tpm1.12, Tpm1.6, Tpm1.7, Tpm1.8, Tpm2.1, Tpm3.1, and Tpm4.2), using well-established actin co-sedimentation and pyrene fluorescence polymerization assays. We observed that the apparent affinity (Kd(app)) to filamentous actin varied in all Tpm isoforms between ∼0.1–5 μM with similar values for both, skeletal and cytoskeletal actin filaments. The data did not indicate any correlation between affinity and size of Tpm molecules, however high molecular weight (HMW) isoforms Tpm1.1, Tpm1.6, Tpm1.7 and Tpm2.1, showed ∼3-fold higher cooperativity compared to low molecular weight (LMW) isoforms Tpm1.12, Tpm1.8, Tpm3.1, and Tpm4.2. The rate of actin filament elongation in the presence of Tpm2.1 increased, while all other isoforms decreased the elongation rate by 27–85 %. Our study shows that the biochemical properties of Tpm isoforms are finely tuned and depend on sequence variations in alternatively spliced regions of Tpm molecules.

Reconstitution of actin filaments on surfaces for observation of filament-associated protein dynamics by fluorescence microscopy is currently an exciting field in biophysics. Here we examine the effects of attaching actin filaments to surfaces on the binding and dissociation kinetics of a fluorescence-labeled tropomyosin, a rod-shaped protein that forms continuous strands wrapping around the actin filament. Two attachment modalities of the actin to the surface are explored: where the actin filament is attached to the surface at multiple points along its length; and where the actin filament is attached at one end and aligned parallel to the surface by buffer flow. To facilitate analysis of actin-binding protein dynamics, we have developed a software tool for the viewing, tracing and analysis of filaments and co-localized species in noisy fluorescence timelapse images. Our analysis shows that the interaction of tropomyosin with actin filaments is similar for both attachment modalities. © 2016 Wiley Periodicals, Inc.

The tropomyosin family of proteins form end-to-end polymers along the actin filament. Tumour cells rely on specific tropomyosin-containing actin filament populations for growth and survival. To dissect out the role of tropomyosin in actin filament regulation we use the small molecule TR100 directed against the C terminus of the tropomyosin isoform Tpm3.1. TR100 nullifies the effect of Tpm3.1 on actin depolymerisation but surprisingly Tpm3.1 retains the capacity to bind F-actin in a cooperative manner. In vivo analysis also confirms that, in the presence of TR100, fluorescently tagged Tpm3.1 recovers normally into stress fibers. Assembling end-to-end along the actin filament is thereby not sufficient for tropomyosin to fulfil its function. Rather, regulation of F-actin stability by tropomyosin requires fidelity of information communicated at the barbed end of the actin filament. This distinction has significant implications for perturbing tropomyosin-dependent actin filament function in the context of anti-cancer drug development.

Mechanisms for transcription factor recognition of specific DNA base sequences are well characterized and recent studies demonstrate that the shape of these cognate binding sites is also important. Here, we uncover a new mechanism where the transcription factor GabR simultaneously recognizes two cognate binding sites and the shape of a 29 bp DNA sequence that bridges these sites. Small- Angle X-ray scattering and multi- Angle laser light scattering are consistent with a model where the DNA undergoes a conformational change to bend around GabR during binding. In silico predictions suggest that the bridging DNA sequence is likely to be bendable in one direction and kinetic analysis of mutant DNA sequences with biolayer interferometry, allowed the independent quantification of the relative contribution of DNA base and shape recognition in the GabR- DNA interaction. These indicate that the two cognate binding sites as well as the bendability of the DNA sequence in between these sites are required to form a stable complex. The mechanism of GabR-DNA interaction provides an example where the correct shape of DNA, at a clearly distinct location fromthe cognate binding site, is required for transcription factor binding and has implications for bioinformatics searches for novel binding sites.

Cells can sense and migrate towards higher concentrations of adhesive cues such as the glycoproteins of the extracellular matrix and soluble cues such as growth factors. Here, we outline a method to create opposing gradients of adhesive and soluble cues in a microfluidic chamber, which is compatible with live cell imaging. A copolymer of poly-L-lysine and polyethylene glycol (PLL-PEG) is employed to passivate glass coverslips and prevent non-specific adsorption of biomolecules and cells. Next, microcontact printing or dip pen lithography are used to create tracks of streptavidin on the passivated surfaces to serve as anchoring points for the biotinylated peptide arginine-glycine-aspartic acid (RGD) as the adhesive cue. A microfluidic device is placed onto the modified surface and used to create the gradient of adhesive cues (100% RGD to 0% RGD) on the streptavidin tracks. Finally, the same microfluidic device is used to create a gradient of a chemoattractant such as fetal bovine serum (FBS), as the soluble cue in the opposite direction of the gradient of adhesive cues.

Autopsie nach Ex. der ULB Sachsen-Anhalt, Vorlageform des Erscheinungsvermerks: Halle, gedruckt bey Johann Gottfried Trampe.

Autopsie nach Ex. der ULB Sachsen-Anhalt, Vorlageform des Erscheinungsvermerks: Halle, gedruckt bey Johann Gottfried Trampe.