Single-Molecule Analysis of Genome Uncoating from Individual Human Rhinovirus Particles, and Modulation by Antiviral Drugs

Infection of humans by many viruses is typically initiated by the internalization of a single virion in each of a few susceptible cells. Thus, the outcome of the infection process may depend on stochastic single-molecule events. A crucial process for viral infection, and thus a target for developing antiviral drugs, is the uncoating of the viral genome. Here a force spectroscopy procedure using an atomic force microscope is implemented to study uncoating for individual human rhinovirus particles. Application of an increasing mechanical force on a virion led to a high force-induced structural transition that facilitated extrusion of the viral RNA molecule without loss of capsid integrity. Application of force to virions that h ad previously extruded the RNA, or to RNA-free capsids, led to a lower force-induced event associated with capsid disruption. The kinetic parameters are determined for each reaction. The high-force event is a stochastic process governed by a moderate free energy barrier (≈20 kcal mol-1 ), which results in a heterogeneous population of structurally weakened virions in which different fractions of the RNA molecule are externalized. The effects of antiviral compounds or capsid mutation on the kinetics of this reaction reveal a correlation between the reaction rate and virus infectivity.

Electrostatic Screening, Acidic pH and Macromolecular Crowding Increase the Self-Assembly Efficiency of the Minute Virus of Mice Capsid In Vitro

The hollow protein capsids from a number of different viruses are being considered for multiple biomedical or nanotechnological applications. In order to improve the applied potential of a given viral capsid as a nanocarrier or nanocontainer, specific conditions must be found for achieving its faithful and efficient assembly in vitro. The small size, adequate physical properties and specialized biological functions of the capsids of parvoviruses such as the minute virus of mice (MVM) make them excellent choices as nanocarriers and nanocontainers. In this study we analyzed the effects of protein concentration, macromolecular crowding, temperature, pH, ionic strength, or a combination of some of those variables on the fidelity and efficiency of self-assembly of the MVM capsid in vitro. The results revealed that the in vitro reassembly of the MVM capsid is an efficient and faithful process. Under some conditions, up to ~40% of the starting virus capsids were reassembled in vitro as free, non aggregated, correctly assembled particles. These results open up the possibility of encapsidating different compounds in VP2-only capsids of MVM during its reassembly in vitro, and encourage the use of virus-like particles of MVM as nanocontainers.

Equilibrium Dynamics of a Biomolecular Complex Analyzed at Single-amino Acid Resolution by Cryo-electron Microscopy

The biological function of macromolecular complexes depends not only on large-scale transitions between conformations, but also on small-scale conformational fluctuations at equilibrium. Information on the equilibrium dynamics of biomolecular complexes could, in principle, be obtained from local resolution (LR) data in cryo-electron microscopy (cryo-EM) maps. However, this possibility had not been validated by comparing, for a same biomolecular complex, LR data with quantitative information on equilibrium dynamics obtained by an established solution technique. In this study we determined the cryo-EM structure of the minute virus of mice (MVM) capsid as a model biomolecular complex. The LR values obtained correlated with crystallographic B factors and with hydrogen/deuterium exchange (HDX) rates obtained by mass spectrometry (HDX-MS), a gold standard for determining equilibrium dynamics in solution. This result validated a LR-based cryo-EM approach to investigate, with high spatial resolution, the equilibrium dynamics of biomolecular complexes. As an application of this approach, we determined the cryo-EM structure of two mutant MVM capsids and compared their equilibrium dynamics with that of the wild-type MVM capsid. The results supported a previously suggested linkage between mechanical stiffening and impaired equilibrium dynamics of a virus particle. Cryo-EM is emerging as a powerful approach for simultaneously acquiring information on the atomic structure and local equilibrium dynamics of biomolecular complexes.

Antiviral compounds modulate elasticity, strength and material fatigue of a virus capsid framework

This study investigates whether the biochemical and antiviral effects of organic compounds that bind different sites in the mature human immunodeficiency virus capsid may be related to the modulation of different mechanical properties of the protein lattice from which the capsid is built. Mechanical force was used as a probe to quantify, in atomic force microscopy experiments at physiological pH and ionic strength, ligand-mediated changes in capsid lattice elasticity, breathing, strength against local dislocation by mechanical stress, and resistance to material fatigue. The results indicate that the effects of the tested compounds on assembly or biochemical stability can be linked, from a physics-based perspective, to their interference with the mechanical behavior of the viral capsid framework. The antivirals CAP-1 and CAI-55 increased the intrinsic elasticity and breathing of the capsid protein lattice and may entropically decrease the probability of the capsid protein to assemble into a functionally competent conformation. Antiviral PF74 increased the resistance of the capsid protein lattice to disruption by mechanical stress and material fatigue and may enthalpically strengthen the basal capsid lattice against breakage and disintegration. This study provides proof of concept that the interrogation of the mechanical properties of the nanostructured protein material that makes a virus capsid may provide fundamental insights into the biophysical action of capsid-binding antiviral agents. The implications for drug design by specifically targeting the biomechanics of viruses are discussed.

A Genetically Engineered, Chain Mail-Like Nanostructured Protein Material with Increased Fatigue Resistance and Enhanced Self-Healing

Protein-based nanostructured materials are being developed for many biomedical and nanotechnological applications. Despite their many desirable features, protein materials are highly susceptible to disruption by mechanical stress and fatigue. This study is aimed to increase fatigue resistance and enhance self-healing of a natural protein-based supramolecular nanomaterial through permanent genetic modification. The authors envisage the conversion of a model nanosheet, formed by a regular array of noncovalently bound human immunodeficiency virus capsid protein molecules, into a supramolecular "chain mail." Rationally engineered mutations allow the formation of a regular network of disulfide bridges in the protein lattice. This network links each molecule in the lattice to each adjacent molecule through one covalent bond, analogous to the rivetting of interlinked iron rings in the chain mail of a medieval knight. The engineered protein nanosheet shows greatly increased thermostability and resistance to mechanical stress and fatigue in particular, as well as enhanced self-healing, without undesirable stiffening compared to the original material. The results provide proof of concept for a genetic design to permanently increase fatigue resistance and enhance self-healing of protein-based nanostructured materials. They also provide insights into the molecular basis for fatigue of protein materials.

Visualization of Single Molecules Building a Viral Capsid Protein Lattice through Stochastic Pathways

Direct visualization of pathways followed by single molecules while they spontaneously self-assemble into supramolecular biological machines may provide fundamental knowledge to guide molecular therapeutics and the bottom-up design of nanomaterials and nanodevices. Here, high-speed atomic force microscopy is used to visualize self-assembly of the bidimensional lattice of protein molecules that constitutes the framework of the mature human immunodeficiency virus capsid. By real-time imaging of the assembly reaction, individual transient intermediates and reaction pathways followed by single molecules could be revealed. As when assembling a jigsaw puzzle, the capsid protein lattice is randomly built. Lattice patches grow independently from separate nucleation events whereby individual molecules follow different paths. Protein subunits can be added individually, while others form oligomers before joining a lattice or are occasionally removed from the latter. Direct real-time imaging of supramolecular self-assembly has revealed a complex, chaotic process involving multiple routes followed by individual molecules that are inaccessible to bulk (averaging) techniques.

Structural determinants of mechanical resistance against breakage of a virus-based protein nanoparticle at a resolution of single amino acids

Virus particles and other protein-based supramolecular complexes have a vast nanotechnological potential. However, protein nanostructures are "soft" materials prone to disruption by force. Whereas some non-biological nanoparticles (NPs) may be stronger, for certain applications protein- and virus-based NPs have potential advantages related to their structure, self-assembly, production, engineering, and/or inbuilt functions. Thus, it may be desirable to acquire the knowledge needed to engineer protein-based nanomaterials with a higher strength against mechanical breakage. Here we have used the capsid of the minute virus of mice to experimentally identify individual chemical groups that determine breakage-related properties of a virus particle. Individual amino acid side chains that establish interactions between building blocks in the viral particle were truncated using protein engineering. Indentation experiments using atomic force microscopy were carried out to investigate the role of each targeted side chain in determining capsid strength and brittleness, by comparing the maximum force and deformation each modified capsid withstood before breaking apart. Side chains with major roles in determining capsid strength against breakage included polar groups located in solvent-exposed positions, and did not generally correspond with those previously identified as determinants of mechanical stiffness. In contrast, apolar side chains buried along the intersubunit interfaces that generally determined capsid stiffness had, at most, a minor influence on strength against disruption. Whereas no correlated variations between strength and either stiffness or brittleness were found, brittleness and stiffness were quantitatively correlated. Implications for developing robust protein-based NPs and for acquiring a deeper physics-based perspective of viruses are discussed.

Thermostability of the Foot-and-Mouth Disease Virus Capsid Is Modulated by Lethal and Viability-Restoring Compensatory Amino Acid Substitutions

Infection by viruses depends on a balance between capsid stability and dynamics. This study investigated biologically and biotechnologically relevant aspects of the relationship in foot-and-mouth disease virus (FMDV) between capsid structure and thermostability and between thermostability and infectivity. In the FMDV capsid, a substantial number of amino acid side chains at the interfaces between pentameric subunits are charged at neutral pH. Here a mutational analysis revealed that the essential role for virus infection of most of the 8 tested charged groups is not related to substantial changes in capsid protein expression or processing or in capsid assembly or stability against a thermally induced dissociation into pentamers. However, the positively charged side chains of R2018 and H3141, located at the interpentamer interfaces close to the capsid 2-fold symmetry axes, were found to be critical both for virus infectivity and for keeping the capsid in a state of weak thermostability. A charge-restoring substitution (N2019H) that was repeatedly fixed during amplification of viral genomes carrying deleterious mutations reverted both the lethal and capsid-stabilizing effects of the substitution H3141A, leading to a double mutant virus with close to normal infectivity and thermolability. H3141A and other thermostabilizing substitutions had no detectable effect on capsid resistance to acid-induced dissociation into pentamers. The results suggest that FMDV infectivity requires limited local stability around the 2-fold axes at the interpentamer interfaces of the capsid. The implications for the mechanism of genome uncoating in FMDV and the development of thermostabilized vaccines against foot-and-mouth disease are discussed.IMPORTANCE This study provides novel insights into the little-known structural determinants of the balance between thermal stability and instability in the capsid of foot-and-mouth disease virus and into the relationship between capsid stability and virus infectivity. The results provide new guidelines for the development of thermostabilized empty capsid-based recombinant vaccines against foot-and-mouth disease, one of the economically most important animal diseases worldwide.

Mechanical stiffening of human rhinovirus by cavity-filling antiviral drugs

Emerging studies at the nanoscale on the relationships between the structure, mechanical properties and infectivity of virus particles are revealing important physics-based foundations of virus biology that may have biomedical and nanotechnological applications. Human rhinovirus (HRV) is the major causative agent of common colds leading to important economic losses, and is also associated with more severe diseases. There is renewed interest in developing effective anti-HRV drugs, but none have been approved so far. We have chosen HRV to explore a possible link between virus mechanics and infectivity and the antiviral effect of certain drugs. In particular, we have investigated a suggestion that the antiviral action of drugs that bind to capsid cavities (pockets) may be related to changes in virus stiffness. Mechanical analysis using atomic force microscopy shows that filling the pockets with drugs or genetically introducing bulkier amino acid side chains into the pockets stiffen HRV virions to different extents. Drug-mediated stiffening affected some regions distant from the pockets and involved in genome uncoating, and may be caused by a subtle structural rearrangement of the virus particle. The results also revealed for HRV a quantitative, logarithmic relationship between mechanical stiffening, achieved either by drug binding or introducing bulkier amino acid side chains into the pockets, and reduced infectivity. From a fundamental physics perspective, these drugs may exert their biological effect by decreasing the deformability of the virion, thus impairing its equilibrium dynamics. The results encourage the design of novel antiviral drugs that inhibit infection by mechanically stiffening the viral particles.

Correction: Corrigendum: Conformational rearrangements in the transmembrane domain of CNGA1 channels revealed by single-molecule force spectroscopy

Nature Communications 8: Article number: 7093 (2015); Published: 12 May 2015; Updated: 29 September 2017 An error was inadvertently introduced into the fifth sentence of the Abstract in the original version of this Article. The sentence should have stated ‘Force spectra determined that the S4 transmembrane domain is mechanically coupled to S5 in the open state, but S3 in the closed state’ and not ‘Force spectra determined that the S4 transmembrane domain is mechanically coupled to S5 in the closed state, but S3 in the open state’.

Kinetics of Surface-Driven Self-Assembly and Fatigue-Induced Disassembly of a Virus-Based Nanocoating

Self-assembling protein layers provide a "bottom-up" approach for precisely organizing functional elements at the nanoscale over a large solid surface area. The design of protein sheets with architecture and physical properties suitable for nanotechnological applications may be greatly facilitated by a thorough understanding of the principles that underlie their self-assembly and disassembly. In a previous study, the hexagonal lattice formed by the capsid protein (CA) of human immunodeficiency virus (HIV) was self-assembled as a monomolecular layer directly onto a solid substrate, and its mechanical properties and dynamics at equilibrium were analyzed by atomic force microscopy. Here, we use atomic force microscopy to analyze the kinetics of self-assembly of the planar CA lattice on a substrate and of its disassembly, either spontaneous or induced by materials fatigue. Both self-assembly and disassembly of the CA layer are cooperative reactions that proceed until a phase equilibrium is reached. Self-assembly requires a critical protein concentration and is initiated by formation of nucleation points on the substrate, followed by lattice growth and eventual merging of CA patches into a continuous monolayer. Disassembly of the CA layer showed hysteresis and appears to proceed only after large enough defects (nucleation points) are formed in the lattice, whose number is largely increased by inducing materials fatigue that depends on mechanical load and its frequency. Implications of the kinetic results obtained for a better understanding of self-assembly and disassembly of the HIV capsid and protein-based two-dimensional nanomaterials and the design of anti-HIV drugs targeting (dis)assembly and biocompatible nanocoatings are discussed.

Structural basis for biologically relevant mechanical stiffening of a virus capsid by cavity-creating or spacefilling mutations

Recent studies reveal that the mechanical properties of virus particles may have been shaped by evolution to facilitate virus survival. Manipulation of the mechanical behavior of virus capsids is leading to a better understanding of viral infection, and to the development of virus-based nanoparticles with improved mechanical properties for nanotechnological applications. In the minute virus of mice (MVM), deleterious mutations around capsid pores involved in infection-related translocation events invariably increased local mechanical stiffness and interfered with pore-associated dynamics. To provide atomic-resolution insights into biologically relevant changes in virus capsid mechanics, we have determined by X-ray crystallography the structural effects of deleterious, mechanically stiffening mutations around the capsid pores. Data show that the cavity-creating N170A mutation at the pore wall does not induce any dramatic structural change around the pores, but instead generates subtle rearrangements that propagate throughout the capsid, resulting in a more compact, less flexible structure. Analysis of the spacefilling L172W mutation revealed the same relationship between increased stiffness and compacted capsid structure. Implications for understanding connections between virus mechanics, structure, dynamics and infectivity, and for engineering modified virus-based nanoparticles, are discussed.

Structural Analysis of a Temperature-Induced Transition in a Viral Capsid Probed by HDX-MS

Icosahedral viral capsids are made of a large number of symmetrically organized protein subunits whose local movements can be essential for infection. In the capsid of the minute virus of mice, events required for infection that involve translocation of peptides through capsid pores are associated with a subtle conformational change. In vitro, this change can be reversibly induced by overcoming the energy barrier through mild heating of the capsid, but little is known about the capsid regions involved in the process. Here, we use hydrogen-deuterium exchange coupled to mass spectrometry to analyze the dynamics of the minute virus of mice capsid at increasing temperatures. Our results indicate that the transition associated with peptide translocation involves the structural rearrangement of regions distant from the capsid pores. These alterations are reflected in an increased dynamics of some secondary-structure elements in the capsid shell from which spikes protrude, and a decreased dynamics in the long intertwined loops that form the large capsid spikes. Thus, the translocation events through capsid pores involve a global conformational rearrangement of the capsid and a complex alteration of its equilibrium dynamics. This study additionally demonstrates the potential of hydrogen-deuterium exchange coupled to mass spectrometry to explore in detail temperature-dependent structural dynamics in large macromolecular protein assemblies. Most importantly, it paves the way for undertaking novel studies of the relationship between structure, dynamics, and biological function in virus particles and other large protein cages.

Amino Acid Side Chains Buried along Intersubunit Interfaces in a Viral Capsid Preserve Low Mechanical Stiffness Associated with Virus Infectivity

Single-molecule experimental techniques and theoretical approaches reveal that important aspects of virus biology can be understood in biomechanical terms at the nanoscale. A detailed knowledge of the relationship in virus capsids between small structural changes caused by single-point mutations and changes in mechanical properties may provide further physics-based insights into virus function; it may also facilitate the engineering of viral nanoparticles with improved mechanical behavior. Here, we used the minute virus of mice to undertake a systematic experimental study on the contribution to capsid stiffness of amino acid side chains at interprotein interfaces and the specific noncovalent interactions they establish. Selected side chains were individually truncated by introducing point mutations to alanine, and the effects on local and global capsid stiffness were determined using atomic force microscopy. The results revealed that, in the natural virus capsid, multiple, mostly hydrophobic, side chains buried along the interfaces between subunits preserve a comparatively low stiffness of most (S2 and S3) regions. Virtually no point mutation tested substantially reduced stiffness, whereas most mutations increased stiffness of the S2/S3 regions. This stiffening was invariably associated with reduced virus yields during cell infection. The experimental evidence suggests that a comparatively low stiffness at S3/S2 capsid regions may have been biologically selected because it facilitates capsid assembly, increasing infectious virus yields. This study demonstrated also that knowledge of individual amino acid side chains and biological pressures that determine the physical behavior of a protein nanoparticle may be used for engineering its mechanical properties.

Imaging and Quantitation of a Succession of Transient Intermediates Reveal the Reversible Self-Assembly Pathway of a Simple Icosahedral Virus Capsid

Understanding the fundamental principles underlying supramolecular self-assembly may facilitate many developments, from novel antivirals to self-organized nanodevices. Icosahedral virus particles constitute paradigms to study self-assembly using a combination of theory and experiment. Unfortunately, assembly pathways of the structurally simplest virus capsids, those more accessible to detailed theoretical studies, have been difficult to study experimentally. We have enabled the in vitro self-assembly under close to physiological conditions of one of the simplest virus particles known, the minute virus of mice (MVM) capsid, and experimentally analyzed its pathways of assembly and disassembly. A combination of electron microscopy and high-resolution atomic force microscopy was used to structurally characterize and quantify a succession of transient assembly and disassembly intermediates. The results provided an experiment-based model for the reversible self-assembly pathway of a most simple (T = 1) icosahedral protein shell. During assembly, trimeric capsid building blocks are sequentially added to the growing capsid, with pentamers of building blocks and incomplete capsids missing one building block as conspicuous intermediates. This study provided experimental verification of many features of self-assembly of a simple T = 1 capsid predicted by molecular dynamics simulations. It also demonstrated atomic force microscopy imaging and automated analysis, in combination with electron microscopy, as a powerful single-particle approach to characterize at high resolution and quantify transient intermediates during supramolecular self-assembly/disassembly reactions. Finally, the efficient in vitro self-assembly achieved for the oncotropic, cell nucleus-targeted MVM capsid may facilitate its development as a drug-encapsidating nanoparticle for anticancer targeted drug delivery.

Quantification and modification of the equilibrium dynamics and mechanics of a viral capsid lattice self-assembled as a protein nanocoating

Self-assembling, protein-based bidimensional lattices are being developed as functionalizable, highly ordered biocoatings for multiple applications in nanotechnology and nanomedicine. Unfortunately, protein assemblies are soft materials that may be too sensitive to mechanical disruption, and their intrinsic conformational dynamism may also influence their applicability. Thus, it may be critically important to characterize, understand and manipulate the mechanical features and dynamic behavior of protein assemblies in order to improve their suitability as nanomaterials. In this study, the capsid protein of the human immunodeficiency virus was induced to self-assemble as a continuous, single layered, ordered nanocoating onto an inorganic substrate. Atomic force microscopy (AFM) was used to quantify the mechanical behavior and the equilibrium dynamics ("breathing") of this virus-based, self-assembled protein lattice in close to physiological conditions. The results uniquely provided: (i) evidence that AFM can be used to directly visualize in real time and quantify slow breathing motions leading to dynamic disorder in protein nanocoatings and viral capsid lattices; (ii) characterization of the dynamics and mechanics of a viral capsid lattice and protein-based nanocoating, including flexibility, mechanical strength and remarkable self-repair capacity after mechanical damage; (iii) proof of principle that chemical additives can modify the dynamics and mechanics of a viral capsid lattice or protein-based nanocoating, and improve their applied potential by increasing their mechanical strength and elasticity. We discuss the implications for the development of mechanically resistant and compliant biocoatings precisely organized at the nanoscale, and of novel antiviral agents acting on fundamental physical properties of viruses.

Mechanical properties of β-catenin revealed by single-molecule experiments

β-catenin is a central component of the adaptor complex that links cadherins to the actin cytoskeleton in adherens junctions and thus, it is a good candidate to sense and transmit mechanical forces to trigger specific changes inside the cell. To fully understand its molecular physiology, we must first investigate its mechanical role in mechanotransduction within the cadherin system. We have studied the mechanical response of β-catenin to stretching using single-molecule force spectroscopy and molecular dynamics. Unlike most proteins analyzed to date, which have a fixed mechanical unfolding pathway, the β-catenin armadillo repeat region (ARM) displays low mechanostability and multiple alternative unfolding pathways that seem to be modulated by its unstructured termini. These results are supported by steered molecular dynamics simulations, which also predict its mechanical stabilization and unfolding pathway restrictions when the contiguous α-helix of the C-terminal unstructured region is included. Furthermore, simulations of the ARM/E-cadherin cytosolic tail complex emulating the most probable stress geometry occurring in vivo show a mechanical stabilization of the interaction whose magnitude correlates with the length of the stretch of the cadherin cytosolic tail that is in contact with the ARM region.

Common features at the start of the neurodegeneration cascade

A single-molecule study reveals that neurotoxic proteins share common structural features that may trigger neurodegeneration, thus identifying new targets for therapy and diagnosis.

Amyloidogenic neurodegenerative diseases are incurable conditions with high social impact that are typically caused by specific, largely disordered proteins. However, the underlying molecular mechanism remains elusive to established techniques. A favored hypothesis postulates that a critical conformational change in the monomer (an ideal therapeutic target) in these “neurotoxic proteins” triggers the pathogenic cascade. We use force spectroscopy and a novel methodology for unequivocal single-molecule identification to demonstrate a rich conformational polymorphism in the monomer of four representative neurotoxic proteins. This polymorphism strongly correlates with amyloidogenesis and neurotoxicity: it is absent in a fibrillization-incompetent mutant, favored by familial-disease mutations and diminished by a surprisingly promiscuous inhibitor of the critical monomeric β-conformational change, neurotoxicity, and neurodegeneration. Hence, we postulate that specific mechanostable conformers are the cause of these diseases, representing important new early-diagnostic and therapeutic targets. The demonstrated ability to inhibit the conformational heterogeneity of these proteins by a single pharmacological agent reveals common features in the monomer and suggests a common pathway to diagnose, prevent, halt, or reverse multiple neurodegenerative diseases.

Neurodegenerative diseases like Alzheimer's or Parkinson's are currently incurable. They are caused by different proteins that, under certain circumstances, aggregate and become toxic as we grow older, but the molecular events underlying this process remain unclear. The lack of a well-defined structure, and the tendency of these “neurotoxic proteins” to aggregate make them difficult to study using conventional techniques. Here, we use an established single-molecule manipulation technique combined with a new protein-engineering strategy to show that all these proteins can adopt a rich collection of structures (conformers) that includes a high proportion of mechanostable conformers, which are associated with toxicity and disease. We also find that a known drug can block the formation of these mechanostable structures in different neurotoxic proteins. We suggest that the most mechanostable conformers, or their precursors, may trigger the pathogenic cascade that results in toxicity. We thus propose that these mechanostable structures are ideal targets for early diagnosis, prevention, and treatment of these fatal diseases.

Unequivocal single-molecule force spectroscopy of intrinsically disordered proteins

Intrinsically disordered proteins (IDPs) are predicted to represent about one third of the eukaryotic proteome. The dynamic ensemble of conformations of this steadily growing class of proteins has remained hardly accessible for bulk biophysical techniques. However, single-molecule techniques provide a useful means of studying these proteins. Atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS) is one of such techniques, which has certain peculiarities that make it an important methodology to analyze the biophysical properties of IDPs. However, several drawbacks inherent to this technique can complicate such analysis. We have developed a protein engineering strategy to overcome these drawbacks such that an unambiguous mechanical analysis of proteins, including IDPs, can be readily performed. Using this approach, we have recently characterized the rich conformational polymorphism of several IDPs. Here, we describe a simple protocol to perform the nanomechanical analysis of IDPs using this new strategy, a procedure that in principle can also be followed for the nanomechanical analysis of any protein.

Nanomechanics of the cadherin ectodomain: "canalization" by Ca2+ binding results in a new mechanical element

Cadherins form a large family of calcium-dependent cell-cell adhesion receptors involved in development, morphogenesis, synaptogenesis, differentiation, and carcinogenesis through signal mechanotransduction using an adaptor complex that connects them to the cytoskeleton. However, the molecular mechanisms underlying mechanotransduction through cadherins remain unknown, although their extracellular region (ectodomain) is thought to be critical in this process. By single molecule force spectroscopy, molecular dynamics simulations, and protein engineering, here we have directly examined the nanomechanics of the C-cadherin ectodomain and found it to be strongly dependent on the calcium concentration. In the presence of calcium, the ectodomain extends through a defined ("canalized") pathway that involves two mechanical resistance elements: a mechanical clamp from the cadherin domains and a novel mechanostable component from the interdomain calcium-binding regions ("calcium rivet") that is abolished by magnesium replacement and in a mutant intended to impede calcium coordination. By contrast, in the absence of calcium, the mechanical response of the ectodomain becomes largely "decanalized" and destabilized. The cadherin ectodomain may therefore behave as a calcium-switched "mechanical antenna" with very different mechanical responses depending on calcium concentration (which would affect its mechanical integrity and force transmission capability). The versatile mechanical design of the cadherin ectodomain and its dependence on extracellular calcium facilitate a variety of mechanical responses that, we hypothesize, could influence the various adhesive properties mediated by cadherins in tissue morphogenesis, synaptic plasticity, and disease. Our work represents the first step toward the mechanical characterization of the cadherin system, opening the door to understanding the mechanical bases of its mechanotransduction.

Quasi-simultaneous imaging/pulling analysis of single polyprotein molecules by atomic force microscopy

Most of studies in protein nanomechanics have used the atomic force microscope (AFM) in its force-measuring mode on immobilized protein repeats (polyproteins) as single-molecule markers. Here, we add imaging capabilities to a standard, state-of-the-art AFM "puller" and integrate the most powerful programs of analysis available for both AFM modes. This unique instrument allows high-resolution, quasi-simultaneous imaging/force spectroscopy in aqueous solution. We demonstrate its capabilities using polyproteins of a model system (titin I27 domain). This tool should greatly facilitate the development of a much needed universal functionalization system for AFM, one that should allow better sample control and an improved efficiency of protein immobilization.

SCE analysis in peripheral blood lymphocytes of a group of filling station attendants

Petroleum derivatives constitute a complex mixture of chemicals which contain known genotoxicants. Thus, chronic occupational exposure to such derivatives may be considered to possess genotoxic risk. Service station attendants are workers exposed to petroleum derivatives and, as consequence, biomonitoring studies designed to evaluate the genetic risk are required. Here we present the data on sister chromatid exchange (SCE) frequencies, proliferation rate index (PRI) and cells with high frequency of SCE (HFC) from a group of 42 attendant workers. In addition, data from a group of 45 unexposed (controls) are presented. The results obtained indicate that there are no significant differences in SCE values between the exposed workers when compared to the non-exposed individuals. The only difference is found when smoking habit is considered, the smokers having significant increases in the SCE frequency. There is a slight increase in the HFC frequency of the occupationally exposed group but the difference did not attain statistical significance.

Temporary variations in chromosomal aberrations in a group of agricultural workers exposed to pesticides

The induction of chromosomal aberrations (CA) was studied in the peripheral lymphocytes of 29 male agricultural workers occupationally exposed to several pesticides. To investigate possible exposure-related changes in the frequency of CA, a longitudinal study has been conducted. Two blood samples were taken from each individual: one in a period of high exposure (spring-summer) and the other in a period of lower exposure (autumn-winter). Simultaneously, two matched control groups constituted by 29 and 24 healthy men, without indication of exposure to pesticides, were analysed. During the period of major exposure, the group of agricultural workers showed a significant increase in the frequency of CA, mainly of chromatid-type, when compared to the unexposed control group; nevertheless, this increase in the expression of CA was not found in the period of minor exposure. This finding could indicate that the frequency of CA is related to the intensity of the pesticide exposure, and that CA have a relatively short-life, recovering the control value a few months later. In addition to the cytogenetic analysis, biochemical and haemotological blood parameters were also analysed and no significant variations were detected.