Cryoatomic force microscopy of filamentous actin

Dual responsive specifically labelled carbogenic fluorescent nanodots for super resolution and electron microscopy

Due to their high biocompatibility and nontoxic nature, carbogenic fluorescent nanodots (FNDs) have already shown their application in bioimaging. However, their non-specific labeling has restricted their application in live cell super resolution microscopy (SRM). Here we introduce, for the first time, an orange emissive FND, specifically conjugated to the HeLa cell actin filament, for successful single molecule stochastic optical reconstruction microscopy (STORM) and super resolution radial fluctuation (SRRF) microscopy. The resolution obtained in SRRF (∼35 nm) was almost an order of magnitude less than the diffraction limited spot. Interestingly, in addition, the FND also showed electron microscope (EM) contrast inside the cell. We hope that this FND will not only replace some of the common dyes used for SRM, but will also be used as a dual responsive marker in correlative super resolution microscopy (CLEM).

Semiflexible Biopolymers in Bundled Arrangements

Bundles and networks of semiflexible biopolymers are key elements in cells, lending them mechanical integrity while also enabling dynamic functions. Networks have been the subject of many studies, revealing a variety of fundamental characteristics often determined via bulk measurements. Although bundles are equally important in biological systems, they have garnered much less scientific attention since they have to be probed on the mesoscopic scale. Here, we review theoretical as well as experimental approaches, which mainly employ the naturally occurring biopolymer actin, to highlight the principles behind these structures on the single bundle level.

Nanostructured self-assembly of inverted formin 2 (INF2) and F-actin-INF2 complexes revealed by atomic force microscopy

Self-organization of cytoskeletal proteins such as actin and tubulin into filaments and microtubules is frequently assisted by the proteins binding to them. Formins are regulatory proteins that nucleate the formation of new filaments and are essential for a wide range of cellular functions. The vertebrate inverted formin 2 (INF2) has both actin filament nucleating and severing/depolymerizing activities connected to its ability to encircle actin filaments. Using atomic force microscopy, we report that a formin homology 2 (FH2) domain-containing construct of INF2 (INF2-FH1-FH2-C or INF2-FFC) self-assembles into nanoscale ringlike oligomeric structures in the absence of actin filaments, demonstrating an inherent ability to reorganize from a dimeric to an oligomeric state. A construct lacking the C-terminal region (INF2-FH1-FH2 or INF2-FF) also oligomerizes, confirming the dominant role of FH2-mediated interactions. Moreover, INF2-FFC domains were observed to organize into ringlike structures around single actin filaments. This is the first demonstration that formin FH2 domains can self-assemble into oligomers in the absence of filaments and has important implications for observing unaveraged decoration and/or remodeling of filaments by actin binding proteins.

Microscopy basics and the study of actin-actin-binding protein interactions

Actin is a multifunctional eukaryotic protein with a globular monomer form that polymerizes into a thin, linear microfilament in cells. Through interactions with various actin-binding proteins (ABPs), actin plays an active role in many cellular processes, such as cell motility and structure. Microscopy techniques are powerful tools for determining the role and mechanism of actin-ABP interactions in these processes. In this article, we describe the basic concepts of fluorescent speckle microscopy, total internal reflection fluorescence microscopy, atomic force microscopy, and cryoelectron microscopy and review recent studies that utilize these techniques to visualize the binding of actin with ABPs.

Correlative nanoscale imaging of actin filaments and their complexes

Actin remodeling is an area of interest in biology in which correlative microscopy can bring a new way to analyze protein complexes at the nanoscale. Advances in EM, X-ray diffraction, fluorescence, and single molecule techniques have provided a wealth of information about the modulation of the F-actin structure and its regulation by actin binding proteins (ABPs). Yet, there are technological limitations of these approaches to achieving quantitative molecular level information on the structural and biophysical changes resulting from ABPs interaction with F-actin. Fundamental questions about the actin structure and dynamics and how these determine the function of ABPs remain unanswered. Specifically, how local and long-range structural and conformational changes result in ABPs induced remodeling of F-actin needs to be addressed at the single filament level. Advanced, sensitive and accurate experimental tools for detailed understanding of ABP-actin interactions are much needed. This article discusses the current understanding of nanoscale structural and mechanical modulation of F-actin by ABPs at the single filament level using several correlative microscopic techniques, focusing mainly on results obtained by Atomic Force Microscopy (AFM) analysis of ABP-actin complexes.

Heteroepitaxial streptavidin nanocrystals reveal critical role of proton "fingers" and subsurface atoms in determining adsorbed protein orientation

Characterization of noncovalent interactions between nanometer-sized structures, such as proteins, and solid surfaces is a subject of intense interest of late owing to the rapid development of numerous solid materials for medical and technological applications. Yet the rational design of these surfaces to promote the adsorption of specific nanoscale complexes is hindered by a lack of an understanding of the noncovalent interactions between nanostructures and solid surfaces. Here we take advantage of the unexpected observation of two-dimensional nanocrystals of streptavidin on muscovite mica to provide details of the streptavidin-mica interface. Analysis of atomic force microscopic images together with structural modeling identifies six positively charged residues whose terminal amine locations match the positions of the single atom-sized anionic cavities in the basal mica surface to within 1 Å. Moreover, we find that the streptavidin crystallites are oriented only along a single direction on this surface and not in either of three different directions as they must be if the protein interacted solely with the 3-fold symmetric basal surface atoms. Hence, this broken symmetry indicates that the terminal amine protons must also interact directly with the subsurface hydroxide atoms that line the bottom of these anionic cavities and generate only a single axis of symmetry. Thus, in total, these results reveal that subsurface atoms can have a significant influence on protein adsorption and orientation and identify the insertion of proton "fingers" as a means by which proteins may generally interact with solid surfaces.

3D nanometer images of biological fibers by directed motion of gold nanoparticles

Using near-infrared femtosecond pulses, we move single gold nanoparticles (AuNPs) along biological fibers, such as collagen and actin filaments. While the AuNP is sliding on the fiber, its trajectory is measured in three dimensions (3D) with nanometer resolution providing a high-resolution image of the fiber. Here, we systematically moved a single AuNP along nanometer-size collagen fibers and actin filament inside chinese hamster ovary K1 living cells, mapping their 3D topography with high fidelity.

Reconstructing adhesion structures in tissues by cryo-electron tomography of vitrified frozen sections

Cryo-electron tomography enables three-dimensional insights into the macromolecular architecture of cells in a close-to-life state. However, it is limited to thin specimens, <1.0 μm in thickness, typically restricted to the peripheral areas of intact eukaryotic cells. Analysis of tissue ultrastructure, on the other hand, requires physical sectioning approaches, preferably cryo-sectioning, following which electron tomography (ET) may be performed. Nevertheless, cryo-electron microscopy of vitrified sections is a demanding technique and typically cannot be used to examine thick sections, >80-100 nm, due to surface crevasses. Here, we explore the potential use of cryo-ET of vitrified frozen sections (VFSs) for imaging cell adhesions in chicken smooth muscle and mouse epithelial tissues. By investigating 300-400 nm thick sections, which are collected on the EM grid and re-vitrified, we resolved fine 3D structural details of the membrane-associated dense plaques and flanking caveoli in smooth muscle tissue, and desmosomal adhesions in stratified epithelium. Technically, this method offers a simple approach for reconstructing thick volumes of hydrated frozen sections.

Atomic force microscopy reveals drebrin induced remodeling of f-actin with subnanometer resolution

We show by high-resolution atomic force microscopy analysis that drebrin A (a major neuronal actin binding protein) induced F-actin structural and mechanical remodeling involves significant changes in helical twist and filament stiffness (+55% persistence length). These results provide evidence of a unique mechanical role of drebrin in the dendrites, contribute to current molecular-level understanding of the properties of the neuronal cytoskeleton, and reflect the role of biomechanics at the nanoscale, to modulate nanofilament-structure assemblies such as F-actin.

The human IgM pentamer is a mushroom-shaped molecule with a flexural bias

The textbook planar model of pentameric IgM, a potent activator of complement C1q, is based upon the crystallographic structure of IgG. Although widely accepted, key predictions of this model have not yet been directly confirmed, which is particularly important since IgG lacks a major Ig fold domain in its Fc region that is present in IgM. Here, we construct a homology-based structural model of the IgM pentamer using the recently obtained crystallographic structure of IgE Fc, which has this additional Ig domain, under the constraint that all of the cysteine residues known to form disulfide bridges both within each monomer and between monomers are bonded together. In contrast to the planar model, this model predicts a non-planar, mushroom-shaped complex, with the central portion formed by the C-terminal domains protruding out of the plane formed by the Fab domains. This unexpected conformation of IgM is, however, directly confirmed by cryo-atomic force microscopy of individual human IgM molecules. Further analysis of this model with free energy calculations of out-of-plane Fab domain rotations reveals a pronounced asymmetry favoring flexions toward the central protrusion. This bias, together with polyvalent attachment to cell surface antigen, would ensure that the IgM pentamer is oriented on the cell membrane with its C1q binding sites fully exposed to the solution, and thus provides a mechanistic explanation for the first steps of C1q activation by IgM.

A Review of Scanning Probe Microscopy Investigations of Liposome-DNA Complexes

Liposome-DNA complexes are one of the most promising systems for the protection and delivery of nucleic acids to combat neoplastic, viral, and genetic diseases. In addition, they are being used as models in the elucidation of many biological phenomena such as viral infection and transduction. In order to understand these phenomena and to realize the mechanism of nucleic acid transfer by liposome-DNA complexes, studies at the molecular level are required. To this end, scanning probe microscopy (SPM) is increasingly being used in the characterization of lipid layers, lipid aggregates, liposomes, and their complexes with nucleic acid molecules. The most attractive attributes of SPM are the potential to image samples with subnanometer spatial resolution under physiological conditions and provide information on their physical and mechanical properties. This review describes the application of scanning tunneling microscopy and atomic force microscopy, the two most commonly applied SPM techniques, in the characterisation of liposome-DNA complexes.

Application of calorimetry, sub-ambient atomic force microscopy and dynamic mechanical analysis to the study of frozen aqueous trehalose solutions

PURPOSE

Disaccharides such as trehalose are widely used as cryo-protectants to maintain the activity of proteinaceous drugs during freezing. One unresolved issue is the double transition that is observed very commonly in DSC experiments on disaccharide solutions in the frozen state; the assignment of these transitions remains disputed. Here we use calorimetry and two new techniques to shed light on the true nature of these transitions.

METHODS

Modulated Temperature DSC (MTDSC), cryo atomic force microscopy (AFM) and a novel DMA technique were used to study these transitions.

RESULTS

MTDSC identified the two transitions Tr1 and Tr2 at -35.4 and -27.9 degrees C respectively in the reversing heat flow signal, an exotherm and endotherm were observed in the non-reversing signal at circa -32 and -29 degrees C respectively. It is shown for the first time that AFM images can be obtained of a softening and melting sample without damaging it. A force modulation imaging technique showed a softening at Tr1 and a loss of ice crystals at Tr2. These observations were supported by the DMA results.

CONCLUSIONS

The results indicate Tr1 is associated with a glass transition while Tr2 is associated with the onset of loss of crystallinity.

Packing defects and the width of biopolymer bundles

The formation of bundles composed of actin filaments and cross-linking proteins is an essential process in the maintenance of the cells' cytoskeleton. It has also been recreated by in-vitro experiments, where actin networks are routinely produced to mimic and study the cellular structures. It has been observed that these bundles seem to have a well-defined width distribution, which has not been adequately described theoretically. We propose here that packing defects of the filaments, quenched and random, contribute an effective repulsion that counters the cross-linking adhesion energy and leads to a well-defined bundle width. This is a two-dimensional strain-field version of the classic Rayleigh instability of charged droplets.

Minimal F-actin cytoskeletal system for planar supported phospholipid bilayers

Preferential binding of F-actin to lipid bilayers containing ponticulin was investigated on both planar supported bilayers and on a cholesterol-based tethering system. The transmembrane protein ponticulin in Dictyostelium discoideum is known to provide a direct link between the actin cytoskeleton and the cell membrane ( Wuestehube, L. J. ; Luna, E. J. J. Cell Biol. 1987, 105, 1741- 1751 ). Purification of ponticulin has allowed an in vitro model of the F-actin cytoskeletal scaffold system to be formed and investigated by AFM, epi-fluorescence microscopy, surface plasmon resonance (SPR), and quartz crystal microbalance with dissipation (QCM-D). Single filament features of F-actin bound to the ponticulin containing lipid bilayer are shown by AFM to have a pitch of 37.3 +/- 1.1 nm and a filament height of 7.0 +/- 1.6 nm. The complementary techniques of QCM-D and SPR were used to obtain dissociation constants for the interaction of F-actin with ponticulin containing bilayers, giving 10.5 +/- 1.7 microM for a physisorbed bilayer and 10.8 +/- 3.6 microM for a tethered bilayer, respectively.

Chirality and equilibrium biopolymer bundles

We use continuum theory to show that chirality is a key thermodynamic control parameter for the aggregation of biopolymers: chirality produces a stable disperse phase of hexagonal bundles under moderately poor solvent conditions, as has been observed in in vitro studies of F actin [O. Pelletier et al., Phys. Rev. Lett. 91, 148102 (2003)]. The large characteristic radius of these chiral bundles is not determined by a mysterious long-range molecular interaction but by in-plane shear elastic stresses generated by the interplay between a chiral torque and an unusual, but universal, nonlinear gauge term in the strain tensor of ordered chains that is imposed by rotational invariance.

The role of high-resolution imaging in the evaluation of nanosystems for bioactive encapsulation and targeted nanotherapy

Nanotechnology has already started to significantly impact many industries and scientific fields including biotechnology, pharmaceutics, food technology and semiconductors. Nanotechnology-based tools and devices, including high-resolution imaging techniques, enable characterization and manipulation of materials at the nanolevel and further elucidate nanoscale phenomena and equip us with the ability to fabricate novel materials and structures. One of the most promising impacts of nanotechnology is in the area of nanotherapy. Employing nanosystems such as dendrimers, nanoliposomes, niosomes, nanotubes, emulsions and quantum dots, nanotherapy leads toward the concept of personalized medicine and the potential for early diagnoses coupled with efficient targeted therapy. The development of smart targeted nanocarriers that can deliver bioactives at a controlled rate directly to the designated cells and tissues will provide better efficacy and reduced side effects. Nanocarriers improve the solubility of bioactives and allow for the delivery of not only small-molecule drugs but also the delivery of nucleic acids and proteins. This review will focus on nanoscale bioactive delivery and targeting mechanisms and the role of high-resolution imaging techniques in the evaluation and development of nanocarriers.

Biochemical implications of a three-dimensional model of monomeric actin bound to magnesium-chelated ATP

Actin structure is of intense interest in biology due to its importance in cell function and motility mediated by the spatial and temporal regulation of actin monomer-filament interconversions in a wide range of developmental and disease states. Despite this interest, the structure of many functionally important actin forms has eluded high-resolution analysis. Due to the propensity of actin monomers to assemble into filaments structural analysis of Mg-bound actin monomers has proven difficult, whereas high-resolution structures of actin with a diverse array of ligands that preclude polymerization have been quite successful. In this work, we provide a high-resolution structural model of the Mg-ATP-actin monomer using a combination of computational methods and experimental footprinting data that we have previously published. The key conclusion of this study is that the structure of the nucleotide binding cleft defined by subdomains 2 and 4 is essentially closed, with specific contacts between two subdomains predicted by the data.

Molecular scale imaging of F-actin assemblies immobilized on a photopolymer surface

A photo-immobilization based process is presented for direct imaging of hierarchical assemblies of biopolymers using atomic force microscopy (AFM). The technique was used to investigate the phase behavior of F-actin aggregates as a function of concentration of the divalent cation Mg2+. The data provided direct experimental evidence of a coil-on-coil (braided) structure of F-actin bundles formed at high Mg2+ concentrations. At intermediate Mg2+ concentrations, the data showed the first images of the two-dimensional nematic rafts discovered by recent x-ray studies and theoretical treatments.

Architecture and high-resolution structure of Bacillus thuringiensis and Bacillus cereus spore coat surfaces

We have utilized atomic force microscopy (AFM) to visualize the native surface topography and ultrastructure of Bacillus thuringiensis and Bacillus cereus spores in water and in air. AFM was able to resolve the nanostructure of the exosporium and three distinctive classes of appendages. Removal of the exosporium exposed either a hexagonal honeycomb layer (B. thuringiensis) or a rodlet outer spore coat layer (B. cereus). Removal of the rodlet structure from B. cereus spores revealed an underlying honeycomb layer similar to that observed with B. thuringiensis spores. The periodicity of the rodlet structure on the outer spore coat of B. cereus was approximately 8 nm, and the length of the rodlets was limited to the cross-patched domain structure of this layer to approximately 200 nm. The lattice constant of the honeycomb structures was approximately 9 nm for both B. cereus and B. thuringiensis spores. Both honeycomb structures were composed of multiple, disoriented domains with distinct boundaries. Our results demonstrate that variations in storage and preparation procedures result in architectural changes in individual spore surfaces, which establish AFM as a useful tool for evaluation of preparation and processing "fingerprints" of bacterial spores. These results establish that high-resolution AFM has the capacity to reveal species-specific assembly and nanometer scale structure of spore surfaces. These species-specific spore surface structural variations are correlated with sequence divergences in a spore core structural protein SspE.

Atomic force microscopy of thermally treated myosin filaments

Heat-induced morphological change in myosin filaments was observed using atomic force microscope. The thickness of fixed native myosin filament was estimated to be 95 +/- 5 nm. When myosin filaments in 0.1 M NaCl at pH 6.0 were heated at 40, 55, and 70 degrees C for 10 min, the particulate structure appeared spirally on the surface of the filament at 40 degrees C, and the thickness of the filament was 75 +/- 10 nm. When myosin filaments were treated at 55 degrees C, several filaments were formed associated with side-by-side interaction through projected myosin heads to form a strand. The surface of the strand looked knobby. The thickness of thermally denatured filaments at 55 degrees C was 48 +/- 5 nm, and that of strands was about 80-110 nm, indicating the involvement of several filaments in a strand. The strands became to be rope-like at 70 degrees C, and the individual filaments in a strand were not distinguishable.

The high-resolution architecture and structural dynamics of Bacillus spores

The capability to image single microbial cell surfaces at nanometer scale under native conditions would profoundly impact mechanistic and structural studies of pathogenesis, immunobiology, environmental resistance, and biotransformation. Here, using in vitro atomic force microscopy, we have directly visualized high-resolution native structures of bacterial endospores, including the exosporium and spore coats of four Bacillus species in air and water environments. Our results demonstrate that the mechanisms of spore coat self-assembly are similar to those described for inorganic and macromolecular crystallization. The dimensions of individual Bacillus atrophaeus spores decrease reversibly by 12% in response to a change in the environment from fully hydrated to air-dried state, establishing that the dormant spore is a dynamic physical structure. The interspecies distributions of spore length and width were determined for four species of Bacillus spores in water and air environments. The dimensions of individual spores differ significantly depending upon species, growth regimes, and environmental conditions. These findings may be useful in the reconstruction of environmental and physiological conditions during spore formation and for modeling the inhalation and dispersal of spores. This study provides a direct insight into molecular architecture and structural variability of bacterial endospores as a function of spatial and developmental organizational scales.

Chemical treatment of mica for atomic force microscopy can affect biological sample conformation

An important aspect in the preparation of substrate materials to use in atomic force microscopy lies in the question of interactions introduced by treatments designed to immobilize the sample over the substrate. Here we used a mica substrate that was chemically modified with cationic nickel to immobilize actin filaments (F-actin). Chemical modification could be followed quantitatively by measuring the interaction force between the scanning tip and the mica surface. This approach allowed us to observe polymeric F-actin in a structure that resembles an actin gel. It also improved sample throughput and conferred sample stability as well as repeatability from run to run.

An overview of the biophysical applications of atomic force microscopy

The potentialities of the atomic force microscopy (AFM) make it a tool of undeniable value for the study of biologically relevant samples. AFM is progressively becoming a usual benchtop technique. In average, more than one paper is published every day on AFM biological applications. This figure overcomes materials science applications, showing that 17 years after its invention, AFM has completely crossed the limits of its traditional areas of application. Its potential to image the structure of biomolecules or bio-surfaces with molecular or even sub-molecular resolution, study samples under physiological conditions (which allows to follow in situ the real time dynamics of some biological events), measure local chemical, physical and mechanical properties of a sample and manipulate single molecules should be emphasized.

MgATP-induced conformational changes in a single myosin molecule observed by atomic force microscopy: periodicity of substructures in myosin rods

This paper discusses the conformational changes in a single myosin molecule directly observed using atomic force microscopy (AFM). The myosin molecules were pretreated in rigor solutions without MgATP or in relaxed solutions with various concentrations of MgATP. The images of these molecules were obtained using a tapping mode AFM. The results indicate that the orientation of the myosin's heads and tail strongly depend on the MgATP concentration. Without using MgATP, almost all of the myosin molecules are in the extended form; however, when MgATP is used, the molecules bend according to the level of MgATP concentration. The mean-square end-to-end distance of the myosin molecules is significantly shorter with p[MgATP] = 4 than with p[MgATP] = 6. The rod region did not show the same level of intensity along their length in the extended form. The rods exhibited clusters of discontinuity, which were identified as substructures. The size of these substructures change at intervals that are multiples of 14.3-14.5 nm, which reflects the periodicity of the alpha-helical coiled coils. The substructure clusters also correspond to the myosin crossbridge spacing in muscles (14.3 or 43 nm). These results suggest that the myosin's head bends in conjunction with the bending or tilting in the helical substructures. Conformational changes of the myosin molecule induced by MgATP seem to mimic the molecular motions in a muscle's force generation process.

Direct observations of freeze-etching processes of ice-embedded biomembranes by atomic force microscopy

We have fabricated a cryogenic atomic force microscope that is designed for structural investigation of freeze-fractured biological specimens. The apparatus is operated in liquid nitrogen gas at atmospheric pressure. Freeze-fracturing, freeze-etching and subsequent imaging are carried out in the same chamber, so that the surface topography of a fractured plane is easily visualized without ice contamination. A controlled superficial sublimation of volatile molecules allows us to obtain three-dimensional views of ultrastructures of biological membranes.

Probing the machinery of intracellular trafficking with the atomic force microscope

Atomic force microscopy has emerged as a powerful tool for characterizing single biological macromolecules, macromolecular assemblies, and whole cells in aqueous buffer, in real time, and at molecular-scale spatial and force resolution. Many of the central elements of intracellular transport are tens to hundreds of nanometers in size and highly dynamic. Thus, atomic force microscopy provides a valuable means of addressing questions of structure and mechanism in intracellular transport. We begin this review of recent efforts to apply atomic force microscopy to problems in intracellular transport by discussing the technical principles behind atomic force microscopy. We then turn to three specific areas in which atomic force microscopy has been applied to problems with direct implications for intracellular trafficking: cytoskeletal structure and dynamics, vesicular transport, and receptor-ligand interactions. In each case, we discuss studies which use both intact cellular elements and reconstituted models. While many technical challenges remain, these studies point to several areas where atomic force microscopy can be used to provide valuable insight into intracellular transport at exquisite spatial and energetic resolution.

Micromechanics and ultrastructure of actin filament networks crosslinked by human fascin: a comparison with alpha-actinin

Fascin is an actin crosslinking protein that organizes actin filaments into tightly packed bundles believed to mediate the formation of cellular protrusions and to provide mechanical support to stress fibers. Using quantitative rheological methods, we studied the evolution of the mechanical behavior of filamentous actin (F-actin) networks assembled in the presence of human fascin. The mechanical properties of F-actin/fascin networks were directly compared with those formed by alpha-actinin, a prototypical actin filament crosslinking/bundling protein. Gelation of F-actin networks in the presence of fascin (fascin to actin molar ratio >1:50) exhibits a non-monotonic behavior characterized by a burst of elasticity followed by a slow decline over time. Moreover, the rate of gelation shows a non-monotonic dependence on fascin concentration. In contrast, alpha-actinin increased the F-actin network elasticity and the rate of gelation monotonically. Time-resolved multiple-angle light scattering and confocal and electron microscopies suggest that this unique behavior is due to competition between fascin-mediated crosslinking and side-branching of actin filaments and bundles, on the one hand, and delayed actin assembly and enhanced network micro-heterogeneity, on the other hand. The behavior of F-actin/fascin solutions under oscillatory shear of different frequencies, which mimics the cell's response to forces applied at different rates, supports a key role for fascin-mediated F-actin side-branching. F-actin side-branching promotes the formation of interconnected networks, which completely inhibits the motion of actin filaments and bundles. Our results therefore show that despite sharing seemingly similar F-actin crosslinking/bundling activity, alpha-actinin and fascin display completely different mechanical behavior. When viewed in the context of recent microrheological measurements in living cells, these results provide the basis for understanding the synergy between multiple crosslinking proteins, and in particular the complementary mechanical roles of fascin and alpha-actinin in vivo.