The viscoelasticity of entangled actin networks: the influence of defects and modulation by talin and vinculin

Simulations of dynamically cross-linked actin networks: Morphology, rheology, and hydrodynamic interactions

Cross-linked actin networks are the primary component of the cell cytoskeleton and have been the subject of numerous experimental and modeling studies. While these studies have demonstrated that the networks are viscoelastic materials, evolving from elastic solids on short timescales to viscous fluids on long ones, questions remain about the duration of each asymptotic regime, the role of the surrounding fluid, and the behavior of the networks on intermediate timescales. Here we perform detailed simulations of passively cross-linked non-Brownian actin networks to quantify the principal timescales involved in the elastoviscous behavior, study the role of nonlocal hydrodynamic interactions, and parameterize continuum models from discrete stochastic simulations. To do this, we extend our recent computational framework for semiflexible filament suspensions, which is based on nonlocal slender body theory, to actin networks with dynamic cross linkers and finite filament lifetime. We introduce a model where the cross linkers are elastic springs with sticky ends stochastically binding to and unbinding from the elastic filaments, which randomly turn over at a characteristic rate. We show that, depending on the parameters, the network evolves to a steady state morphology that is either an isotropic actin mesh or a mesh with embedded actin bundles. For different degrees of bundling, we numerically apply small-amplitude oscillatory shear deformation to extract three timescales from networks of hundreds of filaments and cross linkers. We analyze the dependence of these timescales, which range from the order of hundredths of a second to the actin turnover time of several seconds, on the dynamic nature of the links, solvent viscosity, and filament bending stiffness. We show that the network is mostly elastic on the short time scale, with the elasticity coming mainly from the cross links, and viscous on the long time scale, with the effective viscosity originating primarily from stretching and breaking of the cross links. We show that the influence of nonlocal hydrodynamic interactions depends on the network morphology: for homogeneous meshworks, nonlocal hydrodynamics gives only a small correction to the viscous behavior, but for bundled networks it both hinders the formation of bundles and significantly lowers the resistance to shear once bundles are formed. We use our results to construct three-timescale generalized Maxwell models of the networks.

Mimicking Active Biopolymer Networks with a Synthetic Hydrogel

Stiffening due to internal stress generation is of paramount importance in living systems and is the foundation for many biomechanical processes. For example, cells stiffen their surrounding matrix by pulling on collagen and fibrin fibers. At the subcellular level, molecular motors prompt fluidization and actively stiffen the cytoskeleton by sliding polar actin filaments in opposite directions. Here, we demonstrate that chemical cross-linking of a fibrous matrix of synthetic semiflexible polymers with thermoresponsive poly( N-isopropylacrylamide) (PNIPAM) produces internal stress by induction of a coil-to-globule transition upon crossing the lower critical solution temperature of PNIPAM, resulting in a macroscopic stiffening response that spans more than 3 orders of magnitude in modulus. The forces generated through collapsing PNIPAM are sufficient to drive a fluid material into a stiff gel within a few seconds. Moreover, rigidified networks dramatically stiffen in response to applied shear stress featuring power law rheology with exponents that match those of reconstituted collagen and actomyosin networks prestressed by molecular motors. This concept holds potential for the rational design of synthetic materials that are fluid at room temperature and rapidly rigidify at body temperature to form hydrogels mechanically and structurally akin to cells and tissues.

Two-Photon Correlation Spectroscopy in Single Dendritic Spines Reveals Fast Actin Filament Reorganization during Activity-Dependent Growth

Two-photon fluorescence correlation spectroscopy (2P-FCS) within single dendritic spines of living hippocampal pyramidal neurons was used to resolve various subpopulations of mobile F-actin during activity-dependent structural changes such as potentiation induced spine head growth. Two major classes of mobile F-actin were discovered: very dynamic and about a hundred times less dynamic F-actin. Spine head enlargement upon application of Tetraethylammonium (TEA), a protocol previously used for the chemical induction of long-term potentiation (cLTP) strictly correlated to changes in the dynamics and filament numbers in the different actin filament fractions. Our observations suggest that spine enlargement is governed by a mechanism in which longer filaments are first cut into smaller filaments that cooperate with the second, increasingly dynamic shorter actin filament population to quickly reorganize and expand the actin cytoskeleton within the spine head. This process would allow a fast and efficient spine head enlargement using a major fraction of the actin filament population that was already present before spine head growth.

Building an artificial actin cortex on microscopic pillar arrays

Eukaryotic cells obtain their morphology and mechanical strength from the cytoskeleton and in particular from the cross-linked actin network that branches throughout the whole cell. This actin cortex lies like a quasi-two-dimensional (2D) biopolymer network just below the cell membrane, to which it is attached. In the quest for building an artificial cell, one needs to make a biomimetic model of the actin cortex and combine this in a bottom-up approach with other "synthetic" components. Here, we describe a reconstitution method for such an artificial actin cortex, which is freely suspended on top of a regular array of pillars. By this immobilization method, the actin network is only attached to a surface at discrete points and can fluctuate freely in between. By discussing the method to make the micropillars and the way to reconstitute a quasi-2D actin network on top, we show how one can study an isolated, reconstituted part of a cell. This allows the study of fundamental interaction mechanisms of actin networks, providing handles to design a functional actin cortex in an artificial cell.

Form-finding model shows how cytoskeleton network stiffness is realized

In eukaryotic cells the actin-cytoskeletal network provides stiffness and the driving force that contributes to changes in cell shape and cell motility, but the elastic behavior of this network is not well understood. In this paper a two dimensional form-finding model is proposed to investigate the elasticity of the actin filament network. Utilizing an initially random array of actin filaments and actin-cross-linking proteins the form-finding model iterates until the random array is brought into a stable equilibrium configuration. With some care given to actin filament density and length, distance between host sites for cross-linkers, and overall domain size the resulting configurations from the form-finding model are found to be topologically similar to cytoskeletal networks in real cells. The resulting network may then be mechanically exercised to explore how the actin filaments deform and align under load and the sensitivity of the network’s stiffness to actin filament density, length, etc. Results of the model are consistent with the experimental literature, e.g. actin filaments tend to re-orient in the direction of stretching; and the filament relative density, filament length, and actin-cross-linking protein’s relative density, control the actin-network stiffness. The model provides a ready means of extension to more complicated domains and a three-dimensional form-finding model is under development as well as models studying the formation of actin bundles.

Local Viscoelasticity of Biopolymer Solutions

We describe a new, high-resolution technique for determining the local viscoelastic response of polymer gels on a micrometer scale. This is done by monitoring thermal fluctuations of embedded probe particles. We derive the relationship between the amplitude of fluctuations and the low-frequency storage modulus G′, as well as the relationship between the fluctuation power spectrum, measured between 0.1 Hz and 25kHz, and the complex shear modulus G((ω). For both, semiflexible F-actin solutions and flexible polyacrylamide (PAAm) gels we observe high-frequency power-law dependence in the spectra, which reflects the behavior of the shear modulus. However, we observe distinctly different scaling exponents for G((ω) in F-actin and PAAm gels—presumably due to the semiflexible nature of the actin filaments.

Transient anomalous subdiffusion: effects of specific and nonspecific probe binding with actin gels

When signaling molecules diffuse through the cytosol, they encounter a wide variety of obstacles that hinder their mobility in space and time. Some of those factors include, but are not limited to, interactions with mobile and immobile targets or obstacles. Besides finding a crowded environment inside the cell, macromolecules assemble into molecular complexes that drive specific biological functions adding additional complexity to their diffusion. Thus, simple models of diffusion often fail to explain mobility through the cell interior, and new approaches are needed. Here we used fluorescent correlation spectroscopy to measure diffusion of three molecules of similar size with different surface properties diffusing in actin gels. The fluorescent probes were (a) quantum dots, (b) yellow-green fluorescent spheres, and (c) the beta isoform of Ca(2+) calmodulin-dependent protein kinase II tagged with green fluorescent protein. We compared various models for fitting the autocorrelation function (ACF) including single component, two-component, and anomalous diffusion. The two-component and anomalous diffusion models were superior and were largely indistinguishable based on a goodness of fit criteria. To better resolve differences between these two models, we modified the ACF to observe temporal variations in diffusion. We found in both simulated and experimental data a transient anomalous subdiffusion between two freely diffusing regimes produced by binding interactions of the diffusive tracers with actin gels.

Viscoelasticity of entangled actin networks studied by long-pulse magnetic bead microrheometry

We studied the viscoelastic response of entangled actin networks using embedded microbeads driven by force pulses with amplitudes in the range from 3 to 120 pN and durations up to 60 s. We distinguished three regimes in the time dependence of the compliance J(t) of the network. These were characterized by specific power laws J(t) approximately t(alpha)(i) (i=1, 2, 3). In the short-time regime (i=1), we observed the exponent alpha1 approximately 0.75. In the long-time regime (i=3), we find that alpha3 approximately 1. For the intermediate-time interval (i=2), we observed a novel dynamic regime: for all actin concentrations and all applied forces, it was characterized by the exponent alpha3 approximately 0.5. In both regimes i=2 and i=3, the compliance depended upon the actin concentration c, such as J approximately c(-gamma)(i) with gamma2 approximately 1.1 and gamma 3 approximately 1.4. Using these results, we calculated the shear modulus in the frequency domain and found that the intermediate-time regime in the t domain corresponds to its plateau behavior.

Mechanical properties of Xenopus egg cytoplasmic extracts

Cytoplasmic extracts prepared from Xenopus laevis eggs are used for the reconstitution of a wide range of processes in cell biology, and offer a unique environment in which to investigate the role of cytoplasmic mechanics without the complication of preorganized cellular structures. As a step toward understanding the mechanical properties of this system, we have characterized the rheology of crude interphase extracts. At macroscopic length scales, the extract forms a soft viscoelastic solid. Using a conventional mechanical rheometer, we measure the elastic modulus to be in the range of 2-10 Pa, and loss modulus in the range of 0.5-5 Pa. Using pharmacological and immunological disruption methods, we establish that actin filaments and microtubules cooperate to give mechanical strength, whereas the intermediate filament cytokeratin does not contribute to viscoelasticity. At microscopic length scales smaller than the average network mesh size, the response is predominantly viscous. We use multiple particle tracking methods to measure the thermal fluctuations of 1 microm embedded tracer particles, and measure the viscosity to be approximately 20 mPa-s. We explore the impact of rheology on actin-dependent cytoplasmic contraction, and find that although microtubules modulate contractile forces in vitro, their interactions are not purely mechanical.

The effect of alpha-actinin on the length distribution of F-actin

Actin filament length distribution in cells is often regulated to fit specific tasks. In comparison to the well-studied regulation of the average filament length (e.g., using capping proteins), controlling the width of the distribution is less well understood. We utilize two complementary methods to measure the effect of alpha-actinin on the width of the distribution of lengths of F-actin in vitro. Analyzing transmission electron micrographs shows that crosslinking by alpha-actinin reduces the width of the length distribution of F-actin, decreasing the coefficient of variation by two- to threefold. Analysis of fluorescence data from depolymerization assays confirms this observation. We suggest a mechanistic molecular model in which a local (weak) stabilization of crosslinked monomers in the filament is the physical origin of the decrease in the variance of lengths. Although alpha-actinin is known to bind reversibly to F-actin, our model shows that even weak binding can produce this effect, and that in fact it persists throughout a wide range of binding strengths.

Scaling of F-actin network rheology to probe single filament elasticity and dynamics

The linear and nonlinear viscoelastic response of networks of cross-linked and bundled cytoskeletal filaments demonstrates remarkable scaling with both frequency and applied prestress, which helps elucidate the origins of the viscoelasticity. The frequency dependence of the shear modulus reflects the underlying single-filament relaxation dynamics for 0.1-10 rad/sec. Moreover, the nonlinear strain stiffening of such networks exhibits a universal form as a function of prestress; this is quantitatively explained by the full force-extension relation of single semiflexible filaments.

Relating microstructure to rheology of a bundled and cross-linked F-actin network in vitro

The organization of individual actin filaments into higher-order structures is controlled by actin-binding proteins (ABPs). Although the biological significance of the ABPs is well documented, little is known about how bundling and cross-linking quantitatively affect the microstructure and mechanical properties of actin networks. Here we quantify the effect of the ABP scruin on actin networks by using imaging techniques, cosedimentation assays, multiparticle tracking, and bulk rheology. We show how the structure of the actin network is modified as the scruin concentration is varied, and we correlate these structural changes to variations in the resultant network elasticity.

Elastic behavior of cross-linked and bundled actin networks

Networks of cross-linked and bundled actin filaments are ubiquitous in the cellular cytoskeleton, but their elasticity remains poorly understood. We show that these networks exhibit exceptional elastic behavior that reflects the mechanical properties of individual filaments. There are two distinct regimes of elasticity, one reflecting bending of single filaments and a second reflecting stretching of entropic fluctuations of filament length. The mechanical stiffness can vary by several decades with small changes in cross-link concentration, and can increase markedly upon application of external stress. We parameterize the full range of behavior in a state diagram and elucidate its origin with a robust model.

Microrheometry of semiflexible actin networks through enforced single-filament reptation: frictional coupling and heterogeneities in entangled networks

Magnetic tweezers are applied to study the enforced motion of single actin filaments in entangled actin networks to gain insight into friction-mediated entanglement in semiflexible macromolecular networks. Magnetic beads are coupled to one chain end of test filaments, which are pulled by 5 to 20 pN force pulses through entangled solutions of nonlabeled actin, the test filaments thus acting as linear force probes of the network. The transient filament motion is analyzed by microfluorescence, and the deflection-versus-time curves of the beads are evaluated in terms of a mechanical equivalent circuit to determine viscoelastic parameters, which are then interpreted in terms of viscoelastic moduli of the network. We demonstrate that the frictional coefficient characterizing the hydrodynamic coupling of the filaments to the surrounding network is much higher than predicted by the tube model, suggesting that friction-mediated interfilament coupling plays an important role in the entanglement of non-cross-linked actin networks. Furthermore, the local tube width along the filament contour (measured in terms of the root-mean-square displacement characterizing the lateral Brownian motion of the test filament) reveals strong fluctuations that can lead to transient local pinching of filaments.

Tube diameter in tightly entangled solutions of semiflexible polymers

A statistical mechanical treatment is given of the confinement of a wormlike polymer in an entangled solution to a tube, yielding quantitative predictions for the average tube diameter D(e) and macroscopic plateau modulus G, in the tightly entangled regime in which D(e) is much less than the persistence length L(p). Three approaches are pursued. A self-consistent binary collision approximation, which explicitly describes the topological constraints imposed by neighboring chains, yields predictions consistent with the scaling laws D(e)proportional to rho(-3/5) and G proportional to rho(7/5) proposed previously, where rho is the contour length per unit volume. An effective medium approximation, which treats the network as a continuum with a modulus G, instead yields D(e) proportional to rho(-1/3) and G proportional to rho(4/3), which is found to be the correct scaling in the limit rhoL(2)(p)>>1. An elastic network approximation treats the displacement of a test chain as the sum of a collective displacement of the network, which is treated as a continuum, plus a local displacement, which is treated in a binary collision approximation. Predictions are compared to measurements of both D(e) and G in actin protein filament (F-actin) solutions.

Binding of tropomyosin-troponin to actin increases filament bending stiffness

Rheologic measurements show that the association of tropomyosin-troponin with actin filaments is responsible for the reduction of the internal chain dynamic and increase in the mechanical rigidity of actin filaments. Basing calculations on the linear relation between the plateau modulus, G(N)('), and bending modulus, kappa, I find that tropomyosin-troponin at r(AT) = 7 increases actin filament stiffness by approximately 50%. This is confirmed by dynamic light scattering. Further increases are observed at rising F-actin and constant tropomyosin-troponin concentrations. Tropomyosin-troponin also delays actin assembly and subsequent network formation and increases filament stiffness over time.

Microrheometry underestimates the values of the viscoelastic moduli in measurements on F-actin solutions compared to macrorheometry

We present a systematic comparison of microrheological and macrorheological measurements of the viscoelastic storage and loss moduli, G'(f) and G"(f), respectively, of solutions of the semiflexible biopolymer F-actin. Using magnetic tweezers microrheometry and rotating disk macrorheometry, we show that microscopic values for G'(f) and G"(f) are significantly smaller than macroscopic results over the frequency range f = 0.004-4 Hz, whereas the qualitative shape of the spectra is similar. These findings confirm recent theoretical predictions [A. C. Maggs, Phys. Rev. E 57, 2091 (1998)]. The discrepancy affects not only absolute values of G'(f) and G"(f): although microscopic and macroscopic plateau regime are found in the same frequency range, the two methods yield different values for the entanglement time which determines the high-frequency end of the plateau. By investigating F-actin solutions of different mean filament lengths, we show that microscopic and macroscopic G'(f) and G"(f) converge, if the probe particle used in microrheometry becomes large compared to the length of actin filaments.

Thiol oxidation of actin produces dimers that enhance the elasticity of the F-actin network

Slow oxidation of sulfhydryls, forming covalently linked actin dimers and higher oligomers, accounts for increases in the shear elasticity of purified actin observed after aging. Disulfide-bonded actin dimers are incorporated into F-actin during polymerization and generate cross-links between actin filaments. The large gel strength of oxidized actin (>100 Pa for 1 mg/ml) in the absence of cross-linking proteins falls to within the theoretically predicted order of magnitude for uncross-linked actin filament networks (1 Pa) with the addition of sufficient concentrations of reducing agents such as 5 mM dithiothreitol or 10 mM beta-mercaptoethanol. As little as 1 gelsolin/1000 actin subunits also lowers the high storage modulus of oxidized actin. The effects of gelsolin may be both to increase filament number as it severs F-actin and to cover the barbed end of an actin filament, which otherwise might cross-link to the side of another filament via an actin dimer. These new findings may explain why previous studies of actin rheology report a wide range of values when purified actin is polymerized without added regulatory proteins.

The effect of intact talin and talin tail fragment on actin filament dynamics and structure depends on pH and ionic strength

We employed quasi-elastic light scattering and electron microscopy to investigate the influence of intact talin and talin tail fragment on actin filament dynamics and network structure. Using these methods, we confirm previous reports that intact talin induces cross-linking as well as filament shortening on actin networks. We now show that the effect of intact talin as well as talin tail fragment on actin networks is controlled by pH and ionic strength. At pH 7.5, actin filament dynamics in the presence of intact talin and talin tail fragment are characterized by a rapid decay of the dynamic structure factor and by a square root power law for the stretched exponential decay which is in contrast with the theory for pure actin solutions. At pH 6 and low ionic strength, intact talin cross-links actin filaments more tightly than talin tail fragment. Talin head fragment showed no effect on actin networks, indicating that the actin binding sites reside probably exclusively within the tail domain.

Mechanical properties of actin filament networks depend on preparation, polymerization conditions, and storage of actin monomers

This study investigates possible sources for the variance of more than two orders of magnitude in the published values for the shear moduli of purified actin filaments. Two types of forced oscillatory rheometers used in some of our previous work agree within a factor of three for identical samples. Polymers assembled in EGTA and Mg2+ from fresh, gel-filtered ATP-actin at 1 mg/ml typically have an elastic storage modulus (G') of approximately 1 Pa at a deformation frequency of 0.1-1 Hz. G' is slightly higher when actin is polymerized in KCl with Ca2+ and Mg2+. Gel filtration removes minor contaminants from actin but has little effect on G' for most preparations of actin from acetone powder. Storage of actin monomers without frequent changes of buffer containing fresh ATP and dithiothreitol can result in changes that increase the G' of filaments by more than a factor of 10. Frozen storage can preserve the properties of monomeric actin, but care is necessary to prevent protein denaturation or aggregation due to freezing or thawing.

Interaction of cytoskeletal proteins with membrane lipids

Rapid and significant progress has been made in understanding lipid/protein interactions involving cytoskeletal components and the plasma membrane. Covalent and noncovalent lipid modifications of cytoskeletal proteins mediate their interaction with lipid bilayers. The application of biophysical techniques such as differential scanning colorimetry, neutron reflection, electron spin resonance, CD spectroscopy, nuclear magnetic resonance, and hydrophobic photolabeling, allow various folding stages of proteins during electrostatic adsorption and hydrophobic insertion into lipid bilayers to be analyzed. Reconstitution of proteins into planar lipid films and liposomes help to understand the architecture of biological interfaces. During signaling events at plasma membrane interfaces, lipids are important for the regulation of catalytic protein functions. Protein/lipid interactions occur selectively and with a high degree of specificity and thus have to be considered as physiologically relevant processes with gaining impact on cell functions.

Examining F-actin interaction with intact talin and talin head and tail fragment using static and dynamic light scattering

We examined the binding kinetics of intact talin and talin head and tail fragment with F-actin at pH 7.0 and at low ionic strength. We observed by a transient kinetic method a fast followed by a slower binding process for intact talin and talin tail fragment with filamentous actin. The latter can be attributed to F-actin cross-linking and/or bundling, which was observed in cosedimentation assays as well as by low shear viscometry and electron microscopy [Zhang, J., Robson, R. M., Schmidt, J. M. & Stromer, M. H. (1996) Biochem. Biophys. Res. Commun. 218, 530-537]. This finding is supported by dynamic light scattering measurements, indicating changes in internal actin filament dynamics due to cross-linking/bundling events with intact talin and talin tail fragment. No binding of the talin head fragment with F-actin was detected by either method.

Viscoelasticity of actin-gelsolin networks in the presence of filamin

Cross-linking of actin filaments by filamin by means of frequency-dependent rheology yields an increase in the filament's elasticity and stiffness. Higher cross-linker (filamin) ratios are required for mean actin-filament lengths of 5-6 microm than for random-length distribution of actin filaments. The loss modulus (i.e. the viscous portion) in the region of the internal-chain dynamics [G"(omega) approximately omega(alpha)] is influenced by the cross-linking of filaments, and with an increasing molar ratio of filamin/actin a reduction of alpha is observed. Rheological measurements reveal that actin networks are already formed at the polymerizing stage at a molar ratio of filamin/actin of less than 1:100, and electron micrographs show phase separation of actin/filament networks of low density and of actin/filament bundles.

Bundling of actin filaments by elongation factor 1 alpha inhibits polymerization at filament ends

Elongation factor 1 alpha (EF1 alpha) is an abundant protein that binds aminoacyl-tRNA and ribosomes in a GTP-dependent manner. EF1 alpha also interacts with the cytoskeleton by binding and bundling actin filaments and microtubules. In this report, the effect of purified EF1 alpha on actin polymerization and depolymerization is examined. At molar ratios present in the cytosol, EF1 alpha significantly blocks both polymerization and depolymerization of actin filaments and increases the final extent of actin polymer, while at high molar ratios to actin, EF1 alpha nucleates actin polymerization. Although EF1 alpha binds actin monomer, this monomer-binding activity does not explain the effects of EF1 alpha on actin polymerization at physiological molar ratios. The mechanism for the inhibition of polymerization is related to the actin-bundling activity of EF1 alpha. Both ends of the actin filament are inhibited for polymerization and both bundling and the inhibition of actin polymerization are affected by pH within the same physiological range; at high pH both bundling and the inhibition of actin polymerization are reduced. Additionally, it is seen that the binding of aminoacyl-tRNA to EF1 alpha releases EF1 alpha's inhibiting effect on actin polymerization. These data demonstrate that EF1 alpha can alter the assembly of F-actin, a filamentous scaffold on which non-membrane-associated protein translation may be occurring in vivo.

F-actin, a model polymer for semiflexible chains in dilute, semidilute, and liquid crystalline solutions

Single actin filaments were analyzed in solutions ranging from dilute (0.2 microgram/ml), where filaments interact only with solvent, to concentrations (4.0 mg/ml) at which F-actin forms a nematic phase. A persistence length of approximately 1.8 microns and an average length of approximately 22 microns (Kaufmann et al., 1992) identify actin as a model for studying the dynamics of semiflexible polymers. In dilute solutions the filaments exhibit thermal bending undulations in addition to diffusive motion. At higher semidilute concentrations (1.4 mg/ml) three-dimensional reconstructions of confocal images of fluorescently labeled filaments in a matrix of unlabeled F-actin reveal steric interactions between filaments, which account for the viscoelastic behavior of these solutions. The restricted undulations of these labeled chains reveal the virtual tube formed around a filament by the surrounding actin. The average tube diameter <a> scales with monomer concentration c as <a> varies; is directly proportional to c-(0.5 +/- 0.15). The diffusion of filaments in semidilute solutions (c = (0.1-2.0) mg/ml) is dominated by diffusion along the filament contour (reptation), and constraint release by remodeling of the surrounding filaments is rare. The self-diffusion coefficient D parallel along the tube decreases linearly with the chain length for semidilute solutions. For concentrations > 2.5 mg/ml a transition occurs from an isotropic entangled phase to a coexistence between isotropic and nematic domains. Analysis of the molecular motions of filaments suggests that the filaments in the aligned domains are in thermal equilibrium and that the diffusion coefficient parallel to the director D parallel is nearly independent of filament length. We also report the novel direct observation of u-shaped defects, called hairpins, in the nematic domains.

Determination of the affinity of talin and vinculin to charged lipid vesicles: a light scatter study

Recent experimental findings have demonstrated that both talin and vinculin bind to phospholipids and insert into the hydrophobic region of lipid membranes. Here, we show that the light scatter method can be used for measuring the affinity of proteins to phospholipid membranes. Large unilamellar DMPC/DMPG vesicles were produced by the extrusion technique (LUVETs). We have used repeated heating/cooling scans between 15 degrees C and 35 degrees C to ensure protein-lipid interaction/insertion. A molar affinity of talin, K = 2.9 x 10(6) M-1 and of vinculin, K = 3.3 x 10(5) M-1 to lipid vesicles, respectively, was determined from the plot; light scatter signal at 380 nm against protein concentrations by fitting the term, ln (Io/I-1) = A-K x c to the data.

Interaction of the 47-kDa talin fragment and the 32-kDa vinculin fragment with acidic phospholipids: a computer analysis

In recent in vitro experiments, it has been demonstrated that the 47-kDa fragment of the talin molecule and the 32-kDa fragment of the vinculin molecule interact with acidic phospholipids. By using a computer analysis method, we determined the hydrophobic and amphipathic stretches of these fragments and, by applying a purpose-written matrix method, we ascertained the molecular amphipathic structure of alpha-helices. Calculations for the 47-kDa mouse talin fragment (residues 1-433; NH2-terminal region) suggest specific interactions of residues 21-39, 287-342, and 385-406 with acidic phospholipids and a general lipid-binding domain for mouse talin (primary amino acid sequence 385-401) and for Dictyostelium talin (primary amino acid sequence 348-364). Calculations for the 32-kDa chicken embryo vinculin fragment (residues 858-1066; COOH-terminal region) and from nematode vinculin alignment indicate for chicken embryo vinculin residues 935-978 and 1020-1040 interactions with acidic phospholipids. Experimental confirmation has been given for vinculin (residues 916-970), and future detailed experimental analyses are now needed to support the remaining computational data.

Cell-substrate interactions and locomotion of Dictyostelium wild-type and mutants defective in three cytoskeletal proteins: a study using quantitative reflection interference contrast microscopy

Reflection interference contrast microscopy combined with digital image processing was applied to study the motion of Dictyostelium discoideum cells in their pre-aggregative state on substrata of different adhesiveness (glass, albumin-covered glass, and freshly cleaved mica). The temporal variations of the size and shape of the cell/substratum contact area and the time course of advancement of pseudopods protruding in contact with the substratum were analyzed. The major goal was to study differences between the locomotion of wild-type cells and strains of triple mutants deficient in two F-actin cross-linking proteins (alpha-actinin and the 120-kDa gelation factor) and one F-actin fragmenting protein (severin). The size of contact area, AC, of both wild-type and mutant cells fluctuates between minimum and maximum values on the order of minutes, pointing toward an intrinsic switching mechanism associated with the mechanochemical control system. The fluctuation amplitudes are much larger on freshly cleaved mica than on glass. Wild-type and mutant cells exhibit remarkable differences on mica but not on glass. These differences comprise the population median of AC and alterations in pseudopod protrusion. AC is smaller by a factor of two or more for all mutants. Pseudopods protrude slower and shorter in the mutants. It is concluded that cell shape and pseudopods are destabilized by defects in the actin-skeleton, which can be overcompensated by strongly adhesive substrata. Several features of amoeboid cell locomotion on substrata can be understood on the basis of the minimum bending energy concept of soft adhering shells and by assuming that adhesion induces local alterations of the composite membrane consisting of the protein/lipid bilayer on the cell surface and the underlying actin-cortex.

Internal actin filament dynamics in the presence of vinculin: a dynamic light scattering study

Analyses of dynamic light scattering data by stretched exponential fit show that vinculin has a negligible influence on internal actin filament dynamics and actin bending stiffness which contrasts with our previous observations with talin, another actin and vinculin-binding protein from focal adhesions. The results here agree with kinetic and rheologic measurements.

Identification of functional domains in the cytoskeletal protein talin

The cytoskeletal protein talin potentially plays a key role in actin-membrane linkage. It is able to nucleate actin filament growth in vitro while binding simultaneously to lipid bilayers. Thrombin digestion of human platelet talin yields tow polypeptide domains of 200 kDa and 47 kDa. We have purified these fragments and analyzed their functional properties: the 200-kDa fragment was active in nucleating actin filament formation and reduced the viscosity of filamentous actin, comparable to the effects of the intact protein. The 47-kDa fragment was inactive in this respect. However, the 47-kDa polypeptide, but not the 200-kDa fragment, interacted specifically with large liposomes containing acidic phospholipids. This is demonstrated by selective, hydrophobic photolabeling of the 47-kDa fragment using phosphatidylserine liposomes containing trace amounts of a photoactivable phospholipid analogue and by selective co-sedimentation of this domain with the liposomes. The 200-kDa fragment, whether alone or in conjunction with the small fragment, neither incorporated significant amounts of label nor co-sedimented with the liposomes. We thus are able to attribute specialized functions to distinct domains on the talin molecule. These enable the protein to interact simultaneously with actin filaments and lipid membranes.

Local measurements of viscoelastic moduli of entangled actin networks using an oscillating magnetic bead micro-rheometer

A magnetically driven bead micro-rheometer for local quantitative measurements of the viscoelastic moduli in soft macromolecular networks such as an entangled F-actin solution is described. The viscoelastic response of paramagnetic latex beads to external magnetic forces is analyzed by optical particle tracking and fast image processing. Several modes of operation are possible, including analysis of bead motion after pulse-like or oscillatory excitations, or after application of a constant force. The frequency dependencies of the storage modulus, G'(omega), and the loss modulus, G''(omega), were measured for frequencies from 10(-1) Hz to 5 Hz. For low actin concentrations (mesh sizes epsilon > 0.1 micron) we found that both G'(omega) and G''(omega) scale with omega 1/2. This scaling law and the absolute values of G' and G'' agree with conventional rheological measurements, demonstrating that the magnetic bead micro-rheometer allows quantitative measurements of the viscoelastic moduli. Local variations of the viscoelastic moduli (and thus of the network density and mesh size) can be probed in several ways: 1) by measurement of G' and G'' at different sites within the network; 2) by the simultaneous analysis of several embedded beads; and 3) by evaluation of the bead trajectories over macroscopic distances. The latter mode yields absolute values and local fluctuations of the apparent viscosity eta(x) of the network.