Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session U31: Polymers and Filaments for the Cytoskeleton |
Hide Abstracts |
Sponsoring Units: DPOLY DBP Chair: David Morse, U Minnesota Room: LACC 503 |
Thursday, March 24, 2005 8:00AM - 8:12AM |
U31.00001: The response to point forces in cytoskeletal networks Alex J. Levine, Davd Head, Fred C. MacKintosh Networks of semiflexible polymers that are cross-linked densely on the scale of their thermal persistence length form the structural basis of the cytoskeleton. These cytoskeletal networks, together with various cross-linking and other associated proteins largely determine the (visco-)elastic response of cells. We have found that semiflexible networks show a much more complex elastic response than traditional gels constructed of flexible polymers. In particular the both geometry of the deformation field under uniformly imposed shear stress and the effective shear modulus depend sensitively on the length of the constituent filaments relative the ``nonaffinity length'' that is a function of both the filament bending modulus and cross-linker density. In this talk I discuss the elastic Greens function in semiflexible networks, i.e. the response of these networks to localized forces. These investigations further highlight the role of the nonaffinity length and will improve our understanding of the action of molecular motors in the cytoskeleton. They will also facilitate the interpretation of microrheology data in semiflexible networks and in the cell. [Preview Abstract] |
Thursday, March 24, 2005 8:12AM - 8:24AM |
U31.00002: Mechanical Response Study of Collagen by means of Molecular Simulation Pieter J. in 't Veld, Mark J. Stevens We developed a coarse-grained model to study mechanical behavior of collagen fibrils as a function of their degree of cross-linking. A collagen molecule is represented by Lennard-Jones beads, which intra-molecularly are connected through harmonic springs on both bond length and angle. In this model each bead represents a helical turn in a collagen molecule. Triple-helical collagen molecules, which are 300 \textit{nm} long, are packed within fibrils in a staggered fashion with an axial spacing of 67 \textit{nm} in the absence of a load on the tendon. We treat the outer layer or shell different from the core by assuming the shell has the maximum amount of available cross-links. The core has a variable amount of cross-links by allowing cross-link formation and breakage depending on a reaction-type criterion. We study the stress-strain behavior of a single fibril through tensile deformation along the principal axis and a three-point bend perpendicular to the principal axis. [Preview Abstract] |
Thursday, March 24, 2005 8:24AM - 8:36AM |
U31.00003: Forced unfolding of protein domains determines cytoskeletal rheology John Crocker, Brenton Hoffman, Gladys Massiera Cells have recently been shown to have a power-law dynamic shear modulus over wide frequency range; the value of the exponent being non-universal, varying from 0.1-0.25 depending on cell type. This observation has been interpreted as evidence for the Soft Glassy Rheology (SGR) model, a trap-type glass model with an effective granular temperature. We propose a simple, alternative model of cytoskeletal mechanics based on the thermally activated, forced unfolding of domains in proteins cross-linking a stressed semi-flexible polymer gel. It directly relates a cell’s mechanical response to biophysical parameters of the cytoskeleton’s molecular constituents. Simulations indicate that unfolding events in a random network display a collective self-organization, giving rise to an exponential distribution of crosslink stress that can reproduce cell viscoelasticity. The model suggests natural explanations for the observed correlation between cell rheology and intracellular static stress, including those previously explained using the tensegrity concept. Moreover, our model provides insight into potential mechanisms of mechanotransduction as well as cell shape sensing and maintenance. [Preview Abstract] |
Thursday, March 24, 2005 8:36AM - 8:48AM |
U31.00004: Structure and Interactions in Neurofilament Networks Jayna Jones, M. Ojeda-Lopez, C.R. Safinya Neurofilaments (NFs) are a major constituent of nerve cell axons that assemble from three subunit proteins of low (NF-L), medium (NF-M), and high molecular weight (NF-H) to form a 10 nm diameter rod with radiating sidearms. The sidearm interactions result in an oriented network of NFs running parallel to the axon. Here, we reassemble NFs \textit{in vitro} from varying weight ratios of two of the subunit proteins, NF-L and NF-M, purified from bovine spinal cord. We demonstrate the formation of the NF network where synchrotron x-ray scattering (SSRL) reveals a well-defined interfilament spacing, while the defect structure in polarized optical microcopy shows the liquid crystalline nature. The interfilament spacing varies depending on NF-M sidearm density and we relate this change to sidearm interactions. We show that at a low density of sidearms, repulsive forces dominate creating a lattice spacing that is regulated by the buffer volume. With an increasing sidearm density, the equilibrium interfilament spacing decreases as a result of competing repulsive and attractive forces. Supported by NIH GM-59288, NSF DMR- 0203755, {\&} CTS-0404444. [Preview Abstract] |
Thursday, March 24, 2005 8:48AM - 9:00AM |
U31.00005: Electrostatic self-assembly between biological polymers \& macroions: Interactions of F-actin \& DNA with lysozyme Lori K. Sanders, Thomas E. Angelini, Wujing Xian, Brian W. Matthews, Gerard C.L. Wong The pathological self-assembly of polyelectrolytes such as DNA and F-actin with cationic antimicrobial proteins such as lysozyme may have significant clinical consequences in Cystic Fibrosis (CF) lung infections. Wild-type lysozyme is a compact, cationic, globular protein which carries a net charge of +9e at neutral pH. Our Small Angle X-ray Scattering (SAXS) experiments on F-actin-lysozyme complexes indicate that the wild-type lysozyme close packs into 1-D columns between hexagonally organized F-actin filaments. We will present SAXS results of the interactions of F-actin and DNA with genetically engineered lysozyme mutants that carry a reduced charge of +5e. We have also used fluorescence microscopy to investigate the morphologies and sizes of such bundles induced with divalent cations, wild-type lysozyme, and mutant lysozymes. [Preview Abstract] |
Thursday, March 24, 2005 9:00AM - 9:12AM |
U31.00006: Phase Behavior of F-actin Glenna Z. Sowa, David S. Cannell, Andrea J. Liu, Emil Reisler To better understand the close spatial proximity of F-actin (filamentous actin) bundles to other structures comprised of F-actin in cellular environments, we have measured the phase boundary between F-actin and F-actin bundles as a function of spermine concentration. To do this, we first grew actin filaments by adding MgCl$_{2}$ to G-actin (globular actin). F-actin was then incubated with spermine (a low-binding-energy linker and actin-bundling factor) overnight, and then the samples were spun at low speeds to separate bundles from unbundled F-actin. The relative amounts of actin in the pellet and supernatant were determined via gel electrophoresis, yielding a description of the bundling transition as a function of actin and spermine concentrations. With this approach, we are constructing a phase diagram for the F-actin/spermine system. Surprisingly, the dependence of bundle formation on actin concentration is small to non-existent. At the actin concentrations we studied (4.5, 9, 18 and 36$\mu $M), actin tends to form bundles at the same spermine concentration. This observation calls for the evaluation of the effect of the ambient Mg$^{2+}$ in solution (added to polymerize actin) on actin bundling by spermine. [Preview Abstract] |
Thursday, March 24, 2005 9:12AM - 9:24AM |
U31.00007: Phase behavior of semidilute polyelectrolyte mixtures of F-actin and DNA Scott Slimmer, John C. Butler, Olena V. Zribi, Ramin Golastanian, Gerard C. L. Wong We investigate the phase behavior of semidilute mixtures of polyelectrolyte DNA coils and F-actin rods. F-actin has a persistence length of $\sim $10 microns and a linear charge density of -1e/0.25nm. DNA has a persistence length of $\sim $50nm and a linear charge density of --1e/0.17nm. Confocal and polarized microscopy data show that actin-DNA phase separates into ribbon-like birefringent domains of nematic F-actin and a disordered mesh of DNA coils. Synchrotron Small Angle X-ray Scattering (SAXS) show that DNA compresses F-actin into an ultradense dense nematic phase. The spacing between nematic F-actin domains shows a power-law dependence on DNA concentration. [Preview Abstract] |
Thursday, March 24, 2005 9:24AM - 9:36AM |
U31.00008: Fluorescent Speckle Microrheology of F-actin Networks Margaret Gardel, Dinah Loerke, Gaudenz Danuser, Clare Waterman-Storer We present a non-invasive technique to probe the mechanical properties of F-actin cytoskeletal networks at sub-micron to micron length scales by using fluorescent speckle microscopy (FSM) to directly image the thermally-driven strain fluctuations of filaments in the network. In FSM, cytoskeletal polymers are labeled with a low concentration ratio of fluorescent:non-fluorescent cytoskeletal subunits and stochastic, spatial variations in fluorescence intensity result in diffraction-limited intensity peaks in high magnification, high resolution images called `speckles'. Using TIRF microscopy and a fast, sensitive cooled CCD camera with on-chip multiplication gain, we were able to image speckles in \textit{in vitro} F-actin networks cross-linked with \textit{$\alpha $}-actinin at 30 frames/sec for nearly 120 seconds. We then track the thermally driven spatial trajectories of the speckles with subpixel accuracy and cross-correlate the displacements of pairs of speckles to directly map the strain fluctuations of the networks and use a generalized Stokes-Einstein relation to interpret these fluctuations in terms of the mechanical properties. Fluorescent speckle microrheology will be a powerful, highly spatiotemporally resolved method for non-invasively probing cytoskeletal mechanics in living cells during morphogenic processes such as migration or division. [Preview Abstract] |
Thursday, March 24, 2005 9:36AM - 9:48AM |
U31.00009: Entanglement of Semiflexiible Polymers: A Brownian Dynamics Study Shriram Ramanathan, David Morse We report extensive Brownian dynamics simulations of very tightly entangled solutions of semiflexible rods, of length $L$ comparable to their persistence length $L_{p}$, at concentrations comparable to those in recent experiments on Fd-virus and filamentous actin. We find a clear crossover with increasing number concentration $c$ from a regime of loosely entangled rods, in which rotational diffusion is hindered by topological constraints but transverse bending fluctuations are not, to a tightly entangled regime in which bending fluctuations are also restricted, and can relax only by reptation along a wormlike tube. This crossover occurs at a dimensionless concentration $c^{**}L^{3} \sim 500$ for chains with $L = L_{p}$. The tube radius $R_{e}$ is found to depend upon $c$ and $L_{p}$ with the predicted scaling relation $R_{e}\propto c^{-3/5} L_{p}^{-1/5}$ for $c > c^{**}$. The dynamic modulus $G(t)$ has been obtained from simulations of the relaxation of stress after a small amplitude step extension of the simulation unit cell. An elastic plateau in $G(t)$ that is absent at lower concentrations also appears for $c \geq c^{**}$. [Preview Abstract] |
Thursday, March 24, 2005 9:48AM - 10:00AM |
U31.00010: Polyelectrolyte Bundles: Finite size at thermodynamic equilibrium? Mehmet Sayar, Hans J. Limbach, Christian Holm Experimental observation of finite size aggregates formed by polyelectrolytes such as DNA and F-actin, as well as synthetic polymers like poly(p-phenylene), has created a lot of attention in recent years. Here, bundle formation in rigid rod-like polyelectrolytes is studied via computer simulations. For the case of hydrophobically modified polyelectrolytes finite size bundles are observed even in the presence of only monovalent counterions. Furthermore, in the absence of a hydrophobic backbone, we have also observed formation of finite size aggregates via multivalent counterion condensation. The size distribution of such aggregates and the stability is analyzed in this study. [Preview Abstract] |
Thursday, March 24, 2005 10:00AM - 10:12AM |
U31.00011: Growth of Attached Actin Filaments Jie Zhu, A. E. Carlsson Actin filaments in cells extend themselves by polymerizing free actin monomers onto their growing ends. The growing filaments can push obstacles and thus do mechanical work. It is known [1] that if the filaments are not attached to the obstacle, new monomers can be added when the obstacle fluctuates away from the growing filament ends. However, experiments [2, 3] show that the growing ends of actin filaments are firmly attached to the obstacle. Based on the idea of the Brownian ratchet model, we develop an energy-based model to investigate the growth of attached actin filaments. In this model, the force field describing the interaction between the actin filament and surface proteins (such as ActA) on the obstacle's surface is given a simplified but plausible analytic form. We use both Brownian-dynamics simulations and analytical approaches to calculate the attachment time and the growth rate. Our results show that a high binding energy ($\sim$28kT) is required for the binding of an actin filament to the obstacle, and the actin filament can remain attached to a 25 nm bead for about 30 s, while still growing at about 50\% of the free-filament growth velocity.\newline *Supported by NSF grant number DMS-0240770.\newline [1] Peskin, Odell and Oster, Biophys. J. \textbf{65}, 316 (1993). [2] Kuo and McGrath, Nature \textbf{407}, 1026 (2000). [3] Gerbal, et al. Eur. Biophys. J. \textbf{29}, 134 (2000). [Preview Abstract] |
Thursday, March 24, 2005 10:12AM - 10:24AM |
U31.00012: Structure and stability of self-assembled actin-lysozyme complexes studied via computer simulation Camilo Guaqueta, Erik Luijten Using both molecular dynamics and grand-canonical Monte Carlo simulations, we have studied the structure and stability of complexes of filamentous actin (an anionic polyampholyte) and lysozyme (a cationic globular protein) in aqueous solution. We find that lysozyme initially bridges pairs of filaments, which then relax into hexagonally-coordinated bundles comprised of actin rods held together by one-dimensional arrays of lysozyme macroions. In order to connect to small-angle x-ray scattering results, we have examined the role of the concentration of monovalent salt. We find that exclusion of salt from the bundled phase is essential for bundle stability, and we address with our simulation results the different competing effects which could be responsible for this salt repartitioning. [Preview Abstract] |
Thursday, March 24, 2005 10:24AM - 10:36AM |
U31.00013: Hierarchical Self Assembly of Actin Bundle Networks Linda Hirst, Cyrus Safinya The network-like structure of actin bundles formed with the cross-linking protein $\alpha $-actinin has been investigated on different length scales via small angle x-ray scattering and confocal fluorescence microscopy. We describe the hierarchical structure of aggregates formed at different ratios of cross-linker using both $\alpha $-actinin and also the non-specific polyelectrolyte, polylysine. The effects of different lengths of F-actin are also discussed. An interesting feature of this system is the formation of a dense layer on the surface of the actin gel. This layer exhibits interesting morphologies and can be formed to have a defined shape. Biologically based structures such as this have the potential to generate interesting biological scaffolds for applications in cell encapsulation and tissue engineering. [Preview Abstract] |
Thursday, March 24, 2005 10:36AM - 10:48AM |
U31.00014: Elastic actin comet tails: shape, stresses and propulsion Ajay Gopinathan, Andrea Liu Actin based motility is a recurring theme in a variety of biological systems ranging from keratocytes that use their dynamically re-arranging cytoskeleton for motility to bacterial pathogens like Listeria that hijack the host cell's actin machinery and are propelled by actin comet tails. The basic principle behind all these processes is the conversion of free energy of polymerization into a protrusive force. Recent experimental observations have suggested several distinctive features of such propulsion especially in the case of Listeria motion. We model the process by a finite element simulation of the actin comet tail which is treated as a continuum elastic material that is tethered to the rear of the bacterium. We investigate steady state properties such as the shape of the comet tail, stresses generated and also the time dependence of the motion. [Preview Abstract] |
Thursday, March 24, 2005 10:48AM - 11:00AM |
U31.00015: The orientational order parameter of nematic liquid crystalline phase of F-actin Jorge Viamontes, Jay X. Tang The cytoskeletal protein actin self-assembles to form long and stiff filaments, F-actin, which serves essential functions in cells, such as control of cell shape, division, and motility. Suspensions of F-actin form either entangled isotropic networks or a nematic liquid crystalline phase. Depending on the average filament length, the isotropic-nematic (I-N) liquid crystalline transition occurs at a concentration of 2 mg/ml or above. We have measured the orientational order parameter of F-actin traversing the I-N phase transition using a combination of techniques, including fluorescence microscopy, local birefringence measurement, and x-ray scattering. With actin concentrations above the region of I-N transition, the order parameter approaches a saturated value of 0.75. This value implies significant extent of misalignment or entanglement among long actin filaments even in the nematic phase. At concentrations slightly below the I-N transition, non zero values of the order parameter were detected within a time window on the order of an hour following the sample preparation, which tends to cause unintended initial alignment. This result shows extremely slow rotational kinetics of F-actin in the entangled networks. [Preview Abstract] |
|
U31.00016: Actin Filamin networks and stress criticality Brian DiDonna, Alex Levine, John Crocker, Brenton Hoffman We study critical behavior in a model biopolymer network comprised of semiflexible polymers crosslinked by extensible proteins with unfolding domains. The domains unfold reversibly at a critical pulling force. The force extension curve of such a crosslinker resembles a sawtooth function, with another domain unfolding and thus adding entropic compliance each time a critical pulling force is reached. Filamin and alpha-actinin are both biological crosslinkers which exhibit this sawtooth behavior. We demonstrate through theory and simulation that our model network exhibits critical pileup in the distribution of crosslinker lengths when it is sheared. This is to say, the population fraction of crosslinkers at a given tension dies exponentially away from the unfolding force of the unfolding domains. This leads to a novel force relaxation time scaling as crosslinkers are thermally excited over the unfolding threshold. [Preview Abstract] |
|
U31.00017: Order-Order Transition of Size-mismatched Ions on F-actin Polyelectrolytes Robert Coridan, Lori K. Sanders, Wujing Xian, Brian W. Matthews, Gerard C. L. Wong Multivalent ions induce condensation of like-charged F-actin polyelectrolytes into close-packed bundles, in which multivalent ions organize into 1-D density waves. We examine the condensation behavior of anionic F-actin using multivalent cations with a large size mismatch, Ba$^{2+}$ and lysozyme(+9), a small globular protein (2.5nm x 2.5nm x 4.5nm). An unexpected first-order phase transition on the F-actin surface between a Ba$^{2+}$ counterion charge density wave state and 1-D close-packed lysozyme chains is found as the lysozyme-actin ratio is varied. By comparing wild-type lysozyme with genetically-engineered lysozyme with reduced charge, we show that this transition shifts with the actin-lysozyme isoelectric point. [Preview Abstract] |
|
U31.00018: Fingers and Comet Tails--Motility and Morphology in growing actin gels Ariel Balter, Allan Bower, Jay Tang Actin-based cell motility has proven to be a useful system for studying the dynamic system of polymer gel formation in the cytoskeleton. There is still no consensus regarding the exact method for the transduction of chemical energy to mechanical energy during actin based motility. Also under debate is the ``symmetry breaking'' which occurs in a biomimetic system used to simulate and study actin based motility. An enzyme coated bead immersed in real or synthetic cell extract will first grow a symmetric cloud of actin gel. Then the gel will spontaneously differentiate into one or more ``tails.'' A symmetry breaking stochastic model has been proposed for the formation of one tail. We propose a model based on an elasto-chemical instability at the outer edge of the gel. Our theoretical model allows us predict when one or more tails will form as a function of system parameters, explain the observed shape of an actin ``comet tail'' and predict when the instability will take place. Our model is supported by finite element simulations. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700