Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Z4: Biopolymers |
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Sponsoring Units: DPOLY DBP Chair: Michael Lang and Zuowei Wang, University of North Carolina Room: Baltimore Convention Center 308 |
Friday, March 17, 2006 11:15AM - 11:51AM |
Z4.00001: Bonds that strengthen under force Invited Speaker: While the adhesive strength of most receptor-ligand interactions is exponentially reduced if strained, some receptor-ligand complexes exist that strengthen under force which is the hallmark of catch bonds. Although the existence of catch bonds was theoretically predicted, the first experimental demonstrations of their existence were given only recently, i.e. for the bacterial adhesin FimH that is located at the tip of type I fimbriae of \textit{E. coli} and for p-selectin. In a major collaborative effort, we studied the structural origin by which the FimH-mannose bond is switched by force to a high binding state. Mutational studies were thereby combined with steered molecular dynamic simulations to decipher how force might affect protein conformation. Force-activation of FimH leads to a complex `stick-and-roll' bacterial adhesion behavior in which \textit{E. coli} preferentially rolls over mannosylated surfaces at low shear but increasingly sticks firmly as the shear is increased. Interesting similarities are further seen if comparing the structural mechanisms by which liganded FimH and liganded integrins are switched to a high binding state. This comparison was made possible by docking fibronectin's 10$^{th}$ type III module (fnIII$_{10})$ to $\alpha _{V}\beta _{3}$ integrin. $\alpha _{V}\beta _{3}$ can switch from the ``closed'' $\alpha _{V}\beta _{3}$ integrin headpiece to the ``open'' conformation by opening the hinge angle between the $\beta $A domain and the hybrid domain of the $\beta $-integrin. The ``open'' state has been implicated by many experimental laboratories to correspond to the activated state of integrins. \newline \newline W. E. Thomas, E. Trintchina, M. Forero, V. Vogel, E. Sokurenko, Bacterial adhesion to target cells enhanced by shear-force, Cell, 109 (2002) 913. \newline W. E. Thomas, L. M. Nilsson, M. Forero, E. V. Sokurenko, V. Vogel, Shear-dependent `stick-and-roll' adhesion of type 1 fimbriated \textit{Escherichia coli}, Molecular Microbiology 53 (2004) 1545. \newline W. Thomas, M. Forero, O. Yakovenko, L. Nilsson, P. Vicini, E. Sokurenko, V. Vogel, Catch Bond Model Derived from Allostery Explains Force-Activated Bacterial Adhesion, Biophys. J, in press \newline E. Puklin-Faucher, M. Gao, K. Schulten, V. Vogel, How the opening of the $\beta $A/hybrid domain hinge angle in the $\alpha _{v}\beta _ {3}$ integrin headpiece is regulated by the liganded MIDAS conformation and by ligand-mediated mechanical force, submitted. [Preview Abstract] |
Friday, March 17, 2006 11:51AM - 12:27PM |
Z4.00002: Cellular Force, and Geometry Sensing (Over Time) Can Detect Matrix Rigidity: Local Modules Produce Global Signals Invited Speaker: The shape and behavior of mammalian cells is defined by an interplay between extracellular signals and the cellular responses. Although the chemical nature of the external signals is important, there is a growing realization that the physical aspects of the external environment are equally important. In particular, the stresses, rigidity and form of the external environment have major effects on cell behavior. Of particular importance is rigidity since cancerous cells can often grow on soft agar or in a fluid phase without force production. For most mammalian cells there are relatively few types of motility that are evident from quantitative analyses of rapidly spreading fibroblasts (Dubin-Thaler et al., Biophys. J. 86:1794-1806, 2004). One motile phase that we have studied extensively involves periodic contractions (24 s period) in local regions of the leading edge of the cell (Giannone et al., Cell, 116:431-443, 2004). The periodic signal is carried radially from the cell edge toward the center and is part of a general mechanism for rigidity-directed movement and pathfinding. Another motile phase involves the movement of individual collagen fibers in a hand-over-hand fashion (Meshel et al., Nature Cell Biol. 7:157-164, 2005) where the form of the fiber is being sensed. Rigidity and form sensing in these systems is dependent upon the cytoskeleton and force-dependent tyrosine phosphorylation through oncogenes (Sawada and Sheetz, J Cell Biol. 156:609-15, 2002; Tamada et al., Developmental Cell, 7:706-718, 2004). Recent studies indicate that the cell rigidity sensing occurs preferentially at the leading edges of moving cells and involves forces of 10-20 pN generated by displacements of 50-100 nm (Jiang et al., Biophys J. In Press). We will discuss how cells organize motility tools in motile phases (D\"{o}bereiner et al., Phys. Rev. Letters. 93:108105-1-4, 2004) in a dialogue with the environment to define cell morphology and behavior over time.. [Preview Abstract] |
Friday, March 17, 2006 12:27PM - 1:03PM |
Z4.00003: Looking for steps of individual enzymes moving along DNA Invited Speaker: Understanding the molecular mechanism of any motor activity involves determining the elementary step size with which it moves. RecBCD is a processive, DNA-based motor with both helicase and nuclease activities. To directly measure RecBCD's putative step size of 4 base pairs (1.4 nm), several technical advances were incorporated in a new high-resolution optical trapping instrument capable of resolving 0.1 nm motion. First, mechanical drift was eliminated by developing a differential measurement system based upon improved laser beam-pointing stability and the introduction of a fiducial mark attached to a microscope coverslip (e.g., a stuck bead). To generalize this technique to measure subnanometer vertical motion, we intensity stabilized the detection laser and differentially amplified the vertical signal. We further enhanced this process by actively stabilizing the sample in 3D. In the presence of substantial thermal heating, 3D differential measurements with active stabilization achieved short term (1 s) stabilities of 0.13, 0.08 and 0.22 nm (RMS) in x, y, and z, respectively. Positional stability, as demonstrated by our differential subtraction, does not guarantee subnanometer resolution of an optically trapped bead \textit{under load}. We therefore intensity stabilized our trapping laser to $\sim $0.1{\%} at 100 Hz. Finally, our technique requires a DNA tether to be within a small distance (3 $\mu $m) of a fiducial mark. If the stuck bead is too close or directly along one of the primary axes of the stage motion, it interferes with the measurement. Since the location of stuck beads and DNA tethers is random, this leads to only a few stuck bead/DNA tether pairs that can be successfully used. To overcome these limitations, we developed a regular grid of nanoposts. We will present our progress on integrating these technological advances to measure individual steps of RecBCD. [Preview Abstract] |
Friday, March 17, 2006 1:03PM - 1:39PM |
Z4.00004: Thermodynamics and Structure of Polymerizing Actin Invited Speaker: The polymerization of the globular protein G-actin to form filamentary F-actin is an important cellular process, serving major functions in cell structure and cell motility. This transition from monomeric G-actin to polymeric F-actin can be initiated by the variation of thermodynamic variables such as temperature, pressure, and compositions of G-actin and salts. We use fluorescence spectroscopy to obtain the fraction of monomer converted to polymer, and model these data using a Flory-Huggins type of theory. We measure the size and shape of the actin species by small angle neutron scattering, and find an unexpected spherical shape for G-actin in salt buffer. [Preview Abstract] |
Friday, March 17, 2006 1:39PM - 2:15PM |
Z4.00005: Synthetic and Biopolymer Gels - Similarities and Difference. Invited Speaker: Ion exchange plays a central role in a variety of physiological processes, such as nerve excitation, muscle contraction and cell locomotion. Hydrogels can be used as model systems for identifying fundamental chemical and physical interactions that govern structure formation, phase transition, etc. in biopolymer systems. Polyelectrolyte gels are particularly well-suited to study ion-polymer interactions because their structure and physical-chemical properties (charge density, crosslink density, etc) can be carefully controlled. They are sensitive to different external stimuli such as temperature, ionic composition and pH. Surprisingly few investigations have been made on polyelectrolyte gels in salt solutions containing both monovalent and multivalent cations. We have developed an experimental approach that combines small angle neutron scattering and osmotic swelling pressure measurements. The osmotic pressure exerted on a macroscopic scale is a consequence of changes occurring at a molecular level. The intensity of the neutron scattering signal, which provides structural information as a function of spatial resolution, is directly related to the osmotic pressure. We have found a striking similarity in the scattering and osmotic behavior of polyacrylic acid gels and DNA gels swollen in nearly physiological salt solutions. Addition of calcium ions to both systems causes a sudden volume change. This volume transition, which occurs when the majority of the sodium counterions are replaced by calcium ions, is reversible. Such reversibility implies that the calcium ions are not strongly bound by the polyanion, but are free to move along the polymer chain, which allows these ions to form temporary bridges between negative charges on adjacent chains. Mechanical measurements reveal that the elastic modulus is practically unchanged in the calcium-containing gels, i.e., ion bridging is qualitatively different from covalent crosslinks. [Preview Abstract] |
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