Imaging and Elasticity of the Stalk of the Protozoan Peritrich Vorticellid :
An Ultrafast Biological Supramolecular Spring
Rafael E. Bras
(*Funded by the Dupont-MIT Alliance in conjunction with Profs. Alexander van Oudenaarden (PHYSICS-MIT), Paul T. Matsudaira (Biology/BEH/Whitehead Institute-MIT), and L. Mahadevan (MECHE-MIT/PHYSICS-CAMBRIDGE U.))

(*Contact mode in air on mica)

(*Contact mode in air on polylysine-coated mica)

(*Optical Micrographs BELOW obtained from : http://bioweb.uwlax.edu/zoolab/Table_of_Contents/Lab-02/Vorticella/Live_Vorticella_1/live_vorticella_1.htm)

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              Nature has found various ingenious ways to induce movement on the cellular and molecular scale. One of the most powerful and fastest engines is found in stalked peritrichs of the family Vorticellids (Vorticella, Carchesium, Zoothamnium). These organisms are composed of a zooid (cell body, diameter ~50 microns) and a long stalk (in its extended state 20-200 microns long). One end of the stalk is connected to the zooid and the other end attaches to a solid support such as a submerged leaf. The spasmoneme is the organelle responsible for the remarkable motility and lies within the stalk. After addition of Ca2+ the spasmoneme rapidly contracts at velocities approaching 8 cm/s (Amos, 1971; Moriyama et al., 1998). The contraction is assumed to be a response effective in protecting the cell against dangerous mechanical disturbances such as turbulence in the surrounding water. This rate of contraction is extremely high, the zooid traverse ~1800 times its length per second. This is 120 times larger than a bacterial flagellar engine and 360 times larger than a midsize car engine at 60 mph. The spasmoneme is located asymmetrically with the stalk and stalk sheath is stiffened on the opposite side by complex extracellular fibers (Amos, 1972). Due to this asymmetry the stalk bends rather than collapses when the spasmoneme contracts.

              Birefringence measurements (Amos, 1971) and electron microscopy (Amos, 1972; Allen, 1973; Amos, 1975) indicate that spasmoneme in the presence of Ca2+ (fully extended state) consists of a bundle of roughly parallel filaments (Fig. 2B) that are weakly crosslinked to each other. In the absence of calcium ions, the extended spasmoneme collapses as an entropic spring. An early model (Hoffman-Berling, 1958) assumes that energy is stored by electrostatic repulsion between negatively charged filaments. By adding Ca2+ the negative charges are screened, which results in a collapse into the isotropic state. The structure and chemical composition of the spasmoneme are unknown. Recent experiments indicate the importance a major spasmoneme protein, called spasmin, that potentially binds Ca2+ ions and orchestrates the contraction (Asai et al., 1998; Maciejewski et al., 1999). Although the Vorticella system has been explored for more than three centuries now, starting with the observations of the 17th-century microscopist Leewenhoek (van Leeuwenhoek, 1676), the basic biochemistry and biophysics of this powerful spring remains unresolved. The overall goals of this project is to elucidate the molecular mechanism of spasmoneme contraction by (i) biochemical characterization of the constituents of the spasmoneme; (ii) biophysical characterization of the mechanics and dynamics of the stalk; and (iii) development of theoretical models to quantitatively understand the contraction mechanism. In order to test theoretical predictions of stalk contraction quantitative biomechanical experiments are underway which involve measuring the elastic properties (force-extension curves) of individual Vorticella stalks (Asai et al., 1978) at different Ca2+ concentrations using high-resolution force spectroscopy. The experimental force-extension curves will be fitted to known theoretical models. Relevant models are rubber-like elasticity models (Treloar, 1975; Moriyama et al., 1999), entropic strings models such as the worm-like chain model (Marko et al., 1995), and novel theoretical models. The worm-like chain model is particularly successful in describing collapsed DNA molecules in the presence of multivalent cations (Baumann et al., 2000). Possible conformational changes of protein subunits in the stalk will significantly alter the force-extension curves.

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