Pearling, Tubulation and Coiling: Three Instabilities of Membranes with Anchored Macromolecules.

  I review the results of an experimental and theoretical study of the effects of polymers having a number of hydrophobic anchors grafted along a hydrophilic backbone on phospholipid vesicles of different geometries. Pearling of tubular vesicles, tubulation at the rim of highly oblate vesicles, and finally a coiling instability in highly multilamellar tubes have been observed, above threshold polymer concentrations. These instabilities can be accounted for within a single framework in which the local polymer concentration is coupled to local membrane curvature, and in which polymers effect these changes by inducing curvature in the bilayers on which they anchor. Curvature is induced by two mechanisms: a local deformation due to anchoring, and an increase in the area of one leaflet of a bilayer with respect to the other.


      References

"Coiling of Cylindrical Membrane Stacks with Anchored Polymers"
V. Frette, I. Tsafrir, M.A. Guedeau-Boudeville, L. Jullien, D. Kandel and J. Stavans, Phys. Rev.Lett. 83, 2465 (1999).
"Pearling Instabilities of Membranes with Anchored Polymers"
I. Tsafrir, D. Sagi, T. Arzi, M. A. Guedeau-Boudeville, V. Frette, D. Kandel and J. Stavans, Phys. Rev.Lett. 86, 1138, (2001).
"Coiling Instability of Multilamellar Membrane Tubes with Anchored Polymers" I. Tsafrir, M. A. Guedeau-Boudeville, D. Kandel and Joel Stavans, Phys. Rev. E. 63, 031603-1 (2001).
Movies of some of the phenomena can be found in my website, soon to be revamped: http://www.weizmann.ac.il/~festava/

   Pulling a Nanostring with a Nanomotor: The RuvAB-DNA interaction

      Recombination is a fundamental genetic mechanism playing various roles in the life of a cell. One of them is to bypass DNA damage that cannot be repaired, and thus allow DNA replication to resume and a cell to divide. Holliday junctions form between two homologous DNA tracts as intermediates during a recombination process. To bypass damage, a cell needs to displace the junction point, and thermal diffusionas a displacement mechanism is too inefficient. To actually carry out this displacement efficiently and in a directed fashion, the cell employs a protein complex, the RuvAB motor. This motor enlarges two arms of a junction at the expense of the transversal arms, using ATP as an energy source. A characterization of this motor was presented using single-molecule techniques, and among other features, direct measurements of its rate, processivity and the force it can exert were measured. This work is unpublished as yet.