dictyNews Electronic Edition Volume 27, number 14 November 17, 2006 Please submit abstracts of your papers as soon as they have been accepted for publication by sending them to dicty@northwestern.edu or by using the form at http://dictybase.org/db/cgi-bin/dictyBase/abstract_submit. Back issues of dictyNews, the Dicty Reference database and other useful information is available at dictyBase - http://dictybase.org. ========= Abstracts ========= Active mechanical stabilization of the viscoplastic intracellular space of Dictyostelia cells by microtubule actin cross-talk Doris Heinrich #, Erich Sackmann * # Physik Department, Lehrstuhl fuer Biophysik E22, Technische Universitaet Muenchen, D-85748 Garching, Germany * Sektion Physik, Lehrstuhl fuer Angewandte Physik, Universitaet Muenchen, Amalienstrasse 54, D-80799 Muenchen, Germany [sackmann@ph.tum.de] Acta Biomaterialia, in press The micro-viscoelasticity of the intracellular space of Dictyostelium discoideum cells is studied by evaluating the intracellular transport of magnetic force probes and their viscoelastic responses to force pulses of 20-700 pN. The role of the actin cortex, the microtubule (MT) aster and their crosstalk is explored by comparing the behaviour of wild type cells, myosin II null mutants, latrunculin A and benomyl treated cells. The MT coupled beads perform irregular local and long range directed motions which are characterized by measuring their velocity distributions (P(v)). The correlated motion of the MT and the centrosome are evaluated by microfluorescence of GFP-labelled MTs. P(v) can be represented by log-normal distributions with long tails and it is determined by random sweeping motions (v~0.5µm/s) of the MTs (caused by tangential forces on the filament ends coupled to the actin cortex) and by intermittent bead transports parallel to the MTs (v[max] ~1.5µm/sec). The tails are due to spontaneous filament deflections (with speeds up to 10µm/s) attributed to pre-stressing of the MT by local cortical tensions, generated by dynactin motors generating plus-end directed forces in the MTs. The viscoelastic responses are strongly non-linear and are mostly directed opposite or perpendicular to the force, showing that the cytoplasm behaves as an active viscoplastic body with time and force dependent drag coefficients. Nano-Newton loads exerted on the soft MT are balanced by traction forces arising at the MT ends coupled to the actin cortex and the centrosome, respectively. The mechanical coupling between the soft microtubules and the viscoelastic actin cortex provides cells with high mechanical stability despite the softness of the cytoplasm. Submitted by: Doris Heinrich [doris.heinich@ph.tum.de] -------------------------------------------------------------------------------- Time-resolved Responses to Chemoattractant, Characteristic of the Front and Tail of Dictyostelium Cells Martin Etzrodt, Hellen C.F. Ishikawa, Jeremie Dalous1, Annette Mueller-Taubenberger2, Till Bretschneider, and Guenther Gerisch* Max-Planck-Institut fuer Biochemie, Am Klopferspitz 18, D-82152 Martinsried, Germany 1Permanent address: CEA Grenoble, Departement Reponse et Dynamique Cellulaires, 17 rue des Martyrs, 38054 Grenoble Cedex 09, France. 2Present address: Ludwig Maximilians Universitaet Muenchen, Institut fuer Zellbiologie / ABI, Schillerstrasse 42, D-80336 Muenchen, Germany. *Corresponding author: Dr. Guenther Gerisch, Max-Planck-Institut fuer Biochemie, Am Klopferspitz 18a, D-82152 Martinsried, Germany. Tel.: ++49 (0)89 8578 2326; Fax: ++49 (0)89 8578 3885; gerisch@biochem.mpg.de FEBS Letters, in press In a gradient of chemoattractant, Dictyostelium cells are orientated with their front directed toward the source and their tail pointing into the opposite direction. The front region is specified by the polymerization of actin and the tail by the recruitment of filamentous myosin-II. We have dissected these front and tail responses by exposing cells to an upshift of cyclic AMP. A sharp rise and fall of polymerized actin within 10 seconds is accompanied by the recruitment of proteins involved in turning actin polymerization on or off. The cortical accumulation of myosin-II starts when the front response has declined, supporting the concept of divergent signal transmission and adaptation pathways. Submitted by: Guenther Gerisch [gerisch@biochem.mpg.de] -------------------------------------------------------------------------------- Organization of Actin Networks in Intact Filopodia Ohad Medalia1,2, Martin Beck1, Mary Ecke1, Igor Weber1,3, Ralph Neujahr1, Wolfgang Baumeister1, and Guenther Gerisch1* 1Max-Planck-Institut fuer Biochemie, D-82152 Martinsried, Germany. 2Department of Life Sciences and The NIBN, The Ben-Gurion University, PO Box 653, 84120 Beer-Sheva, Israel. 3present address: Rudjer Boskovic Institute, Bijenicka cesta 54, P.O.B. 180, 10002 Zagreb, Croatia. *Correspondence: gerisch@biochem.mpg.de Current Biology, in press Filopodia are finger-like extensions of the cell surface that are involved in sensing the environment [1], in attachment of particles for phagocytosis [2], in anchorage of cells on a substratum [3], and in the response to chemoattractants or other guidance cues [4-6]. Filopodia present an excellent model for actin-driven membrane protrusion. They grow at their tips by the assembly of actin and are stabilized along their length by a core of bundled actin filaments. To visualize actin networks in their native membrane-anchored state, filopodia of Dictyostelium cells were subjected to cryo-electron tomography. At the site of actin polymerization, a peculiar structure, the Ňterminal coneÓ, is built of short filaments fixed with their distal end to the filopodŐs tip and with their proximal end to the flank of the filopod. The backbone of the filopodia consists of actin filaments that are shorter than the entire filopod and aligned in parallel or obliquely to the filopodŐs axis. We hypothesize that growth of the highly dynamic filopodia of Dictyostelium is accompanied by repetitive nucleation of actin polymerization at the filopod tip, followed by the re-arrangement of filaments within the shaft. Submitted by: Guenther Gerisch [gerisch@biochem.mpg.de] ============================================================= [End dictyNews, volume 27, number 14]