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dictyNews Volume 27 Number 14
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.
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Abstracts
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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]
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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 Òterminal coneÓ, is built of short filaments fixed with their distal
end to the filopodÕs 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 filopodÕs 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]
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[End dictyNews, volume 27, number 14]
