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Knotting Nanoropes
Fig.: Three snapshots of
a bundle formed by twenty filaments as observed
in computer simulations: (a) Loose bundle for
a crosslinker concentration that is only slightly
above the threshold value; (b) and (c) show two
different conformations of the same bundle corresponding
to a segregated conformation with three sub-bundles
and a compact conformation with roughly cylindrical
shape, respectively. Image: Max Planck Institute of Colloids and Interfaces
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Jan Kierfeld, Torsten Kühne, and Reinhard Lipowsky at Max Planck Society Pins down control parameters for the assembly of filament bundles, Nanoropes.
Biomimetic systems that are composed
of rigid polymers or filaments and crosslinking molecules
can be used to assemble Nanoropes, filament networks
and bundles. The bundles represent `nanoropes' and
exhibit material properties that are primarily determined
by the number of plaited filaments. Scientists at
the Max Planck Institute of Colloids and Interfaces
in Potsdam, Germany have now shown that this assembly
of Nanoropes is prevented by the thermal motion of
the filaments, unless the crosslinker concentration
exceeds a certain threshold value. The latter value
depends on the number of filaments, but remains finite
in the limit of a large filament number. As the crosslinker
concentration is lowered, the Nanoropes may segregate
into small sub-bundles, or undergo abrupt unbinding
transitions. (Physical Review Letters 95, 038102,
July, 2005).
Biological cells are mechanically stable
because they contain actin filaments and microtubules
that form networks and bundles. These filament architectures
are determined and controlled by crosslinking proteins,
which have two sticky ends that bind to different
filaments forming Nanoropes. In order to understand
the underlying forces and to optimise the mechanical
properties of these architectures, one must study
biomimetic model systems that are solely composed
of filaments and crosslinking proteins. One important
example is the assembly of several filaments into
thick bundles or ‘nanoropes’ that are
more rigid, and sustain a larger external load, than
single filaments.
The assembly of filaments by molecular
crosslinkers is disturbed by the thermal motion of
the filaments. Scientists at the Max Planck Institute
of Colloids and Interfaces have now shown that this
thermal motion prevents filament assembly unless the
crosslinker concentration exceeds a certain threshold
value. The latter value depends on the filament rigidity,
on the binding energy of the crosslinkers, and on
the temperature. Furthermore, the threshold value
decreases as the number N of filaments within the
bundle is increased, but remains finite in the limit
of large N.
Snapshots of filament bundles, Nanoropes
as observed in computer simulations are displayed
in Fig. 1. The snapshot in Figure 1(a) shows loose
Nanoropes for a crosslinker concentration only slightly
above the threshold value. The simulations also reveal
that these Nanoropes do not always reach their equilibrium
shape, but often segregate into sub-bundles containing
typically five filaments as shown in Figure 1(b).
This bundle morphology differs strongly from the fully
equilibrated bundle shape as shown in Figure 1(c)
for the same system. Which of the two morphologies
is attained depends on the initial arrangement of
the filaments and on the kinetics of the assembly
process.
Biomimetic systems, consisting of solutions
of actin filaments and crosslinking proteins, have
also been studied experimentally by several research
groups. The available experimental data is consistent
with the new theory based on the interplay of molecular
crosslinkers and thermal motion. In particular, there
is some experimental evidence for the threshold concentration
of crosslinkers and the sudden onset of filament bundle
formation above this concentration, but systematic
experimental studies remain to be done that explore
the dependence on the filament number N.
Apart from representing important structural
elements, Nanoropes can also provide strong pushing
forces. These pushing forces arise from the directed
growth of the filaments by the addition of molecular
building blocks. One important problem is to understand
the dependence of these pushing forces on the number
of filaments within the bundle. This research topic
is currently being pursued within the European network
on 'Active Biomimetic Systems'.
Original News can be found here
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