6th Annual Symposium
Physics of Cancer
Leipzig, Germany
September 7-9, 2015
Contributed Talk
Time dependent actin bundling points beyond the classical filament image
Timo Maier1,2, Tamás Haraszti1,2, Joachim Pius Spatz1,2
1Max Planck Institute for Intelligent Systems, Department of New Materials and Biosystems, Heisenberg str. 3, D-70569 Stuttgart, Germany
2University of Heidelberg, Institute of Physical Chemistry, Department of Biophysical Chemistry, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany
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Filamentous actin plays crucial roles in cellular adhesion, motility and determining the elastic properties and the shape of cells. Its interaction to various proteins and to divalent cations is a determinant factor in the mechanical properties of the formed network, thus in the funciton of the actin cytoskeleton.

The fundamental physical chemistry of in vitro actin gels has been studied for about 3 decates. In recent years, it has become clear that there is a contradiction in the description of actin filaments in simple, divalent electrolyte solutions. It is known, that mixing actin with divalent cations during polymerization, bundles are formed above a critical electrolyte concentration (about 27 mM for magnesium). The process is described by counter ion condensation, considering the actin as a linear, charged, semiflexible chain.

We have investigated this process using 2-dimensional networks of prepolymerized actin filaments either on microparticles or on the top of micropillars. The experiment was constructed such, that it allowed for separating the polymerization and bundling processes. In qualitative analysis we have seen stable (Y-shaped) splay configuration of filaments after the bundling. Experiments using holographic optical tweezers, however, indicated forces of the order of 0.1-0.25 pN driving the process [1].
To resolve this contradiction, we investigated the time dependence of the bundling at low, but constant (2-12mM) magnesium concentrations, using the thermal motion of tracer particles attached to the bundles and fluorescence microscopy.

We first characterized the bunles with particle tracking microrheology, solving the problem of spline-like interpolation for converting the creep compliance to the complex shear modulus. (Which tool has more general applications in microrheology.)

Finally, we have found that the divalent magnesium ions preferentially adsorb to the filaments, resulting in bundling even at these low concentrations. The resulted bundling is not reversible even after removing the magnesium and adding EDTA and EGTA to the solution for several hours.
[1]M. Streichfuss, F. Erbs, K. Uhrig, R. Kurre, A. E. Clemen, and H. J. C. Böhm, T. Haraszti, J. P. SpatzMeasuring Forces between Two Single Actin Filaments during Bundle Formation, Nano Letters 11: 3676 – 3680. (2011)
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