Journal Club

Journal Club

Solid friction between soft filaments, Ward et al., Nature Materials (August 2015)



Moderator: Dhananjay T. Tambe

Category: Scientific article/RSF interest

Cellular contraction, migration, division, and barrier function are amongst the processes that involve structural reorganization of actin filament bundles.  Such actin filament bundles are formed due to macromolecular crowding of cellular cytoplasm.  But friction between the actin filaments is widely believed to be similar to the friction between structureless filament bundles that are formed due to high ionic strength or high depletant concentration of suspension medium.  According to this model, the inter-filament frictional force is linearly dependent on both sliding velocity and overlap length, and the magnitude of the frictional force is few fN.  However, with the first direct measurement of frictional forces between actin filaments, Ward et al demonstrated that above model of the actin filament bundle requires qualitatively revision.  The frictional force between the bundled actin filaments exhibited four unanticipated properties.  (1) The friction force and the sliding velocity had a relationship that was not linear but instead logarithmic.  (2) The friction force was not linearly dependent on the overlap length but instead, after a critical value of the force, independent of the overlap length.  (3) The friction force was not independent of the orientation of the filament, but instead the force between antiparallel actin filaments was twice that between the parallel actin filaments.  (4) The magnitude of frictional force was not in fN as dictated by dominant hydrodynamic friction but instead in pN (i.e. 1000 times larger) as dictated by dominant solid-like friction.  In the light of these new findings, the physical processes that govern frictional coupling between the actin filaments and its significance in cellular contraction, migration, division and barrier function will require major reassessment.

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