Inter-filament Attractions Narrow the Length Distribution of Actin   Filaments
  We show that the exponential length distribution that is typical of actin filaments under physiological conditions dramatically narrows in the presence of (i) crosslinker proteins (ii) polyvalent counterions or (iii) depletion mediated attractions. A simple theoretical model shows that in equilibrium, short-range attractions enhance the tendency of filaments to align parallel to each other, eventually leading to an increase in the average filament length and a decrease in the relative width of the distribution of filament lengths. 
David Biron, Elisha Moses, Itamar Borukhov and S. A. Safran (Weizmann   Institute of Science, Israel)