Polymer-induced bundling ofFactin and the depletion force

The inert polymer polyethylene glycol (PEG) induces a “bundling” phenomenon in $F$ actin solutions when its concentration exceeds a critical onset value $C_o$. Over a limited range of PEG molecular weight and ionic strength, $C_o$ can be expressed as a function of these two variables. The process is reversible, but hysteresis is also observed in the dissolution of the bundles, with ionic strength having a large influence. Additional actin filaments are able to join the previously formed bundles. PEG polymers are not incorporated into the actin bundles. Estimates of the Asakura-Oosawa depletion force, Coulomb repulsion, and van der Waals potential are combined in order to explain the bundling effect and hysteresis. Conjectures are presented concerning the apparent limit in bundle size.