Salt-induced condensation in actin-DNA mixtures

Multivalent ions can induce condensation of like-charged polyelectrolytes into compact states, a process that requires different ion valence for different polyelectrolyte species. We have examined the trivalent ion-induced condensation behavior in binary anionic polyelectrolyte mixtures consisting of DNA coils and F-actin rods, and observe a micro-phase separation between the two polyelectrolytes into coexisting finite-sized F-actin bundles and DNA toroids. Further, by increasing the DNA volume fraction in the mixture, condensed F-actin bundles can be completely destabilized, leading to only DNA condensation within the mixture. We examine a number of possible causes and propose a model based on polyelectrolyte competition for ions.