The "in vitro motility assay" and phalloidin-F-actin

We have compared the osmotic properties of the hydrated, native actin filament and of hydrated phalloidin-F-actin. We have found that phalloidin-F-actin interacts much more strongly with water than native F-actin. It is therefore very likely that the interaction with myosin (that requires the expulsion of the protein solvation water) is more problematic for phalloidin-F-actin that for native F-actin. We conclude that phalloidin-F-actin is not a bona fide substitute for native F-actin in the "in vitro motility assay".