Phalloidin affects the myosin-dependent sliding velocities of actin filaments in a bound-divalent cation dependent manner

We examined sliding velocities in vitro of four types of actin filaments, t hat is, filaments with Ca2+ or Mg2+ bound at the high affinity metal bindin g site, each with rhodamine phalloidin bound with a high or low stoichiomet ry. When surfaces coated with a high density of heavy meromyosin (HMM) were used, high stoichiometric concentrations of rhodamine phalloidin reduced s liding velocities of only Ca2+-actin filaments, by 40%. As the HMM density on surfaces was reduced, continuous movement of actin filaments became depe ndent on the presence of methylcellulose and sliding velocities of all four types became progressively slower. Interestingly, Ca2+-actin filaments wit h a high stoichiometric concentration of rhodamine phalloidin were the fast est among the four types of filaments on sparse HMM surfaces. In contrast, phalloidin did not affect steady state ATPase activities of HMM in the pres ence of Ca2+- or Mg2+-actin filaments. We speculate that the reversal of th e order of sliding velocities among the four types of actin filaments betwe en high and low densities of HMM relates with different axial elasticity of the actin filaments, so that stiffer filaments move slower on dense HMM su rfaces, but faster on sparse surfaces, than elastic ones.