Distinctions between mechanisms of cytochalasin D activity for Mg2+- and K+-induced actin assembly.

The fraction of assembled actin and the diffusion coefficients of filamentous and nonfilamentous species have been determined by fluorescence photobleaching recovery. For both Mg2+-induced and K+-induced actin assembly, a higher concentration of cation leads to longer filaments. Cytochalasin D reduces the fraction of actin present as assembled filaments. In the absence of Mg2+, the accompanying increase in diffusion coefficient of the filaments is of an appropriate magnitude to be accounted for by shortening of filaments as a result of net depolymerization. In the presence of Mg2+, cytochalasin D induces a dose-dependent increase in diffusion coefficient up to about a factor of 10. This increase indicates a shortening of filaments consistent with extensive filament cleavage. Under all conditions studied, the unassembled actin is present primarily as monomer.