Cytochalasin B strongly inhibits certain forms of cell motility, such as cytokinesis and the ruffling of growth cones, and it dramatically decreases the viscosity of gels formed with mixtures of actin and a wide variety of actin-binding proteins. These observations suggest that cytochalasin B interferes with the assembly of actin filaments. In the classic experiment that defined its mechanism, short lengths of actin filaments were decorated with myosin heads and then mixed with actin subunits in the presence or absence of cytochalasin B. Assembly of actin filaments was measured by assaying the viscosity of the solution (Fig A) and by examining samples by electron microscopy (Fig B).
A. Suggest a plausible mechanism to explain how cytochalasin B inhibits actin filament assembly. Account for the appearance of the filaments in the electron micrographs and the viscosity measurements (both the altered rate and extent).
B. The normal growth characteristics of an actin filament and the actin-binding properties of cytochalasin B argue that actin monomers undergo a conformational change upon addition to an actin filament. How so?