Cytochalasin B strongly inhibits certain forms of cell motility, and it dramatic
ID: 176320 • Letter: C
Question
Cytochalasin B strongly inhibits certain forms of cell motility, and it dramatically decreases the viscosity of gels formed with actin and actin-binding proteins. In the classic experiment that defined its mechanism of action, short lengths of actin filaments were first treated with the S1 subfragment of myosin, which allows you to determine the length of the actin filaments at the beginning of the experiment. These “decorated” actin filaments were then mixed with actin subunits in the presence or absence of cytochalasin B. Assembly of actin filaments was measured by assaying viscosity of the solution (fig 1) and by examining samples by electron microscopy (fig 2). Suggest a plausible mechanism to explain how cytochalasin B inhibits filament assembly. Account for appearance by electron microscopy and viscosity (rate of change in viscosity and ultimate levels of viscosity reached).
0.8 Z 0.6 0.4 0.2 0.0 -k cytochalasin B cytochalasin B 10 15 20 time (minutes)Explanation / Answer
Cytochalasin B inhibits actin filament polymerization by binding to the growing end of the polymer. Inhibition affects all three stages: nucleation is initiation, elongation and annealing. As the growth of polymer slows down, it no more pushes through the surrounding liquid, thus no increase in viscosity. Thus, no more actin molecules joined once cytochalasin B is present.
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