Figure 9.49 Actin assembly in vitro. (a)Electron micro- graph of a short actin filament that was labeled with S1 myosin and then used to nucleate actin polymerization. The addition of actin subunits occurs much more rapidly at the barbed (plus)end than at the pointed (minus)end of the ex- isting filament. (b) Schematic diagram of the preferential ad dition of actin subunits at the plus end of a microfilament and their preferential loss from the minus end in an in vitro assay. As a result, subunits treadmill through the filament in vitro. (a: Courtesy of M. S. Runge and T. D. Pollard