Fluorescent phalloidin enables visualization of actin without effects on myosin's actin filament sliding velocity and hydrolytic properties in vitro.

Recent reports have demonstrated an activating effect of phalloidin in striated muscle. Furthermore, modeling of X-ray diffraction and crystallographic data suggests that phalloidin binding may induce conformational changes in actin. To determine whether phalloidin affects the mechanics of the actomyosin interaction, the velocity of actin filaments variably labeled with rhodamine-phalloidin was measured. In addition, solution actin-activated myosin subfragment-1 ATPase activity with phalloidin-labeled actin was compared to unlabeled actin. Here we found that phalloidin does not significantly effect actin filament velocity or parameters of ATPase, namely Vmax and K(m). Possible differences between muscle strip data and these in vitro results are discussed.