Fluctuation of local points of F-actin sliding on the surface-fixed H-meromyosin molecules in the presence of ATP Original Research Article

F-actin fragments fluorescently labeled with rhodamine–phalloidin were copolymerized with non-labeled F-actin fragments. F-actin copolymer consisted of several bright (fluorescent) and dark (non-fluorescent) stripes of approximately 1 μm in width. Local motion of individual speckled F-actin was investigated by measuring translocation fluctuation of several tracing points marked on the actin filament. The tracing points included the borders between neighboring bright and dark stripes, as well as the tip and tail of the filament. For speckled F-actin with an average sliding speed of 4.6 μm/s at 23°C, the translocation distance of the tracing points (per 0.1 s) showed significant fluctuation, of the order of ±0.12 μm/s, approximately 25% of the sliding speed. The fluctuation correlation of the translocation distance between two tracing points decreased as the distance between them increased. Statistical analysis of the correlation length of the translocation distance Lc showed that Lc increased with the sliding speed of the actin filament. The sliding speed, however, saturated as the correlation length became close to the persistence length of the bending elasticity of F-actin. On the contrary, the correlation length of change in the translocation direction was essentially equal to the persistence length of F-actin, independent of the sliding speed. These results suggest that elasticity of the actin filament underlies the sliding velocity of F-actin.