Axial Rotation of an Actin Filament Revealed by Single-Fluorophore Imaging
  

















  















  
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A single fluorophore of tetramethylrhodamine was attached to an actin filament, which lay on a surface coated with myosin.  The angle between the long axis of the fluorophore and filament axis happened to be ~45°.  When ATP was added, the filament slid towatd upper left, at ~45° in the image plane.  To detect the orientation of the fluorophore, we decomposed the fluorescence into vertically (V) and horizontally (H) polarized components, and imaged the two simultaneously.  As seen in the movie, the fluorescent spot appeared altenately in the V and H images, showing that the fluorophore assumed vertical and horizontaly orientations alternately.  The alternation was quite regular, and was synchronous with the translational movement toward upper left.  The interpretation is that the actin filament rotated around its axis as it moved past myosin (figure, a).  The filament made one turn per sliding distance of 1 μm.  This rotational pitch is much larger than the helical pitch of actin of 72 nm, suggesting that myosin 'runs' rather than 'walks' on actin.
  
                                                      Sase, I. et al., Proc. Natl. Acad. Sci. USA 94, 5646-5650 (1997).