Membrane-bound myosin IC drives the chiral rotation of the gliding actin filament around its longitudinal axis.

Myosin IC, a single-headed member of the myosin I family, specifically interacts with anionic phosphatidylinositol 4,5-bisphosphate (PI[4,5]P2) in the cell membrane via the pleckstrin homology domain located in the myosin IC tail. Myosin IC is widely expressed and physically links the cell membrane to the actin cytoskeleton; it plays various roles in membrane-associated physiological processes, including establishing cellular chirality, lipid transportation, and mechanosensing. In this study, we evaluated the motility of full-length myosin IC of Drosophila melanogaster via the three-dimensional tracking of quantum dots bound to actin filaments that glided over a membrane-bound myosin IC-coated surface. The results revealed that myosin IC drove a left-handed rotational motion in the gliding actin filament around its longitudinal axis, indicating that myosin IC generated a torque perpendicular to the gliding direction of the actin filament. The quantification of the rotational motion of actin filaments on fluid membranes containing different PI(4,5)P2 concentrations revealed that the rotational pitch was longer at lower PI(4,5)P2 concentrations. These results suggest that the torque generated by membrane-bound myosin IC molecules can be modulated based on the phospholipid composition of the cell membrane.

Three-dimensional tracking of the ciliate Tetrahymena reveals the mechanism of ciliary stroke-driven helical swimming.

Helical swimming in free-space is a common behavior among microorganisms, such as ciliates that are covered with thousands hair-like motile cilia, and is thought to be essential for cells to orient directly to an external stimulus. However, a direct quantification of their three-dimensional (3D) helical trajectories has not been reported, in part due to difficulty in tracking 3D swimming behavior of ciliates, especially Tetrahymena with a small, transparent cell body. Here, we conducted 3D tracking of fluorescent microbeads within a cell to directly visualize the helical swimming exhibited by Tetrahymena. Our technique showed that Tetrahymena swims along a right-handed helical path with right-handed rolling of its cell body. Using the Tetrahymena cell permeabilized with detergent treatment, we also observed that influx of Ca2+ into cilia changed the 3D-trajectory patterns of Tetrahymena swimming, indicating that the beating pattern of cilia is the determining factor in its swimming behavior.