Myosins are biological molecular motors that glide along long filaments called actin to transport cargo within cells. Now researchers at the University of Pennsylvania School of Medicine have determined how a biological motor called myosin I responds to applied force [Laakso, et al., Science (2008) 321, 133]. The team attached each end of a filament to beads which were grabbed by optical tweezers. By moving the optical traps in opposite directions, tension could be applied to the actin which interacted with a myosin.

E. Michael Ostap explained how the single molecule measurements compare to more traditional experiments. “In the absence of load, the results are nearly identical to bulk measurements,” he said. “However, the single molecule measurements allow us to examine the mechanical properties in the presence of force, which was not possible in bulk measurements. We have found that the mechanical properties of myosin I are extremely sensitive to tension. That is, the kinetic properties of the motor activity are highly regulated by forces on the motor.”

The research group have determined that myosin I molecules translate along actin filaments in two substeps. One is a 5.1 nm substep that is insensitive to force, while the other is a 3.3 nm substep that is highly force sensitive. Ostep explained why scientists are keen to know more - humans alone express 39 different myosin genes. “The particular type of myosin we are studying has roles in controlling cell shape”.