MgATP binding changes neither the shape nor the flexibility of the heads of single myosin molecules

The structural mechanism by which myosin heads exert force is unknown. One possibility is that the tight binding of the heads to actin drives them into a force-generating configuration. Another possibility is that the force-generating conformational change is inherent to the myosin heads. In this case the heads would make force by changing their shape according to the species of nucleotide in their active sites, the tight attachment to actin serving only to provide traction. To test this latter possibility, we used negative stain electron microscopy to search for a MgATP-induced shape change in the heads of single myosin molecules. We compared the heads of 10S smooth muscle myosin monomers (wherein MgATP is trapped at the active site) with the MgATP-free heads of 6S monomers. We found that to a resolution of about 2 nm, MgATP binding to the unrestrained myosin head does not drive it to change its shape or its flexibility. This result suggests that the head makes force by virtue of an induced fit to actin.