Half a century has passed since the cross-bridge structure was recognized as the molecular machine that generates muscle tension. Despite various approaches by a number of scientists, information on the structural changes in the
myosin heads, particularly its transient configurations, remains scant even now, in part because of their small size and rapid stochastic movements during the power
stroke. Though progress in cryo-electron microscopy is eagerly awaited as the ultimate means to elucidate structural details, the introduction of some unconventional methods that provide high-contrast raw images of the target
protein assemblies is quite useful, if available, to break the current impasse. Quick-freeze deep⁻etch⁻replica electron microscopy coupled with dedicated image analysis procedures, and high-speed atomic-force microscopy are two such candidates. We have applied the former to visualize actin-associated
myosin heads under in vitro motility assay conditions, and found that they take novel configurations similar to the SH1⁻SH2-crosslinked
myosin that we characterized recently. By incorporating biochemical and biophysical results, we have revised the cross-bridge mechanism to involve the new conformer as an important main player. The latter “microscopy” is unique and advantageous enabling continuous observation of various
protein assemblies as they function. Direct observation of
myosin-V’s movement along actin filaments revealed several unexpected behaviors such as foot-stomping of the leading head and unwinding of the coiled-coil tail. The potential contribution of these methods with intermediate spatial resolution is discussed.