Three-dimensional image analysis of myosin head in function as captured by quick-freeze deep-etch replica electron microscopy.

Quick-freeze deep-etch replica electron microscopy combined with mica-flake technique provides high contrast, high time- and spatial-resolution images of protein molecules in solution, whose three-dimensional structure is well preserved. Thus, it might be quite useful to obtain structural information of individual functioning molecules, such as myosin crossbridges under in vitro motility assay conditions. With that method, we could actually show that both heads of heavy meromyosin (HMM) crossbridges are mostly straight and bound to actin filaments with about 45 degree tilt-angle under rigor conditions, whereas they attached to actin through only one head with a wide variety of angles under in vitro sliding conditions. We also demonstrated that free HMM heads are strongly kinked in the presence of ATP or ADP/inorganic vanadate (Vi) in contrast to almost straight configuration in the absence of nucleotide. To examine more detailed structure of individual crossbridges, we tried to reconstruct the three-dimensional architecture of intramolecular subdomains of single HMM molecule. We took a series of tilted images of single HMM-ADP/Vi particle and successfully obtained its 3-D image by filtered back-projection, even with restricted range of tilt-angles. By comparison of the reconstruction with the atomic model of subfragment-1 (S1) without nucleotide, we found some great structural difference, which partly might be attributable to the conformational change by nucleotide binding.