An Obsevation of Native Skeletal Thin Filament Structure at High Calcium Condition by Electron Cryomicroscopy

It is hampering our understanding on the molecular mechanism of muscle contraction thin filament-linked modulation lack of electron cryomicryoscopy data supporting the steric model (Nature 368,64-67,1994). Recently well-oriented sols of flamentous (F)-actin formed by Ca2+-actin and length controlled by gelsolin give detailed X-ray fibre diffraction patterns led to the flat-actin conformation resolved in F-actin (Nature 457,441-550,2009). von der Eckon et al (Nature,519,114-119,2015) defined a three-dimensional structure of F-actin at a resolution of 3.7A and in synthetic complex with tropomyosin (no troponin complex) at a resolution of 6.5A using electron cryomicroscopy. Here is the first report about a well-defined structure of native skeletal thin filament at high calcium condition by electron cryomicroscopy with Iterative Helical Real-Space Reconstruction (IHRSR): tropomyosin locates C-state on actin, spreads over one of myosin subfragment-1 (S1) strongly binding sites between subdomain_I and_III of the actin (JMB,266,8-14,1997); one intra-strand contact with strong 3D-reconstructed density and more than one inter-strand contacts are resolved among neighboring actin monomers (n, n+1, n+2), mainly formed through actin hydrophobic plug 256-271 linked to various regions of the opposite actin subunits, different from those of S1 decorated F-actin and the published F-actin and F-actin-Tropomyosin complex, in the native skeletal thin filament suggesting actin conformation diversity—globular-actin conformation dominating in high calcium condition, the flat conformation coexisting, and the two identified actin conformations are dynamic in thin filaments during the modulation of skeletal muscle contraction.