title: Structure and flexibility of the tropomyosin overlap junction.
abstract: To be effective as a gatekeeper regulating the access of binding proteins to the actin filament, adjacent tropomyosin molecules associate head-to-tail to form a continuous super-helical cable running along the filament surface. Chimeric head-to-tail structures have been solved by NMR and X-ray crystallography for N- and C-terminal segments of smooth and striated muscle tropomyosin spliced onto non-native coiled-coil forming peptides. The resulting 4-helix complexes have a tight coiled-coil N-terminus inserted into a separated pair of C-terminal helices, with some helical unfolding of the terminal chains in the striated muscle peptides. These overlap complexes are distinctly curved, much more so than elsewhere along the superhelical tropomyosin cable. To verify whether the non-native protein adducts (needed to stabilize the coiled-coil chimeras) perturb the overlap, we carried out Molecular Dynamics simulations of head-to-tail structures having only native tropomyosin sequences. We observe that the splayed chains all refold and become helical. Significantly, the curvature of both the smooth and the striated muscle overlap domain is reduced and becomes comparable to that of the rest of the tropomyosin cable. Moreover, the measured flexibility across the junction is small. This and the reduced curvature ensure that the super-helical cable matches the contours of F-actin without manifesting localized kinking and excessive flexibility, thus enabling the high degree of cooperativity in the regulation of myosin accessibility to actin filaments.
authors.affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA 02118, USA; Computational Biochemistry Group, Interdisciplinary Center for Scientific Computing (IWR), University of Heidelberg, Im Neuenheimer Feld 368, Heidelberg D69120, Germany.
authors.forename: Xiaochuan Edward
authors.initials: XE
authors.lastname: Li
authors.affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA 02118, USA; Computational Biochemistry Group, Interdisciplinary Center for Scientific Computing (IWR), University of Heidelberg, Im Neuenheimer Feld 368, Heidelberg D69120, Germany.
authors.forename: Marek
authors.initials: M
authors.lastname: Orzechowski
authors.affiliation: Department of Physiology and Biophysics, Boston University School of Medicine, 72 East Concord Street, Boston, MA 02118, USA. Electronic address: wlehman@bu.edu.
authors.forename: William
authors.initials: W
authors.lastname: Lehman
authors.affiliation: Computational Biochemistry Group, Interdisciplinary Center for Scientific Computing (IWR), University of Heidelberg, Im Neuenheimer Feld 368, Heidelberg D69120, Germany. Electronic address: stefan.fischer@iwr.uni-heidelberg.de.
authors.forename: Stefan
authors.initials: S
authors.lastname: Fischer
mesh_terms: D000199:Actins / Q000737:chemistry; D000818:Animals; D018360:Crystallography, X-Ray; D006801:Humans; D008958:Models, Molecular; D056004:Molecular Dynamics Simulation; D018482:Muscle, Skeletal / Q000737:chemistry; D009130:Muscle, Smooth / Q000737:chemistry; D019906:Nuclear Magnetic Resonance, Biomolecular; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000737:chemistry; D014335:Tropomyosin / Q000737:chemistry*
chemical_list: D000199:Actins; D011993:Recombinant Fusion Proteins; D014335:Tropomyosin
keywords: Actin; Coiled-coil; Molecular Dynamics; Muscle regulation; Thin filaments