title: Tropomyosin movement is described by a quantitative high-resolution model of X-ray diffraction of contracting muscle.
abstract: Contraction of skeletal and cardiac muscle is controlled by Ca2+ions via regulatory proteins, troponin (Tn) and tropomyosin (Tpm) associated with the thin actin filaments in sarcomeres. In the absence of Ca2+, Tn-C binds actin and shifts the Tpm strand to a position where it blocks myosin binding to actin, keeping muscle relaxed. According to the three-state model (McKillop and Geeves Biophys J 65:693-701, 1993), upon Ca2+binding to Tn, Tpm rotates about the filament axis to a 'closed state' where some myosin heads can bind actin. Upon strong binding of myosin heads to actin, Tpm rotates further to an 'open' position where neighboring actin monomers also become available for myosin binding. Azimuthal Tpm movement in contracting muscle is detected by low-angle X-ray diffraction. Here we used high-resolution models of actin-Tpm filaments based on recent cryo-EM data for calculating changes in the intensities of X-ray diffraction reflections of muscle upon transitions between different states of the regulatory system. Calculated intensities of actin layer lines provide a much-improved fit to the experimental data obtained from rabbit muscle fibers in relaxed and rigor states than previous lower-resolution models. We show that the intensity of the second actin layer line at reciprocal radii from 0.15 to 0.3 nm-1quantitatively reports the transition between different states of the regulatory system independently of the number of myosin heads bound to actin.
authors.affiliation: Institute of Mechanics, M.V. Lomonosov Moscow University, 1 Michurinsky prosp., Moscow, 119192, Russia. natalia@imec.msu.ru.
authors.forename: Natalia A
authors.identifier: http://orcid.org/0000-0002-0238-4920
authors.initials: NA
authors.lastname: Koubassova
authors.affiliation: Institute of Immunology and Physiology, Ural Branch of the Russian Academy of Sciences, Yekaterinburg, Russia.
authors.forename: Sergey Y
authors.initials: SY
authors.lastname: Bershitsky
authors.affiliation: Lee Kong Chian School of Medicine, Nanyang Technological University, Singapore, Singapore.
authors.forename: Michael A
authors.initials: MA
authors.lastname: Ferenczi
authors.affiliation: European Synchrotron Radiation Facility, Grenoble, France.
authors.forename: Theyencheri
authors.initials: T
authors.lastname: Narayanan
authors.affiliation: Institute of Mechanics, M.V. Lomonosov Moscow University, 1 Michurinsky prosp., Moscow, 119192, Russia.
authors.forename: Andrey K
authors.initials: AK
authors.lastname: Tsaturyan
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D000818:Animals; D008958:Models, Molecular; D009068:Movement; D009119:Muscle Contraction; D009218:Myosins / Q000378:metabolism; D011487:Protein Conformation; D011817:Rabbits; D014335:Tropomyosin / Q000737:chemistry / Q000378:metabolism*
chemical_list: D014335:Tropomyosin; D009218:Myosins
keywords: Actin; Modeling; Myosin heads; Tropomyosin; X-ray diffraction