title: The myosin head can bind two actin monomers.
abstract: Force impulse is thought to be generated in muscle when myosin head (S-1), while weakly bound to actin filament, undergoes orientational change to form a strong (rigor) bond with actin. There is ample evidence that this bond involves interaction of 1 myosin head with 1 actin monomer. However, X-ray diffraction data of muscle decorated with S-1, as well as recently proposed model of the thin filaments, suggested that each S-1 molecule interacted with two actin monomers. We reinvestigated this controversy and found that the stoichiometry of acto-S-1 bond depended on the relative amounts of actin and myosin present during titrations: when increasing amounts of actin were added to a fixed amount of S-1 (i.e. when myosin heads were initially in excess over actin), the saturating stoichiometry was 1 mol of S-1 per 1 mol of actin. However, when increasing amounts of S-1 were added slowly to a fixed amount of F-actin (i.e. when actin was initially in excess over S-1), the stoichiometry at saturation was 1 mol of S-1 per 2 mols of actin. The ability of S-1 to bind either one or two actin monomers suggests a way that force could be generated during muscle contraction.
authors.affiliation: Baylor Research Foundation, Baylor University Medical Center, Dallas, TX 75226.
authors.forename: O A
authors.initials: OA
authors.lastname: Andreev
authors.forename: J
authors.initials: J
authors.lastname: Borejdo
mesh_terms: D000199:Actins / Q000378:metabolism*; D000818:Animals; D003432:Cross-Linking Reagents; D005456:Fluorescent Dyes; D007700:Kinetics; D046911:Macromolecular Substances; D009132:Muscles / Q000378:metabolism; D015879:Myosin Subfragments / Q000378:metabolism*; D009282:Naphthalenesulfonates; D009391:Nephelometry and Turbidimetry / Q000379:methods; D011487:Protein Conformation; D011817:Rabbits; D013050:Spectrometry, Fluorescence / Q000379:methods
chemical_list: D000199:Actins; D003432:Cross-Linking Reagents; D005456:Fluorescent Dyes; D046911:Macromolecular Substances; D015879:Myosin Subfragments; D009282:Naphthalenesulfonates; C006421:1,5-I-AEDANS