title: The role of three-state docking of myosin S1 with actin in force generation.
abstract: It has been shown that in solution myosin subfragment 1 binds to actin in three principal steps: [formula: see text] The nucleotide bound to myosin has a major influence on the equilibrium constant of the third of these steps but little effect on the other two. The third step is thought to be coupled to the force-generating event. Three-step binding and structure: The formation of the collision complex is strongly ionic strength dependent but independent of temperature. The isomerization to the A state is not strongly dependent on ionic strength but is affected by organic solvent and temperature. In contrast the isomerization to the R state-is affected by both ionic strength and organic solvent but little affected by temperature. The recent docking of the three-dimensional structures of actin and S1 suggest possible structural correlates of these events. These studies lead to predictions for the docking process, which may be tested using site-directed mutagenesis or peptide inhibitors. Three-step binding and head-head interactions: Studies of HMM binding to actin compared with S1 binding show that binding of two heads in the A state are unlikely presumably because of strain effects. However, binding of two heads as one A and one R state shows little evidence of strain while the isomerization of the second head to give two R states is fivefold weaker than for an isolated S1 head. These results suggest that in a rapidly shortening muscle only one head is likely to be attached at a time. Under isometric conditions, although it is possible for both heads to bind to adjacent actins, it is unlikely that both will be in the force holding R state simultaneously. Three-step binding and regulation by tropomyosin-troponin:Our recent solution studies have established that the thin filament can exist in three calcium-dependent states which we termed blocked, closed and open. A blocked state cannot form the A state with S1 and a closed state cannot form the force holding R state nor accelerate product release from S1. Thus control operates at two distinct points in the docking process. The docking process itself is coupled to hydrolysis of ATP (the A-to-R isomerization is inhibited by the presence of the gamma Pi on ATP), and therefore all of these events are interrelated.The coming together of these different strands provides a biochemical framework that should allow the dynamic properties of the crossbridge in muscle to be understood.
authors.affiliation: Max Planck Institute for Molecular Physiology, Dortmund, Germany.
authors.forename: M A
authors.initials: MA
authors.lastname: Geeves
authors.forename: P B
authors.initials: PB
authors.lastname: Conibear
mesh_terms: D000199:Actins / Q000737:chemistry / Q000378:metabolism* / Q000502:physiology; D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D004734:Energy Metabolism; D066298:In Vitro Techniques; D008954:Models, Biological; D015394:Molecular Structure; D009119:Muscle Contraction / Q000502:physiology*; D009132:Muscles / Q000378:metabolism; D015879:Myosin Subfragments / Q000737:chemistry / Q000378:metabolism* / Q000502:physiology; D009994:Osmolar Concentration; D010710:Phosphates / Q000378:metabolism
chemical_list: D000199:Actins; D015879:Myosin Subfragments; D010710:Phosphates; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate