title: Characterization of the equilibrium between blocked and closed states of muscle thin filaments.
abstract: We recently proposed a three-state model for the regulation of actomyosin interaction by tropomyosin and troponin (Tm.Tn). In this model, the thin filament exists in rapid equilibrium between the following three states: blocked, which cannot bind myosin significantly; closed, which can bind myosin weakly to form the A-state; open, which can bind both to form the A-state and isomerize to the strongly bound R-state. In this study, we demonstrate that the equilibrium between the blocked and closed states is calcium sensitive with an equilibrium constant, KB, of 0.3 and > or = 10 at pCa 8.9 and 4.6, respectively. The pCa dependence of KB is typical of that for calcium binding to thin filaments with a mid-point at pCa 5.6 and a Hill coefficient of 1.8. KB is independent of ionic strength over the range 0.4-0.06 M but increases dramatically below 0.05 M to > or = 10 at 0.01 M suggesting loss of the blocked state at low ionic strength. The blocked state also has reduced occupancy at high temperatures. KB, in the absence of calcium, increases from 0.2 at 5 degrees C to 0.6 at 40 degrees C.
authors.affiliation: Max-Planck-Institut für molekulare Physiologie, Dortmund, Germany.
authors.forename: J G
authors.initials: JG
authors.lastname: Head
authors.forename: M D
authors.initials: MD
authors.lastname: Ritchie
authors.forename: M A
authors.initials: MA
authors.lastname: Geeves
mesh_terms: D000199:Actins / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D009124:Muscle Proteins / Q000737:chemistry / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000378:metabolism*; D015879:Myosin Subfragments / Q000737:chemistry / Q000378:metabolism; D009994:Osmolar Concentration; D011817:Rabbits; D013816:Thermodynamics; D014335:Tropomyosin / Q000378:metabolism; D014336:Troponin / Q000378:metabolism
chemical_list: D000199:Actins; D009124:Muscle Proteins; D015879:Myosin Subfragments; D014335:Tropomyosin; D014336:Troponin; D002118:Calcium