title: Ca2+ and ionic strength dependencies of S1-ADP binding to actin-tropomyosin-troponin: regulatory implications.
abstract: Skeletal and cardiac muscle contraction are inhibited by the actin-associated complex of tropomyosin-troponin. Binding of Ca(2+) to troponin or binding of ATP-free myosin to actin reverses this inhibition. Ca(2+) and ATP-free myosin stabilize different tropomyosin-actin structural arrangements. The position of tropomyosin on actin affects the binding of ATP-free myosin to actin but does not greatly affect myosin-ATP binding. Ca(2+) and ATP-free myosin alter both the affinity of ATP-free myosin for actin and the kinetics of that binding. A parallel pathway model of regulation simulated the effects of Ca(2+) and ATP-free myosin binding on both equilibrium binding of myosin-nucleotide complexes to actin and the general features of ATPase activity. That model was recently shown to simulate the kinetics of myosin-S1 binding but the analysis was limited to a single condition because of the limited data available. We have now measured equilibrium binding and binding kinetics of myosin-S1-ADP to actin at a series of ionic strengths and free Ca(2+) concentrations. The parallel pathway model of regulation is consistent with those data. In that model the interaction between adjacent regulatory complexes fully saturated with Ca(2+) was destabilized and the inactive state of actin was stabilized at high ionic strength. These changes explain the previously observed change in binding kinetics with increasing ionic strength.
authors.affiliation: Laboratory of Biological Modeling, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-5621, USA.
authors.forename: Boris
authors.initials: B
authors.lastname: Gafurov
authors.forename: Yi-Der
authors.initials: YD
authors.lastname: Chen
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Chalovich
mesh_terms: D000199:Actins / Q000737:chemistry* / Q000378:metabolism; D000244:Adenosine Diphosphate / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000737:chemistry; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005786:Gene Expression Regulation; D006868:Hydrolysis; D007477:Ions; D007700:Kinetics; D008958:Models, Molecular; D009218:Myosins / Q000737:chemistry*; D011485:Protein Binding; D011817:Rabbits; D013050:Spectrometry, Fluorescence; D014335:Tropomyosin / Q000737:chemistry*; D014336:Troponin / Q000737:chemistry*
chemical_list: D000199:Actins; D007477:Ions; D014335:Tropomyosin; D014336:Troponin; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D009218:Myosins; D002118:Calcium