title: A mosaic multiple-binding model for the binding of caldesmon and myosin subfragment-1 to actin.
abstract: Binding of caldesmon to actin causes a decrease in the quantity of bound myosin and results in a reduction in the rate of actin-activated adenosine triphosphate hydrolysis. It is generally assumed that the binding of caldesmon and myosin to actin is a pure competitive interaction. However, recent binding studies of enzyme digested caldesmon subfragments directed at mapping the actin binding site of caldesmon have shown that a small 8-kD fragment around the COOH-terminal can compete directly with the myosin subfragment 1 (S-1) binding to actin; at least one other fragment that binds to actin does not inhibit the actin-activated adenosine triphosphate activity of myosin. That is, only a part of the caldesmon sequence may be responsible for directly blocking the binding of S-1 to actin. This prompts us to question the actual mode of binding of intact caldesmon and myosin S-1 to actin: whether the entire intact caldesmon molecule is competing with S-1 binding (pure competitive model) or just a small part of it (mosaic multiple-binding model). To answer this question, we measured the amount of myosin S-1 and caldesmon bound per actin monomer as a function of the total concentration of S-1 added to the system at constant concentrations of actin and caldesmon. A formalism for calculating the titration data based on the pure competitive model and a mosaic multiple-binding model was then developed. When compared with theoretical calculations, it is found that the binding of caldesmon and S-1 to actin cannot be pure competitive if no cooperativity exists between S-1 and caldesmon. In contrast, the mosaic multiple-binding model can fit the binding data rather well regardless of the existence of cooperativity between S-1 and caldesmon.
authors.affiliation: Laboratory of Chemical Physics, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: Y D
authors.initials: YD
authors.lastname: Chen
authors.forename: J M
authors.initials: JM
authors.lastname: Chalovich
mesh_terms: D000199:Actins / Q000737:chemistry / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D002148:Calmodulin-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D005895:Gizzard, Avian; D007700:Kinetics; D008433:Mathematics; D008961:Models, Structural; D008962:Models, Theoretical; D009130:Muscle, Smooth / Q000378:metabolism; D009132:Muscles / Q000378:metabolism; D015879:Myosin Subfragments / Q000737:chemistry / Q000378:metabolism*; D011485:Protein Binding; D051381:Rats; D014335:Tropomyosin / Q000737:chemistry / Q000378:metabolism; D014422:Turkeys
chemical_list: D000199:Actins; D002148:Calmodulin-Binding Proteins; D015879:Myosin Subfragments; D014335:Tropomyosin