title: Two different rigor complexes of myosin subfragment 1 and actin.
abstract: Our previous titration and cross-linking experiments showed that myosin subfragment 1 (S1) can bind to one or two monomers in F-actin [Andreev, O. A., & Borejdo, J. (1991) Biochem. Biophys. Res. Commun. 177, 350-356; (1992a) J. Muscle Res. Cell Motil. 13, 523-533; (1992b) Biochem. Biophys. Res. Commun. 188, 94-101]. In the present work we used a sedimentation method to extend these studies to equilibrium binding and a stopped flow method to investigate its kinetics. Both equilibrium and kinetic data indicated the existence of two different rigor complexes. On the basis of these data we developed a model which suggested that binding of S1 to F-actin occurred in two steps: (i) initial rapid binding to one monomer of F-actin, A + M<==>A.M and (ii) a consequent slow binding to a neighboring monomer, A.M + A<==>A.M.A, where A stands for actin and M for myosin subfragment 1. The second reaction can proceed only if the neighboring actin site is unoccupied. The model fit the equilibrium and kinetic binding data with equilibrium constants K1 = 6 x 10(6) M-1 and K2 = 4 and kinetic constants k+1 = 10.5 x 10(6) M-1 s-1, k-1 = 1.75 s-1, k+2 = 0.8 s-1, and k-2 = 0.2 s-1, where the subscripts refer to the reactions i and ii. These results corroborate our hypothesis that myosin head can make two types of complexes with F-actin and support our speculation that during a power stroke in contracting muscle a myosin head may first bind to one and then to two actins.
authors.affiliation: Baylor Research Institute, Baylor University Medical Center, Dallas, Texas 75226.
authors.forename: O A
authors.initials: OA
authors.lastname: Andreev
authors.forename: A L
authors.initials: AL
authors.lastname: Andreeva
authors.forename: V S
authors.initials: VS
authors.lastname: Markin
authors.forename: J
authors.initials: J
authors.lastname: Borejdo
mesh_terms: D000199:Actins / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D002498:Centrifugation; D007700:Kinetics; D015879:Myosin Subfragments / Q000378:metabolism*; D011817:Rabbits
chemical_list: D000199:Actins; D015879:Myosin Subfragments