title: Actin Sliding Velocities are Influenced by the Driving Forces of Actin-Myosin Binding.
abstract: Unloaded shortening speeds,V, of muscle are thought to be limited by actin-bound myosin heads that resist shortening, orV=a·d·τon-1whereτon-1is the rate at which myosin detaches from actin anddis myosin's step size. Thea-term describes the efficiency of force transmission between myosin heads, and has been shown to become less than one at low myosin densities in a motility assay. Molecules such as inorganic phosphate, Pi, and blebbistatin inhibit bothVand actin-myosin strong binding kinetics suggesting a link betweenVand attachment kinetics. To determine whether these small molecules slowVby increasing resistance to actin sliding or by decreasing the efficiency of force transmission,a, we determine how inhibition ofVby Pi and blebbistatin changes the force exerted on actin filaments during an in vitro sliding assay, measured from changes in the rate,τbreak-1, at which actin filaments break. Upon addition of 30 mM Pito a low (30 μM) [ATP] motility bufferVdecreased from 1.8 to 1.3 μm·sec-1andτbreak-1from 0.029 to 0.018 sec-1. Upon addition of 50 μM blebbistatin to a low [ATP] motility buffer,Vdecreased from 1.0 to 0.7 μm·sec-1andτbreak-1from 0.059 to 0.022 sec-1. These results imply that blebbistatin and PislowVby decreasing force transmission,a, not by increasing resistive forces, implying that actin-myosin attachment kinetics influenceV.
authors.affiliation: University of Nevada Reno School of Medicine, Department of Biochemistry and Molecular Biology, Reno, NV 89557.
authors.forename: Travis J
authors.initials: TJ
authors.lastname: Stewart
authors.forename: Del Ray
authors.initials: DR
authors.lastname: Jackson
authors.forename: Ryan D
authors.initials: RD
authors.lastname: Smith
authors.forename: Steven F
authors.initials: SF
authors.lastname: Shannon
authors.forename: Christine R
authors.initials: CR
authors.lastname: Cremo
authors.forename: Josh E
authors.initials: JE
authors.lastname: Baker