The myosin catalytic domain does not rotate during the working power stroke

Actin Sliding Velocities are Influenced by the Driving Forces of Actin-Myosin Binding

Unloaded shortening speeds, V, of muscle are thought to be limited by actin-bound myosin heads that resist shortening, or V = a·d·τon-1 where τon-1 is the rate at which myosin detaches from actin and d is myosin's step size. The a-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 both V and actin-myosin strong binding kinetics suggesting a link between V and attachment kinetics. To determine whether these small molecules slow V by increasing resistance to actin sliding or by decreasing the efficiency of force transmission, a, we determine how inhibition of V by 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 Pi to a low (30 μM) [ATP] motility buffer V decreased from 1.8 to 1.3 μm·sec-1 and τbreak-1 from 0.029 to 0.018 sec-1. Upon addition of 50 μM blebbistatin to a low [ATP] motility buffer, V decreased from 1.0 to 0.7 μm·sec-1 and τbreak-1 from 0.059 to 0.022 sec-1. These results imply that blebbistatin and Pi slow V by decreasing force transmission, a, not by increasing resistive forces, implying that actin-myosin attachment kinetics influence V.

Polarized fluorescence depletion reports orientation distribution and rotational dynamics of muscle cross-bridges

The method of polarized fluorescence depletion (PFD) has been applied to enhance the resolution of orientational distributions and dynamics obtained from fluorescence polarization (FP) experiments on ordered systems, particularly in muscle fibers. Previous FP data from single fluorescent probes were limited to the 2(nd)- and 4(th)-rank order parameters, <P(2)(cos beta)> and <P(4)(cos beta)>, of the probe angular distribution (beta) relative to the fiber axis and <P(2d)>, a coefficient describing the extent of rapid probe motions. We applied intense 12-micros polarized photoselection pulses to transiently populate the triplet state of rhodamine probes and measured the polarization of the ground-state depletion using a weak interrogation beam. PFD provides dynamic information describing the extent of motions on the time scale between the fluorescence lifetime (e.g., 4 ns) and the duration of the photoselection pulse and it potentially supplies information about the probe angular distribution corresponding to order parameters above rank 4. Gizzard myosin regulatory light chain (RLC) was labeled with the 6-isomer of iodoacetamidotetramethylrhodamine and exchanged into rabbit psoas muscle fibers. In active contraction, dynamic motions of the RLC on the PFD time scale were intermediate between those observed in relaxation and rigor. The results indicate that previously observed disorder of the light chain region in contraction can be ascribed principally to dynamic motions on the microsecond time scale.

Mechanochemical coupling in spin-labeled, active, isometric muscle

Observed effects of inorganic phosphate (P(i)) on active isometric muscle may provide the answer to one of the fundamental questions in muscle biophysics: how are the free energies of the chemical species in the myosin-catalyzed ATP hydrolysis (ATPase) reaction coupled to muscle force? Pate and Cooke (1989. Pflugers Arch. 414:73-81) showed that active, isometric muscle force varies logarithmically with [P(i)]. Here, by simultaneously measuring electron paramagnetic resonance and the force of spin-labeled muscle fibers, we show that, in active, isometric muscle, the fraction of myosin heads in any given biochemical state is independent of both [P(i)] and force. These direct observations of mechanochemical coupling in muscle are immediately described by a muscle equation of state containing muscle force as a state variable. These results challenge the conventional assumption mechanochemical coupling is localized to individual myosin heads in muscle.

Synthesis of a spin-labeled photoaffinity ATP analogue, and its use to specifically photolabel myosin cross-bridges in skeletal muscle fibers

A spin-labeled photoaffinity ATP analogue 3'(2')-O-[4-[4-oxo-(4-amido-2,2,6,6-tetramethyl-piperidino-1-oxyl)]-benz oyl]benzoyl adenosine 5'-triphosphate (SL-Bz2ATP) was synthesized and used to photolabel myosin in muscle fibers. Previous work has shown that 3'(2')-O-(4-benzoyl)benzoyl adenosine 5'-triphosphate (Bz2ATP) photolabeled Ser-324 of the 50 kDa tryptic fragment of skeletal S1 heavy chain. In this work, [alpha-32P]SL-Bz2ATP was hydrolyzed and trapped as the diphosphate analogue with Co2+ and orthovanadate at the active site of myosin in rabbit psoas muscle fibers. After UV irradiation, the myosin heavy chain was the only protein band found to be significantly photolabeled as assayed by gel electrophoresis and radioactivity counting. The labeling was localized after brief trypsin digestion by SDS-PAGE to be on the 50 kDa tryptic fragment of the S1 heavy chain. Ca. 35% of the myosin in fibers was covalently photolabeled. The fibers photolabeled with SL-Bz2ATP had the same active tension and maximum shortening velocity as the control fibers. The resulting spin label on myosin was too mobile to report the orientation of the heads in fibers. Nonetheless, this is the first work to show the feasibility of utilizing active site binding and photoaffinity labeling to place covalent spectroscopic probes at the myosin active site in fibers with high specificity and yield without affecting mechanical function.

A strain-dependent ratchet model for [phosphate]- and [ATP]-dependent muscle contraction

A minimal strain-dependent ratchet model of muscle cross-bridge action is proposed which is broadly compatible with structural and kinetic constraints. Its essential features are: (1) dynamic binding of the S1-products complex to actin through a disorder-order transition coupled to the release of inorganic phosphate; (2) the absence of a force-generating rotation of the myosin head between the two force-holding states A.M.ADP and A.M; (3) strain-control of ADP release and ATP binding, giving net isometric tension and directed motility by the selective dissociation of negatively strained bound states. With a disordered pre-force state, the binding rate to state A.M.ADP need not be symmetric in x, the actin site displacement. With faster binding at positive x, the model predicts many steady-state and transient properties of striated muscle observed experimentally, including phases 2-4 of tension recovery from length changes and their dependence on excess phosphate (which enhances and accelerates phase 3) and reduced ATP (which gives a bimodal phase 2 and slows one mode). The response to large perturbations is often sensitive to the number of actin sites used, and to the inclusion of a 1 nm displacement of the neck region on release of ADP. The latter stabilizes the periodic tension behaviour produced by repeated releases.

Architecture and function in the muscle sarcomere

Striated muscle sarcomeres in vertebrates comprise ordered arrays of actin and myosin filaments, organized by an elaborate protein scaffold. Recent innovative work in a number of laboratories has greatly improved our knowledge of these structures, their organization and their interactions. Structural details have been reported on myosin filaments, actin filaments, Z-bands, M-bands, titin, and nebulin. Time-resolved X-ray diffraction and electron microscopy are revealing the molecular movements involved in force production and regulation.

ADP release produces a rotation of the neck region of smooth myosin but not skeletal myosin

Current theories of muscle cross-bridge function suggest that force is generated by a change in the orientation of the myosin neck region. We attached a paramagnetic probe to a subunit in the neck region and measured the orientation of the probe using electron paramagnetic resonance spectroscopy. The angle of the probes on smooth myosin S1 were changed by 20 degrees +/- 4 degrees on addition of ADP (50% effect at 5 +/- 2 microM), but ADP produced little effect on skeletal S1. The orientation of smooth myosin, +ADP, resembled that of skeletal myosin, +/- ADP, suggesting that the release of ADP generates an extra rotation of the neck region in smooth muscle at the end of its power stroke.