title: Velocities of unloaded muscle filaments are not limited by drag forces imposed by myosin cross-bridges.
abstract: It is not known which kinetic step in the acto-myosin ATPase cycle limits contraction speed in unloaded muscles (V0). Huxley's 1957 model [Huxley AF (1957) Prog Biophys Biophys Chem 7:255-318] predicts that V0 is limited by the rate that myosin detaches from actin. However, this does not explain why, as observed by Bárány [Bárány M (1967) J Gen Physiol 50(6, Suppl):197-218], V0 is linearly correlated with the maximal actin-activated ATPase rate (vmax), which is limited by the rate that myosin attaches strongly to actin. We have observed smooth muscle myosin filaments of different length and head number (N) moving over surface-attached F-actin in vitro. Fitting filament velocities (V) vs. N to a detachment-limited model using the myosin step size d=8 nm gave an ADP release rate 8.5-fold faster and ton (myosin's attached time) and r (duty ratio) ∼10-fold lower than previously reported. In contrast, these data were accurately fit to an attachment-limited model, V=N·v·d, over the range of N found in all muscle types. At nonphysiologically high N, V=L/ton rather than d/ton, where L is related to the length of myosin's subfragment 2. The attachment-limited model also fit well to the [ATP] dependence of V for myosin-rod cofilaments at three fixed N. Previously published V0 vs. vmax values for 24 different muscles were accurately fit to the attachment-limited model using widely accepted values for r and N, giving d=11.1 nm. Therefore, in contrast with Huxley's model, we conclude that V0 is limited by the actin-myosin attachment rate.
authors: Department of Biochemistry and Molecular Biology, University of Nevada School of Medicine, Reno, NV 99557.
authors: Richard K
authors: RK
authors: Brizendine
authors: Department of Biochemistry and Molecular Biology, University of Nevada School of Medicine, Reno, NV 99557.
authors: Diego B
authors: DB
authors: Alcala
authors: Department of Biochemistry and Molecular Biology, University of Nevada School of Medicine, Reno, NV 99557.
authors: Michael S
authors: MS
authors: Carter
authors: Department of Biochemistry and Molecular Biology, University of Nevada School of Medicine, Reno, NV 99557.
authors: Brian D
authors: BD
authors: Haldeman
authors: Department of Biochemistry and Molecular Biology, University of Nevada School of Medicine, Reno, NV 99557.
authors: Kevin C
authors: KC
authors: Facemyer
authors: Department of Biochemistry and Molecular Biology, University of Nevada School of Medicine, Reno, NV 99557.
authors: Josh E
authors: JE
authors: Baker
authors: Department of Biochemistry and Molecular Biology, University of Nevada School of Medicine, Reno, NV 99557 cremo@unr.edu.
authors: Christine R
authors: CR
authors: Cremo
mesh_terms: D008841:Actin Cytoskeleton / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D000199:Actins / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure; D000205:Actomyosin / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D000465:Algorithms; D000818:Animals; D002645:Chickens; D007700:Kinetics; D008854:Microscopy, Electron; D008856:Microscopy, Fluorescence / Q000379:methods; D008954:Models, Biological; D009119:Muscle Contraction; D009130:Muscle, Smooth / Q000378:metabolism; D009218:Myosins / Q000737:chemistry / Q000378:metabolism* / Q000648:ultrastructure; D011485:Protein Binding / Q000187:drug effects; D011817:Rabbits; D012235:Rhodamines / Q000737:chemistry
chemical_list: D000199:Actins; D012235:Rhodamines; D000255:Adenosine Triphosphate; D000205:Actomyosin; D009218:Myosins
keywords: ATPase; actin; kinetics; muscle contraction; myosin