title: The myosin cross-bridge cycle and its control by twitchin phosphorylation in catch muscle.
abstract: The anterior byssus retractor muscle of Mytilus edulis was used to characterize the myosin cross-bridge during catch, a state of tonic force maintenance with a very low rate of energy utilization. Addition of MgATP to permeabilized muscles in high force rigor at pCa > 8 results in a rapid loss of some force followed by a very slow rate of relaxation that is characteristic of catch. The fast component is slowed 3-4-fold in the presence of 1 mM MgADP, but the distribution between the fast and slow (catch) components is not dependent on [MgADP]. Phosphorylation of twitchin results in loss of the catch component. Fewer than 4% of the myosin heads have ADP bound in rigor, and the time course (0.2-10 s) of ADP formation following release of ATP from caged ATP is similar whether or not twitchin is phosphorylated. This suggests that MgATP binding to the cross-bridge and subsequent splitting are independent of twitchin phosphorylation, but detachment occurs only if twitchin is phosphorylated. A similar dependence of detachment on twitchin phosphorylation is seen with AMP-PNP and ATPgammaS. Single turnover experiments on bound ADP suggest an increase in the rate of release of ADP from the cross-bridge when catch is released by phosphorylation of twitchin. Low [Ca(2+)] and unphosphorylated twitchin appear to cause catch by 1) markedly slowing ADP release from attached cross-bridges and 2) preventing detachment following ATP binding to the rigor cross-bridge.
authors.affiliation: Department of Physiology, Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania 19107, USA. tom.butler@mail.tju.edu
authors.forename: T M
authors.initials: TM
authors.lastname: Butler
authors.forename: S R
authors.initials: SR
authors.lastname: Narayan
authors.forename: S U
authors.initials: SU
authors.lastname: Mooers
authors.forename: D J
authors.initials: DJ
authors.lastname: Hartshorne
authors.forename: M J
authors.initials: MJ
authors.lastname: Siegman
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D049872:Bivalvia / Q000502:physiology; D029742:Caenorhabditis elegans Proteins; D002148:Calmodulin-Binding Proteins / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008954:Models, Biological; D009119:Muscle Contraction / Q000187:drug effects / Q000502:physiology; D009124:Muscle Proteins / Q000378:metabolism*; D009126:Muscle Relaxation / Q000187:drug effects / Q000502:physiology; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism*; D010766:Phosphorylation; D010782:Photolysis
chemical_list: D029742:Caenorhabditis elegans Proteins; D002148:Calmodulin-Binding Proteins; D009124:Muscle Proteins; C489233:unc-22 protein, C elegans; D000244:Adenosine Diphosphate; C029273:P(3)-1-(2-nitro)phenylethyladenosine 5'-triphosphate; D000255:Adenosine Triphosphate; D009218:Myosins