title: Myosin cross-bridge kinetics and the mechanism of catch.
abstract: Catch force in molluscan smooth muscle requires little, if any, energy input and is controlled by the phosphorylation state of the thick filament-associated mini-titin, twitchin. The kinetic parameters of myosin cross-bridge turnover in permeabilized catch muscle, and how they are potentially modified by the catch mechanism, were determined by single turnover measurements on myosin-bound ADP. Under isometric conditions, there are fast and slow components of cross-bridge turnover that probably result from kinetic separation of calcium-bound and calcium-free cross-bridge pools. The structure responsible for catch force maintenance at intermediate [Ca+2] does not alter the processes responsible for the fast and slow components under isometric conditions. Also, there is no measurable turnover of myosin-bound ADP during relaxation of catch force by phosphorylation of twitchin at pCa > 8. The only effects of the catch link on myosin-bound ADP turnover are 1), a small, very slow extra turnover when catch force is maintained at very low [Ca+2] (pCa > 8); and 2), attenuation of the shortening-induced increase in turnover at subsaturating [Ca(+2)]. These limited interactions between the catch link and myosin cross-bridge turnover are consistent with the idea that catch force is maintained by a thick and thin filament linkage other than the myosin cross-bridge.
authors.affiliation: Department of Molecular Physiology and Biophysics, Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania, USA.
authors.forename: Aaron S
authors.initials: AS
authors.lastname: Franke
authors.forename: Susan U
authors.initials: SU
authors.lastname: Mooers
authors.forename: Srinivasa R
authors.initials: SR
authors.lastname: Narayan
authors.forename: Marion J
authors.initials: MJ
authors.lastname: Siegman
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Butler
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000818:Animals; D001696:Biomechanical Phenomena; D055592:Biophysical Phenomena; D001703:Biophysics; D002118:Calcium / Q000378:metabolism; D066298:In Vitro Techniques; D007537:Isometric Contraction / Q000502:physiology; D007700:Kinetics; D009124:Muscle Proteins / Q000737:chemistry / Q000502:physiology; D009126:Muscle Relaxation / Q000502:physiology; D009130:Muscle, Smooth / Q000737:chemistry / Q000502:physiology; D009218:Myosins / Q000737:chemistry* / Q000502:physiology*; D049880:Mytilus edulis / Q000502:physiology; D010766:Phosphorylation
chemical_list: D009124:Muscle Proteins; D000244:Adenosine Diphosphate; D009218:Myosins; D002118:Calcium