title: Structural changes in isometrically contracting insect flight muscle trapped following a mechanical perturbation.
abstract: The application of rapidly applied length steps to actively contracting muscle is a classic method for synchronizing the response of myosin cross-bridges so that the average response of the ensemble can be measured. Alternatively, electron tomography (ET) is a technique that can report the structure of the individual members of the ensemble. We probed the structure of active myosin motors (cross-bridges) by applying 0.5% changes in length (either a stretch or a release) within 2 ms to isometrically contracting insect flight muscle (IFM) fibers followed after 5-6 ms by rapid freezing against a liquid helium cooled copper mirror. ET of freeze-substituted fibers, embedded and thin-sectioned, provides 3-D cross-bridge images, sorted by multivariate data analysis into ~40 classes, distinct in average structure, population size and lattice distribution. Individual actin subunits are resolved facilitating quasi-atomic modeling of each class average to determine its binding strength (weak or strong) to actin. ~98% of strong-binding acto-myosin attachments present after a length perturbation are confined to "target zones" of only two actin subunits located exactly midway between successive troponin complexes along each long-pitch helical repeat of actin. Significant changes in the types, distribution and structure of actin-myosin attachments occurred in a manner consistent with the mechanical transients. Most dramatic is near disappearance, after either length perturbation, of a class of weak-binding cross-bridges, attached within the target zone, that are highly likely to be precursors of strong-binding cross-bridges. These weak-binding cross-bridges were originally observed in isometrically contracting IFM. Their disappearance following a quick stretch or release can be explained by a recent kinetic model for muscle contraction, as behaviour consistent with their identification as precursors of strong-binding cross-bridges. The results provide a detailed model for contraction in IFM that may be applicable to contraction in other types of muscle.
authors: Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida, United States of America.
authors: Shenping
authors: S
authors: Wu
authors: Jun
authors: J
authors: Liu
authors: Mary C
authors: MC
authors: Reedy
authors: Robert J
authors: RJ
authors: Perz-Edwards
authors: Richard T
authors: RT
authors: Tregear
authors: Hanspeter
authors: H
authors: Winkler
authors: Clara
authors: C
authors: Franzini-Armstrong
authors: Hiroyuki
authors: H
authors: Sasaki
authors: Carmen
authors: C
authors: Lucaveche
authors: Yale E
authors: YE
authors: Goldman
authors: Michael K
authors: MK
authors: Reedy
authors: Kenneth A
authors: KA
authors: Taylor
mesh_terms: D000199:Actins / Q000378:metabolism; D000818:Animals; D005426:Flight, Animal / Q000502:physiology*; D020062:Heteroptera / Q000502:physiology*; D007537:Isometric Contraction / Q000502:physiology*; D008954:Models, Biological; D018482:Muscle, Skeletal / Q000502:physiology*; D014336:Troponin / Q000378:metabolism
chemical_list: D000199:Actins; D014336:Troponin