Two attached non-rigor crossbridge forms in insect flight muscle

• Published in: Journal of molecular biology Vol. 204; no. 2; pp. 357 - 383
• Main Authors: Reedy, Mary C., Reedy, Michael K., Tregear, Richard T.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 20.11.1988; Elsevier
• Subjects: Adenosine Triphosphate - metabolism; Adenylyl Imidodiphosphate - pharmacology; Animals; Biological and medical sciences; electron microscopy; Ethylene Glycol; Ethylene Glycols - pharmacology; flight; flight muscle; Flight, Animal; Fundamental and applied biological sciences. Psychology; Hemiptera; Hemiptera - physiology; Hemiptera - ultrastructure; Lethocerus; Microscopy, Electron; Molecular biophysics; muscle fibers; muscles; Muscles - drug effects; Muscles - physiology; Muscles - ultrastructure; myosin; Myosins - analysis; rigor; Structure in molecular biology; Supramolecular structure; tension; wings; Flight muscle; Arthropoda; Insecta; Supramolecular structure; Myosin; Crosslinking; Invertebrata; Electron microscopy
• ISSN: 0022-2836; 1089-8638
• DOI: 10.1016/0022-2836(88)90582-7

We have performed thin-section electron microscopy on muscle fibers fixed in different mechanically monitored states, in order to identify structural changes in myosin crossbridges associated with force production and maintenance. Tension and stiffness of fibers from glycerinated Lethocerus flight muscle were monitored during a sequence of conditions using AMPPNP and then AMPPNP plus increasing concentrations of ethylene glycol, which brought fibers through a graded sequence from rigor relaxation. Two intermediate crossbridge forms distinct from the rigor or relaxed forms were observed. The first was produced by AMPPNP at 20 °C, which reduced isometric tension 60 to 70% below rigor level without reducing rigor stiffness. Electron microscopy of these fibers showed that, in spite of the drop in tension, no obvious change from the 45 ° crossbridge angle characteristic of rigor occurred. However, the thick filament ends of the crossbridges were altered from their rigor positions, so that they now marked a 14.5 nm repeat, and formed four separate origins at each crossbridge level. The bridges were also less slewed and bent than rigor bridges, as seen in transverse sections. The second crossbridge form was seen in glycol-AMPPNP at 4 °C, just below the glycol concentration that produced mechanical relaxation. These fibers retained 90% of rigor stiffness at 40 Hz oscillation, but would not bear sustained tension. Stiffness was also high in the presence of calcium at room temperature under similar conditions. Electron microscopy showed crossbridges projecting from the thick filaments at an angle that centered around 90 °, rather than the 45 ° angle familiar from rigor. This coupling of relaxed appearance with persistent stiffness suggests that the 90 ° form may represent a weakly attached crossbridge state like that proposed to precede force development in current models of the crossbridge power stroke.