title: Cross-bridge number, position, and angle in target zones of cryofixed isometrically active insect flight muscle.
abstract: Electron micrographic tomograms of isometrically active insect flight muscle, freeze substituted after rapid freezing, show binding of single myosin heads at varying angles that is largely restricted to actin target zones every 38.7 nm. To quantify the parameters that govern this pattern, we measured the number and position of attached myosin heads by tracing cross-bridges through the three-dimensional tomogram from their origins on 14.5-nm-spaced shelves along the thick filament to their thin filament attachments in the target zones. The relationship between the probability of cross-bridge formation and axial offset between the shelf and target zone center was well fitted by a Gaussian distribution. One head of each myosin whose origin is close to an actin target zone forms a cross-bridge most of the time. The probability of cross-bridge formation remains high for myosin heads originating within 8 nm axially of the target zone center and is low outside 12 nm. We infer that most target zone cross-bridges are nearly perpendicular to the filaments (60% within 11 degrees ). The results suggest that in isometric contraction, most cross-bridges maintain tension near the beginning of their working stroke at angles near perpendicular to the filament axis. Moreover, in the absence of filament sliding, cross-bridges cannot change tilt angle while attached nor reach other target zones while detached, so may cycle repeatedly on and off the same actin target monomer.
authors.affiliation: Medical Research Council Laboratory of Molecular Biology, Cambridge CB2 2QH, United Kingdom. rtl@mrc-lmb.cam.ac.uk
authors.forename: Richard T
authors.initials: RT
authors.lastname: Tregear
authors.forename: Mary C
authors.initials: MC
authors.lastname: Reedy
authors.forename: Yale E
authors.initials: YE
authors.lastname: Goldman
authors.forename: Kenneth A
authors.initials: KA
authors.lastname: Taylor
authors.forename: Hanspeter
authors.initials: H
authors.lastname: Winkler
authors.forename: Clara
authors.initials: C
authors.lastname: Franzini-Armstrong
authors.forename: Hiroyuki
authors.initials: H
authors.lastname: Sasaki
authors.forename: Carmen
authors.initials: C
authors.lastname: Lucaveche
authors.forename: Michael K
authors.initials: MK
authors.lastname: Reedy
mesh_terms: D000199:Actins / Q000737:chemistry* / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000737:chemistry; D000818:Animals; D002118:Calcium / Q000378:metabolism; D005426:Flight, Animal; D006430:Hemiptera; D007091:Image Processing, Computer-Assisted; D008854:Microscopy, Electron; D009119:Muscle Contraction; D018485:Muscle Fibers, Skeletal / Q000166:cytology*; D009132:Muscles / Q000378:metabolism; D015879:Myosin Subfragments / Q000737:chemistry; D009218:Myosins / Q000737:chemistry; D016011:Normal Distribution
chemical_list: D000199:Actins; D015879:Myosin Subfragments; D000255:Adenosine Triphosphate; D009218:Myosins; D002118:Calcium