Cross-Bridges and Periods in Insect Flight Muscle

The periodic structure of the cross-bridge lattice of glycerinatedLethocerus flight muscle has been studied in sections by electronmicroscopy, assisted by optical diffraction, and in unfixedfiber bundles by X-ray diffraction. Diffraction patterns exhibitfirst through ninth orders of 1166 Ä, virtually all ofwhich were found to arise from the lattice of cross-bridges.Diffraction and inspection show that "horizontal" cross-bridgesof relaxation become slanted in rigor, and may push actins towardthe M line in producing the increase in tension seen with theinduction of rigor. Myosin filaments contain unexpected structural features. Cross-bridgeorigins form opposed pairs repeating every 146 Ä; and rotating67.5 degrees with each repeat, thus defining twin, left-handed,helical tracks which require 1 turns (or 8 x 146 Ä) toestablish a meridional repeat of 1166 Ä. Each origin isdual and gives rise to two bridges; thus, the unit groupingof paired origins involves four bridges. One half-turn of themyosin helix requires 388 Ä, matching the actin helix exactlyin pitch. (Actin is, however, right-handed.) The resulting matchseems awkward azimuthally (sixteenfold myosin distributes bridgesto a sixfold envelope of actin filaments), but minimizes axialmismatching between subunits of the myosin and actin and lendscredence to the theory that all bridges may swing synchronouslyduring typical, low-amplitude, oscillatory contractions.