X-ray diffraction and electron microscopy from Lethocerus flight muscle partially relaxed by adenylylimidodiphosphate and ethylene glycol

The low-angle X-ray diffraction pattern from Lethocerus flight muscle fibres was recorded in rigor or under two conditions that modify crossbridge structure and behaviour, aqueous adenylylimidodiphosphate (AMPPNP) and AMPPNP + calcium in an ethylene glycol-water mixture. The effects on the 38.7 nm layer-line peaks ( hk.6) of the diffraction patterns were studied in detail. In aqueous AMPPNP at room temperature, a condition in which rigor tension drops to half without loss of stiffness, the peaks remained nearly as intense as in rigor except for the 10.6, which dropped to half. In 20% ( v/v) ethylene glycol-AMPPNP + 100 μ m-Ca 2+ at 23 °C (gly + pnp + Ca), a condition which removed muscle tension but left stiffness close to the rigor value, the 10.6 and 11.6 peaks greatly decreased but the 31.6 remained relatively high. The 14.5 nm meridional peak (00.16) became stronger on addition of AMPPNP and again on adding glycol + calcium. Considered in terms of constructively interfering filaments and crossbridges, the X-ray data indicated a transfer of diffracting crossbridge mass towards the thick filament as relaxation proceeds. We compared the X-ray diffraction patterns and crossbridge structure seen with electron microscopy (EM) under the same chemical conditions. EM and X-ray observations were mutually quite consistent overall. However, X-ray data indicated that more crossbridge mass was stereospecifically related to actin before fixation in the partially relaxed state (gly + pnp + Ca) than was suggested by the disordered crossbridge profiles seen by EM. We conclude that myosin heads at the start of the power stroke may both be closely related to their thick filament origins and form actin-determined attachments to the thin filament.