Experiments on rigor crossbridge action and filament sliding in insect flight muscle.

We have explored three aspects of rigor crossbridge action: 1. Under rigor conditions, slow stretching (2% per hour) of insect flight muscle (IFM) from Lethocerus causes sarcomere ruptures but never filament sliding. However, in 1 mM AMPPNP, slow stretching (5%/h) causes filament sliding but no sarcomere ruptures, although stiffness equals rigor values. Thus loaded rigor attachments in IFM show no strain relief over several hours, but near-rigor states that allow short-term strain relief indicate different grades of strongly bound bridges, and suggest approaches to annealing the rigor lattice. 2. Sarcomeres of Lethocerus flight muscle, stretched 20-60% and then rigorized, show "hybrid" crossbridge patterns, with overlap zones in rigor, but H-bands relaxed and revealing four-stranded R-hand helical thick filament structure. The sharp boundary exhibits precise phasing between relaxed and rigor arrays along each thick filament. Extrapolating one lattice into the other should allow detailed modeling of the action of each myosin head as it enters rigor. 3. The "A-(bee)-Z problem" exposes a conflict about actin rotational alignment between A-bands and Z-bands of bee IFM, raising the possibility that rigor induction might rotate actins forcefully from one pattern to the other. As Squire noted, 3-D reconstructions of Z-bands in relaxed bee IFM2) imply A-bands where actin target zones form rings rather than helices around thick filaments. However, we confirm Trombitás et al. that rigor crossbridges in bee IFM mark helically arrayed target zones. Moreover, we find that loose crossbridge interactions in relaxed bee IFM mark the same helical pattern. Thus no change of actin rotational alignment by rigor crossbridges seems necessary, but 3-D structure of IFM Z-bands should be re-evaluated regarding the apparent contradiction with A-band symmetry.