Meeting Abstract
The energetic efficiency and tuning of force generation in muscle cells depends on the dynamically changing architecture of the contractile machinery. Contraction is powered by myosin molecular motors that branch off of the thick filament and bind to the actin-containing thin filaments at specific binding sites. But these binding sites are not aligned to the molecular motors. This would result in a low binding probability for most of the myosin motors, except both the thick and thin filaments are compliant. Compliance means that binding sites and cross bridges can realign during contraction, tuning force generation as shown by spatially explicit models of the filament lattice. However the degree to which filament stretching occurs in vivo during cyclic loading remains unknown. Here, we use time-resolved x-ray diffraction techniques to directly measure the strain experienced by myofilaments of the synchronous flight muscles of the hawk moth Manduca sexta during cyclical loading in vivo (i.e. during tethered flapping flight). Any stretching of the thick or thin filaments is manifest as a change in the periodicity of their respective helices. We find that the thick filament backbone experiences strains in the range of 0.12% to 0.61% with a global average of 0.24% (N=7, an average of 0.18 Å in the helical pitch spacing), while layers of myosin crowns show a change in periodicity ranging from 0.03% to 0.42%, with a global average of 0.13% (N=7). Even this seemingly slight compliance can facilitate non-linear tension development, relaxation rates, and elastic energy storage, enabling efficient periodic contraction.
