Project Summary The long term goal of this research project is to understand the molecular mechanism of force production through 3-D visualization of myosin molecular motors in situ in muscle. The research focuses on the structure of the large waterbug Lethocerus sp. because its filament lattice is the best ordered of all known muscles types thereby making it an excellent candidate for 3-D imaging as well as facilitating the trapping of many myosin motors into similar states. Lethocerus, like many insects, utilize a stretch activation mechanism to operate their flight muscles. Stretch activation also occurs in vertebrate striated and cardiac muscle, where in the case of cardiac muscle, it is an important part of the rhythmic contractions. Recent advancements in detector technology, robotic electron microscopes and high throughput data collection, have made it possible to image the filaments themselves at atomic or near atomic resolution thereby providing unprecedented opportunity for atomic level interpretation of muscle. In the current funding period, we have obtained unprecedented resolution and detail of the relaxed state of thick filaments from Lethocerus flight muscle. No coiled-coil protein of the size of myosin has been imaged previously at the resolution we have achieved at the moment (4.3Å) in the backbone of the myosin filament. No assembly of coiled-coiled proteins of the size of the myosin filament backbone has been imaged at this resolution. This advance provides opportunity to investigate the mechanism whereby myosin rod mutations can affect muscle function. The head folding of myosin II filaments of smooth and non-muscle leads to filament instability. This phenomenon has been hypothesized to be due to changes in the rod structure brought on by the head folding. Put simply, the structure of the myosin rod and the myosin heads are coupled in some way, possibly through the transmission of torsional motions of the heads through the coiled-coil rod. Reversing that logic, a change in the coiled-coil structure of packing in the backbone could affect the head folding. Recent muscle research has pointed to the possibility that tension applied either internally by myosin heads or externally by a stretch, can affect the structure of the myosin heads. Thus, the thick filament may function as a tension transducer, but the molecular mechanism by which this occurs in unknown. We hypothesize that tension applied to the thick filament affects the structure of the myosin heads and vice versa, that the myosin heads affect the structure of the myosin rods. We can now test this hypothesis in a naturally formed filament by improving the order of the myosin heads in a relaxed thick filament followed by imaging by cryoEM to obtain better information on the position of the coiled-coil side chains. If the heads and coiled-coil rod structures are coupled, then disordering or removing the relaxed heads of Lethocerus thick filaments, followed by 3D imaging should change the structure of the coiled-coil. The ultimate contribution of this research to muscle physiology will come from the realization that the periodic beats of insect wings and the periodic beats of the human heart have much in common.

Project Narrative The least understood part of the thick filament of muscle is its backbone or shaft in which the coiled-coil myosin rods pack in order to bear the force produced by the myosin motors. Though lacking enzymatic activity, the backbone is the location of almost 1/3 of the known mutations causing various myopathies in humans. This project seeks to understand why this puzzling phenomenon occurs.