The Myosin Relay and Converter Domains Influence Muscle Performance

The role of the myosin heavy chain (MHC) relay and converter domains in controlling the cross bridge cycle during Drosophila indirect flight muscle (IFM) contraction was investigated. The IFM, of transgenic Drosophila lines expressing chimeric MHC versions of the embryonic isoform (EMB) and the indirect flight muscle isoform (IFI) were tested for changes in mechanical performance. IFI-9b and EMB-9a chimeras were created by exchanging alternative versions of relay domains (encoded by exons 9b and 9a of the Drosophila Mhc gene) into IFI and EMB. IFI-9b muscle mechanical properties of maximum power generation (Pmax) and optimal frequency of power generation (fmax) were significantly reduced from that of IFI fibers negatively impacting wing beat frequency and flight ability of these flies. On the other hand, EMB-9a muscle fibers exhibited higher Pmax and fmax of that obtained from EMB fibers. From these observations, it is proposed that the IFI relay domain hastens crossbridge cycling in response to oscillatory load experienced in the IFM fibers. Faster kinetics in EMB-9a fibers, when compared to EMB fibers is due to the reduction of a high activation energy barrier of transition between two crossbridge states. On the other hand, the EMB relay domain is less sensitive to load and slows crossbridge kinetics in the IFM fibers. The MHC relay domain is proposed to critically interact with the converter to generate the myosin power stroke and muscle fiber power. To test this hypothesis mutant IFI containing glutamic acid, instead of arginine at position 759 was created to disrupt relay converter interaction. At 15°C, work loop analysis of mutant fibers revealed a 58% and 31% decrease in maximum power output (Pwl) and frequency of maximum power output (fwl), respectively. Reduction of 30% and 36% in elastic modulus and viscous modulus, respectively of mutant fibers was observed using small amplitude sinusoidal analysis at 15°C. Mutant fibers exhibited a reduction in the apparent rate constant 2pib. Reduced muscle power generation, fiber stiffness and kinetics, and lowered wing beat frequency impaired flight ability of the mutants at 15°C. We propose that relay converter interaction is critical for intra-molecular communication between the lever arm and the myosin catalytic head, muscle fiber power generation and Drosophila flight.