Disrupting Myosin Relay-Converter Domain Communication Impairs Drosophila Muscle Mechanical Performance and Flight Ability

Abstract

We evaluated the mechanical properties of Drosophila indirect flight muscle (IFM) fibers expressing a myosin converter domain R759E mutation. The interaction of R759 with relay loop residue I508 is thought to be critical for relay-converter inter domain communication. By changing the charge on residue 759, we are testing if this inter domain interaction is important for the mechanical performance of muscle fibers. Electron microscopic examination of muscle fibers from young adult R759E flies indicates normal myofibril assembly. Using the work loop analysis technique we found that the maximum power (Pmax) generated by the mutant R759E fibers from two day old flies was significantly reduced by 50% compared to control fibers while the frequency at which maximum power is generated (fmax) was reduced to 67%. Maximum power occurred at peak-to-peak strain amplitude of 2% resting muscle fiber length. Varying ATP concentration at 15°C revealed no significant difference in Km for Pmax or fmax between control and mutant R759E fibers, suggesting that the mutation does not affect ATP affinity. Small amplitude sinusoidal analysis revealed a significant reduction in complex stiffness by 48% compared to control fibers, with elastic modulus, Ee, reduced by 31% and viscous modulus, Ev, reduced by 45%. This reduction in power and mechanical performance of the flight muscle fibers led to a decrease in wing beat frequency from 140 ± 2 Hz for control flies to 127 ± 2 Hz. The reduction in wing beat frequency contributed to a decrease in flight index from 2.31 ± 0.1 for control flies to 1.25 ± 0.1 at 15°C. Thus, this study suggests that the interaction between relay loop I508 and converter domain R759 is critical for myosin inter domain communication, muscle fiber power generation and Drosophila flight performance.