N-Terminal Truncation of Flightin Reduces Oscillatory Power Output without Affecting Cross-Bridge Kinetics in Drosophila Indirect Flight Muscles

Abstract

The asynchronous, indirect flight muscles (IFM) of Drosophila are characterized by a high passive stiffness and exceptionally fast myosin kinetics, two attributes that enhance power output to sustain flight. Flightin is an IFM-specific, 20kDa myosin rod-binding protein required for normal thick filament stiffness, sarcomere integrity, and flight. Previously, we showed that a COOH-terminal truncation of flightin (flnDeltaC44) decreased myofilament lattice order and myosin kinetics, resulting in lower oscillatory power output and flightlessness. Here, we investigate the function of the flightin N-terminal 62 amino acids by creating transgenic Drosophila (flnDeltaN62) expressing a truncated flightin. flnDeltaN62 flies were flight impaired (flight index: 2.8±0.1 vs. 4.2±0.4 for flnDeltaN62 vs. fln+ rescued null control) despite having a normal wing-beat frequency (195±4 vs. 198±2 Hz for fln+). Mechanical analysis of skinned IFM fibers showed that the flightin N-terminal truncation reduced passive, active, and rigor stiffness without affecting cross-bridge kinetics (frequency of maximum power: 205±7 vs. 217±7 Hz for fln+). flnDeltaN62 fibers produced approximately half the isometric tension (passive: 0.9±0.1 vs. 1.7±0.3 kN/m2, active: 0.8±0.1 vs. 1.5±0.2 kN/m2, rigor: 1.1±0.2 vs. 3.1±0.4 kN/m2) and maximum oscillatory power output (38.0±4.6 vs. 89.5±9.6 W/m3) as fln+ fibers. Moreover, about 60% of the flnDeltaN62 fibers tore in rigor, demonstrating mechanical failure near isometric tension values that were sustained by fln+ fibers. Fourier transform analysis of cross-sectional electron micrographs revealed that the flightin N-terminal truncation compromised myofilament lattice crystallinity and reduced inter-thick filament spacing by 10% (44.1±1.3 vs. 49.7±0.4 nm). These results indicate that the flightin N-terminal region enhances myofilament lattice order and mechanical integrity, which in turn is required for effective force transmission, normal oscillatory power output, and flight.