Abstract
Flightin is a myosin binding protein present in Pancrustacea. In Drosophila, flightin is expressed in the indirect flight muscles (IFM), where it is required for the flexural rigidity, structural integrity, and length determination of thick filaments. Comparison of flightin sequences from multiple Drosophila species revealed a tripartite organization indicative of three functional domains subject to different evolutionary constraints. We use atomic force microscopy to investigate the functional roles of the N-terminal domain and the C-terminal domain that show different patterns of sequence conservation. Thick filaments containing a C-terminal domain truncated flightin (flnΔC44) are significantly shorter (2.68 ± 0.06 μm; p < 0.005) than thick filaments containing a full length flightin (fln+; 3.21 ± 0.05 μm) and thick filaments containing an N-terminal domain truncated flightin (flnΔN62; 3.21 ± 0.06 μm). Persistence length was significantly reduced in flnΔN62 (418 ± 72 μm; p < 0.005) compared to fln+ (1386 ± 196μm) and flnΔC44(1128 ± 193 μm). Statistical polymer chain analysis revealed that the C-terminal domain fulfills a secondary role in thick filament bending propensity. Our results indicate that the flightin amino and carboxy terminal domains make distinct contributions to thick filament biomechanics. We propose these distinct roles arise from the interplay between natural selection and sexual selection given IFM’s dual role in flight and courtship behaviors.
Highlights
Thick filaments play a fundamental role in defining the sarcomeric structure and contractile properties of muscle
Our results indicate that the flightin amino and carboxy terminal domains make distinct contributions to thick filament biomechanics
We propose these distinct roles arise from the interplay between natural selection and sexual selection given indirect flight muscles (IFM)’s dual role in flight and courtship behaviors
Summary
Thick filaments play a fundamental role in defining the sarcomeric structure and contractile properties of muscle. The major constituent of thick filaments in most muscle types is myosin II, a highly conserved protein characterized by a globular motor domain responsible for the chemomechanical basis of contraction, and a coiled-coil tail domain responsible for polymerization into highly ordered filaments [1]. Additional species and muscle-type specific proteins confer thick filaments with distinct structural and functional characteristics tailored to the specific operational demands of the muscle as evidenced, for example, in the various forms of animal locomotion such as running, hopping, flying, swimming, and crawling [2]. The predominant role of muscle in these various forms of locomotion often obscures its other important functions (e.g., heat production, sound production, metabolic health) that rely on contractile mechanisms that are operationally distinct from those used for locomotion. How a common thin and thick filament-based sarcomere design has evolved to fulfill different functional roles remains largely unknown.
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