Flightin, a novel myofibrillar protein of Drosophila stretch-activated muscles.

Abstract

Abstract. The indirect flight muscles of Drosophila are adapted for rapid oscillatory movements which depend on properties of the contractile apparatus itself. Flight muscles are stretch activated and the frequency of contraction in these muscles is independent of the rate of nerve impulses. Little is known about the molecular basis of these adaptations. We now report a novel protein that is found only in flight muscles and has, therefore, been named flightin. Although we detect only one gene (in polytene region 76D) for flightin, this protein has several isoforms (relative gel mobilities, 27-30 kD; pls, 4.6-6.0). These isoforms appear to be created by posttranslational modifications. A subset of these isoforms is absent in newly emerged adults but appears when the adult develops the ability to fly. In intact muscles flightin is associated with the A band of the sarcomere, where evidence suggests it interacts with the myosin filaments. Computer database searches do not reveal extensive similarity to any known protein. However, the NH2-terminal 12 residues show similarity to the NH2-terminal sequence of actin, a region that interacts with myosin. These features suggest a role for flightin in the regulation of contraction, possibly by modulating actin-myosin interaction.T HE indirect flight muscles (IFM) ~ of Drosophila melanogaster are a group of thoracic fibers whose oscillatory contractions power wing beats at high frequencies, far greater than the firing rate of motor nerves. With steady neural input intracellular calcium levels remain high in the IFM, and the muscle responds to slight changes in length with delayed changes in tension (35,51). This property, called stretch activation, allows the IFM to drive the mechanically resonant wing/thorax system at its very high natural frequency. Since skinned fibers also do oscillatory work in the presence of calcium and ATE it is assumed that components of the myofibril itself are responsible for this adaptation (18).The mechanism underlying stretch activation is not known. Wray (64) has suggested that a slight longitudinal displacement of thick and thin filaments increases the number of cross-bridges recruited by optimally aligning the matching actin and myosin filament periodicities. Recently, however, Squire (45) has challenged this hypothesis on the basis of detailed analysis of filament organization. Alternative proposals are that longitudinal displacement of filaments might influence tension development by changing the angle of attachment of cross-bridges bound to actin (45, 51, 52), J. Vigoreaux's present address is Department of Zoology, University of Vermont, Burlington, VT 05405-0086.

COOH-terminal truncation of flightin decreases myofilament lattice organization, cross-bridge binding, and power output in Drosophila indirect flight muscle

“…To identify sequences important for flightin function, we conducted a BLAST search of 11 Drosophila genomes using the flightin sequence from D. melanogaster (33). Alignment of the 12 sequences revealed a tripartite organization (Fig.…”

“…In Drosophila, flightin is a ϳ20-kDa (182 amino acids) protein that is expressed exclusively in the IFM (33). Flightin binds the light meromyosin region of myosin, ϳ2/3 of the way down the rod, because substituting aspartic acid 1554 for lysine abolishes flightin's interaction in vitro (1) and accumulation in vivo (18).…”

Flightin maintains myofilament lattice organization required for optimal flight power and courtship song quality inDrosophila

“…Unlike the myosin regulatory light chain that is expressed in all adult muscle, flightin expression in Drosophila is confined to the IFM [18]. We show here that fln 0 males are mute, underscoring the essential contribution of the IFM to song production.…”

“…Upon skinning, flightin remained associated with fln DN62 and fln DC44 muscle fibres, indicating that neither the N-terminal domain nor the C-terminal domain is required for myosin binding [18]. The only shared sequence between the N-terminal truncated flightin and the C-terminal truncated flightin is the central region extending from amino acid 63 to 138.…”

“…Flightin is a myosin-binding protein that in D. melanogaster is expressed exclusively in the IFMs, where it has been shown to be essential for the proper determination of length and stiffness of thick filaments [18][19][20]. Genetic ablation of flightin expression ( fln 0 flies) resulted in sarcomere degradation, fibre hypercontraction, and structurally and mechanically compromised IFMs unable to generate oscillatory power, rendering fln 0 flies flightless but viable [19,20].…”