The Drosophila projectin mutant,bent D,has reduced stretch activation and altered indirect flight muscle kinetics

Projectin is a ca. 900 kDa protein that is a member of the titin protein superfamily. In skeletal muscle titins are involved in the longitudinal reinforcement of the sarcomere by connecting the Z-band to the M-line. In insect indirect flight muscle (IFM), projectin is believed to form the connecting filaments that link the Z-band to the thick filaments and is responsible for the high relaxed stiffness found in this muscle type. The Drosophila mutant bent^D (bt^D) has been shown to have a breakpoint close to the carboxy-terminal kinase domain of the projectin sequence. Homozygotes for bt^D are embryonic lethal but heterozygotes (bt^D/+) are viable. Here we show that bt^D/+ flies have normal flight ability and a slightly elevated wing beat frequency (bt^D/+ 223 ± 13 Hz; +/+203 ± 5 Hz, mean ± SD; P < 0.01). Electron microscopy of bt^D/+ IFM show normal ultrastructure but skinned fiber mechanics show reduced stretch activation and oscillatory work. Although bt^D/+ IFM power output was at wild-type levels, maximum power was achieved at a higher frequency of applied length perturbation (bt^D/+ 151 ± 6 Hz; +/+ 102 ± 14 Hz; P < 0.01). Results were interpreted in the context of a viscoelastic model of the sarcomere and indicate altered cross-bridge kinetics of the power-producing step. These results show that the bt^D mutation reduces oscillatory work in a way consistent with the proposed role of the connecting filaments in the stretch activation response of IFM.