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 su perfamily. In skeletal muscle titins are involved in the longitudinal reinf orcement 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 f or the high relaxed stiffness found in this muscle type. The Drosophila mut ant bent(D) (bt(D)) has been shown to have a breakpoint close to the carbox y-terminal kinase domain of the projectin sequence. Homozygotes for bt(D) a re embryonic lethal but heterozygotes (bt(D)/+) are viable. Here we show th at bt(D)/+ flies have normal flight ability and a slightly elevated wing be at frequency (bt(D)/+ 223 +/- 13 Hz; +/+203 +/- 5 Hz, mean +/- SD; P < 0.01 ). Electron microscopy of bt(D)/+ IFM show normal ultrastructure but skinne d fiber mechanics show reduced stretch activation and oscillatory work. Alt hough bt(D)/+ IFM power output was at wild-type levels, maximum power was a chieved at a higher frequency of applied length perturbation (bt(D)/+ 151 /- 6 Hz; +/+ 102 +/- 14 Hz; P < 0.01). Results were interpreted in the cont ext of a viscoelastic model of the sarcomere and indicate altered cross-bri dge kinetics of the power-producing step. These results show that the bt(D) mutation reduces oscillatory work in a way consistent with the proposed ro le of the connecting filaments in the stretch activation response of IFM.