The Drosophila projectin mutant, bentD, 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 bentD (btD) has been shown to have a breakpoint close to the carboxy-terminal kinase domain of the projectin sequence. Homozygotes for btD are embryonic lethal but heterozygotes (btD/+) are viable. Here we show that btD/+ flies have normal flight ability and a slightly elevated wing beat frequency (btD/+ 223+/-13 Hz; +/+ 203+/-5 Hz, mean +/- SD; P < 0.01). Electron microscopy of btD/+ IFM show normal ultrastructure but skinned fiber mechanics show reduced stretch activation and oscillatory work. Although btD/+ IFM power output was at wild-type levels, maximum power was achieved at a higher frequency of applied length perturbation (btD/+ 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 btD mutation reduces oscillatory work in a way consistent with the proposed role of the connecting filaments in the stretch activation response of IFM.
Moore
J R
JR
Department of Molecular Physiology and Biophysics, University of Vermont, Burlington 05405, USA. Jeffrey.moore@uvm.edu
Vigoreaux
J O
JO
Maughan
D W
DW
D000818:Animals; D015153:Blotting, Western; D004330:Drosophila / Q000235:genetics / Q000502:physiology*; D004591:Electrophoresis, Polyacrylamide Gel; D006579:Heterozygote; D008854:Microscopy, Electron; D009124:Muscle Proteins / Q000032:analysis* / Q000235:genetics; D009132:Muscles / Q000737:chemistry* / Q000648:ultrastructure; D009154:Mutation
D009124:Muscle Proteins; C078033:projectin