Evidence for Unique Structural Change of Thin Filaments upon Calcium Activation of Insect Flight Muscle

Upon activation of living or skinned vertebrate skeletal muscle fibers, the sixth X-ray layer-line reflection from actin (6th ALL) is known to intensify, without a shift of its peak position along the layer line. Since myosin attachment to actin is expected to shift the peak towards the meridian, this intensification is considered to reflect the structural change of individual actin monomers in the thin filament. Here, we show that the 6th ALL of skinned insect flight muscles (IFMs) is rather weakened upon isometric calcium activation, and its peak shifts away from the meridian. This suggests that the actin monomers in the two types of muscles change their structures in substantially different manners. The changes that occurred in the 6th ALL of IFM were not diminished by lowering the temperature from 20 to 5 °C, while active force was greatly reduced. The inclusion of 100 μM blebbistatin (a myosin inhibitor) did not affect the changes either. This suggests that calcium binding to troponin C, rather than myosin binding to actin, causes the structural change of IFM actin.