Calcium induced change in three-dimensional structure of thin filaments of rabbit skeletal muscle as revealed by cryo-electron microscopy

The three-dimensional images of pure actin filaments and the thin filaments reconstituted from rabbit skeletal actin, tropomyosin and troponin were reconstructed from cryo-electron micrographs. The structural difference between on-state thin filaments (in the presence of calcium ions) and that in an off-state (in the absence of calcium ions) was examined carefully by analyzing 50 filaments in each of states. The image analysis using the t-test showed that there is the significant difference between the three-dimensional structure of thin filaments in an on-state and that in an off-state. It was not determined yet whether this change is due to tropomyosin movement or actin transconformation.