LOCALIZATION AND ELASTICITY OF CONNECTIN (TITIN) FILAMENTS IN SKINNED FROG-MUSCLE FIBERS SUBJECTED TO PARTIAL DEPOLYMERIZATION OF THICK FILAMENTS

The localization and elasticity of connectin (titin) filaments in skinned fibres of frog skeletal muscle were examined for changes in the localization of connectin and in resting tension during partial depolymerization of thick filaments with a relaxing solution containing increased KCl concentrations. Immunoelectron microscopic studies revealed that deposites of antibodies against connectin at a sarcomere length of 3.0-mu-m remained at about 0.8-mu-m from the M-line, until the thick filament was depolymerized to the length of approximately 0.4-mu-m. On further depolymerization, the bound antibodies were found to move towards the Z-line and, on complete depolymerization, were observed to be within 0.3-mu-m of the Z-line; a marked decrease in resting tension accompanied this further depolymerization. These results suggest that connectin filament starts from the Z-line, extends to the M-line, and contributes to resting tension. After partial depolymerization of thick filaments, the distances between the anti-connectin deposits and the Z-line and between anti-connectin deposits and M-line increased with sarcomere length, suggesting that connectin filaments are elastic along their entire length.