Extensible and Less-Extensible Domains of Connectin Filaments in Stretched Vertebrate Skeletal Muscle Sarcomeres as Detected by Immunofluorescence and Immunoelectron Microscopy Using Monoclonal Antibodies1

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Abstract
Two kinds of monoclonal antibodies (3B9 and SM1) against connectin, muscle elastic protein, reacted with both α- and β-connectins. Immunofluorescence studies revealed that 3B9 stained both edges of the A band of chicken breast muscle myofibrils and remained as such upon stretching to a sarcomere length of 3.5 µm. On the other hand, SM1 stained the I band very close to the edges of the A band and the SM!-stained stripes moved considerably upon stretching to a sarcomere length of 3.5 µm. Immunoelectron microscopic observations with frog semitendinosus muscle revealed that three distinct stripes bound with 3B9 in the edges of the A band did not move on stretching up to 3.5 µm. On t.he other hand, the two stripes stained with SM1 in the I band clearly moved to the same extent as the stretching. However, when a sarcomere was stretched to 4.0 µm, all the stripes with 3B9 or SM1 disappeared and diffused deposits of the antibodies were observed. Thus it is concluded that connectin filaments in the I band region are more extensible than those at both edges of the A band.