Elastic properties of the titin filament in the Z-line region of vertebrate striated muscle.
The characteristics of the titin filament in the vicinity of the Z-line were investigated using immunoelectron microscopy. We used monoclonal titin antibodies T-11 and T-12 on single fibres of frog skeletal muscle, and on Z-line-extracted fibres. It is well established that the I-band region of titin is elastic. We find, however, that the elastic properties are not uniform. The T-12 epitope, which binds near the Z-line at the N1-line level, hardly changes position relative to the Z-line as the sarcomere is stretched. This demonstrates the functional inextensibility of the N1-Z-line region. After extreme stretch (above 6-microns sarcomere length), this zone finally does elongate; thus, the titin molecule in this region is intrinsically elastic. The functional inextensibility seen at shorter sarcomere lengths may, therefore, be a result of binding of titin to the actin filament in the zone near the Z-line. When the Z-line was extracted, the T-12 epitope remained in the same position as in the unextracted fibres; it did not retract from the Z-line. Failure to retract implies that functional anchoring of titin is not exclusive to the Z-line, but includes some site closer to the A-band. Combined with the results of the above-mentioned stretch experiment, this result implies a likely binding of titin to the thin filament either focally at the N1 line or all along the entire N1-Z region. Thus, this region of titin is functionally stiff, but intrinsically elastic.
Trombitás
K
K
University of Washington, Seattle 98195.
Pollack
G H
GH
D000818:Animals; D000911:Antibodies, Monoclonal / Q000276:immunology; D001001:Anura; D064211:Connectin; D004548:Elasticity; D016253:Microscopy, Immunoelectron; D009124:Muscle Proteins / Q000276:immunology / Q000502:physiology*; D009132:Muscles / Q000737:chemistry* / Q000648:ultrastructure; D011494:Protein Kinases; D012518:Sarcomeres / Q000648:ultrastructure*
D000911:Antibodies, Monoclonal; D064211:Connectin; D009124:Muscle Proteins; D011494:Protein Kinases