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-lin e were investigated using immunoelectron microscopy. We used monoclona l titin antibodies T-11 and T-12 on single fibres of frog skeletal mus de, 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 pr operties are not uniform. The T-12 epitope, which binds near the Z-lin e at the N1-Z-line level, hardly changes position relative to the Z-li ne as the sarcomere is stretched. This demonstrates the functional ine xtensibility of the N1-Z-line region. After extreme stretch (above 6-m um sarcomere length), this zone finally does elongate; thus, the titin molecule in this region is intrinsically elastic. The functional inex tensibility seen at shorter sarcomere lengths may, therefore, be a res ult of binding of titin to the actin filament in the zone near the Z-l ine. When the Z-line was extracted, the T-12 epitope remained in the s ame 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 doser to the A -band. Combined with the results of the above-mentioned stretch experi ment, this result implies a likely binding of titin to the thin filame nt either focally at the N1 line or all along the entire N1-Z region. Thus, this region of titin is functionally stiff, but intrinsically el astic.