Elastic behavior and arrangement of titin filaments in the I-band

Abstract

Immunoelectron microscopic studies have revealed that the I-band domain of the titin filament behaves elastically. We report experiments that suggest that titin forms independent, highly elastic filaments in the I-band, except in the N1-line region near the Z-line.Freshly prepared bundles of rabbit psoas muscle fibers were quickly frozen and broken into small pieces under liquid nitrogen to fracture sarcomeres at the I-band region. The still-frozen specimens were thawed during fixation with paraformaldehyde (3.7%) in PBS to allow elastic filaments to retract. The muscle pieces were then labelled with monoclonal anti-titin (RT 13 and Sigma), followed by treatment with second antibody (goat anti-mouse).This method gave an excellent decoration pattern. The position of the titin epitope is symmetrical relative to the Z-line (Figure 1). However, in regions where the sarcomeres were broken at the A-I junction, the epitopes are no longer symmetrical about the Z-line (Figure 2); the broken thin filaments remained in their original position, but the titin filaments in the broken half retracted, independently of the thin filaments, forming a dense band just near the Zline (arrow).