Quantal sarcomere-length changes in relaxed single myofibrils.

We carried out experiments on single isolated myofibrils in which thin filaments had been functionally removed, leaving the connecting (titin) filaments as the sole agent taking up the length change. With technical advances that gave sub-nanometer detectability we examined the time course of single sarcomere-length change when the myofibril was ramp-released or ramp-stretched by a motor. The sarcomere-length change was stepwise.

Intact connecting filaments change length in 2.3-nm quanta.

In isolated titin molecules, length changes may occur in discrete steps (Tskhovrebova et al., 1997; Rief et al., 1997).

Implications of quantal motor action in biological systems.

We demonstrate in this paper that quantal behavior is a central feature of biological motile and contractile systems. Step-like behavior has been demonstrated in the interaction between single molecules and filaments both in the kinesin-microtubule system and in the myosin-actin filament system. We show here that the step-like molecular features appear also in the single intact sarcomere.