Quantal sarcomere-length changes in relaxed single myofibrils

We carried out experiments on single isolated myofibrils in which thin fila ments had been functionally removed, leaving the connecting (titin) filamen ts as the sole agent taking up the length change. With technical advances t hat gave sub-nanometer detectability we examined the time course of single sarcomere-length change when the myofibril was ramp-released or ramp-stretc hed by a motor. The sarcomere-length change was stepwise. Step sizes follow ed a consistent pattern: the smallest was similar to2.3 nm, and others were integer multiples of that value. The similar to2.3-nm step quantum is the smallest consistent biomechanical event ever demonstrated. Although the len gth change must involve the connecting filament, the size of the quantum is an order of magnitude smaller than anticipated from folding of 1g- or fibr onectin-like domains, implying either that folding occurs in sub-domain uni ts or that other mechanisms are involved.