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). The extent to which such steps are preserved in the intact muscle-filament lattice has remained unclear. We carried out experiments on single isolated insect-flight-muscle myofibrils in which thin filaments had been functionally removed either by stretch beyond overlap or by a "rigor-stretch" protocol, leaving the connecting (titin) filaments as the sole length-absorbing agent. The myofibril was released or stretched by a motor in ramp-like fashion. The time course of length change in the single sarcomere was stepwise. The same was true for half-sarcomere lengths. The presence of steps at the sarcomere level implies that parallel filaments step synchronously, with high cooperativity. Step sizes showed a consistent distribution: The smallest size was approximately 2.3 nm, and others were integer multiples of that value. Similar results were found for stretch and release. To our knowledge, the approximately 2.3-nm step quantum is the smallest consistent biomechanical event ever demonstrated. This quantum is an order of magnitude smaller than anticipated from the folding/unfolding of a complete Ig- or fibronectin-like domain, and may imply that folding occurs in sub-domain increments. The 2.3-nm incremental length change corresponds to a single turn of the domains' beta sheet.
Blyakhman
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F
Dept. of Bioengineering, University of Washington, Seattle, USA.
Tourovskaya
A
A
Pollack
G H
GH
D000818:Animals; D001516:Bees; D005426:Flight, Animal; D018482:Muscle, Skeletal / Q000502:physiology / Q000648:ultrastructure; D009210:Myofibrils / Q000502:physiology / Q000648:ultrastructure*; D011789:Quantum Theory; D012518:Sarcomeres / Q000502:physiology / Q000648:ultrastructure