Loreau, V.; Koolhaas, W.; Chan, E. H.; De Bossier, P.; Brouilly, N.; Avosani, S.; Sane, A.; Pitaval, C.; Reiter, S.; Luis, N. M.; Mangeol, P.; von Philipsborn, A. C.; Rupprecht, J.-F.; Goerlich, D.; Habermann, B. H.; Schnorrer, F.

Sarcomeres are the contractile units of muscles that enable animals to move. Insect muscles are remarkable examples because they use extremely different contraction frequencies (ranging from ~1 to 1000 Hz) and amplitudes for flying, walking and crawling. This is puzzling because sarcomeres are built from essentially the same actin-myosin components. We show here that the giant protein titin is the key to this functional specialisation. I-band titin spans and determines the length of the sarcomeric I-band, and occurs in muscle-type-specific isoforms. Surprisingly, it also rules the length of the force-generating myosin filament in a force feedback mechanism, even though it is not present there. We provide evidence for this model and its validity beyond insects.