Kettin is a high molecular mass protein of insect muscle associated with thin filaments and α-actinin in the Z-disc. It is thought to form a link between thin and thick filaments towards its C-terminus, contributing significantly to passive sarcomere stiffness. Here the elastic properties were characterised by mechanical stretches on an antibody-delimited region of the single molecule using two independent optical traps capable of exerting forces up to 150 pN. Step-like events were observed in the force–extension relationships consistent with the unfolding of Ig domains at moderate force and refolding of these domains at significantly higher forces than have been observed for related modular proteins.
