Complete unfolding of the titin molecule under external force

Titin (also known as connectin) is a giant filamentous protein that spans the distance between the Z- and M-lines of the vertebrate muscle sarcomere. Several earlier studies have implicated titin as playing a fundamental role in maintaining sarcomeric structural integrity and generating the passive force of muscle. The elastic properties of titin were characterized in recent single-molecule mechanical works that described the molecule as an entropic spring in which partial unfolding may take place at high forces during stretch and refolding at low forces during release. In the present work titin molecules were stretched using a laser tweezer with forces above 400 pN. The high external forces resulted in complete mechanical unfolding of the molecule, characterized by the disappearance of force hysteresis at high forces. Titin refolded following complete denaturation, as the hysteresis at low forces reappeared in subsequent stretch-release cycles. The broad force range throughout which unfolding occurred indicates that the various globular domains in titin require different unfolding forces due to differences in the activation energies for their unfolding.