COMPLETE UNFOLDING OF THE TITIN MOLECULE UNDER EXTERNAL FORCE

Titin (also known as connectin) is a giant filamentous protein that sp ans the distance between the Z- and M-lines of the vertebrate muscle s arcomere. Several earlier studies have implicated titin as playing a f undamental role in maintaining sarcomeric structural integrity and gen erating the passive force of muscle. The elastic properties of titin w ere characterized in recent single-molecule mechanical works that desc ribed the molecule as an entropic spring in which partial unfolding ma y take place at high forces during stretch and refolding at low forces during release. In the present work titin molecules were stretched us ing a laser tweezer with forces above 400 pN. The high external forces resulted in complete mechanical unfolding off the molecule, character ized by the disappearance of force hysteresis at high forces. Titin re folded following complete denaturation, as the hysteresis at low force s reappeared in subsequent stretch-release cycles. The broad force ran ge throughout which unfolding occurred indicates that the various glob ular domains in titin require different unfolding forces due to differ ences in the activation energies for their unfolding. (C) 1998 Academi c Press.