Mechanical fatigue in repetitively stretched single molecules of titin

Relaxed striated muscle cells exhibit mechanical fatigue when exposed to re peated stretch and release cycles. To understand the molecular basis of suc h mechanical fatigue, single molecules of the giant filamentous protein tit in, which is the main determinant of sarcomeric elasticity, were repetitive ly stretched and released while their force response was characterized with optical tweezers. During repeated stretch-release cycles titin becomes mec hanically worn out in a process we call molecular fatigue. The process is c haracterized by a progressive shift of the stretch-force curve toward incre asing end-to-end lengths, indicating that repeated mechanical cycles increa se titin's effective contour length. Molecular fatigue occurs only in a res tricted force range (0-25 pN) during the initial part of the stretch half-c ycle, whereas the rest of the force response is repeated from one mechanica l cycle to the other. Protein-folding models fail to explain molecular fati gue on the basis of an incomplete refolding of titin's globular domains. Ra ther, the process apparently derives from the formation of labile nonspecif ic bonds cross-linking various sites along a pre-unfolded titin segment. Be cause titin's molecular fatigue occurs in a physiologically relevant force range, the process may play an important role in dynamically adjusting musc le's response to the recent history of mechanical perturbations.