Stretching Single Protein Molecules: Titin Is a Weird Spring

Titin is a huge protein with many immunoglobulin domains that provide elasticity to muscle. In his Perspective, Erickson discusses three analyses of single titan molecules--two on pp. 1109 and 1112 of this issue and one in this week9s Nature. Molecular pulling on individual titin molecules with laser tweezers and atomic force microscopy reveals that it is an entropic spring that eventually loses its ability to recoil (renature) when large stretches of its immunoglobulin domains unravel.