Low-force transitions in single titin molecules reflect a memory of contractile history.
Creators
- 1. Department of Biophysics and Radiation Biology, Semmelweis University, Tu˝zolto´ u. 37-47, Budapest, H1094 Hungary
- 2. Laboratory of Physiology, BIO c/o Department of Physics, University of Florence, via Sansone 1, 50019 Sesto Fiorentino (FI), Italy
- 3. Laboratory of Molecular Physiology, NHLBI NIH, Building 50/ 3529 50 South Drive, Bethesda, MD 20892-8015, USA
- 4. Department of Biophysics and Radiation Biology, Semmelweis University, Tu˝zolto´ u. 37-47, Budapest, H1094 Hungary.
Description
Titin is a giant elastomeric muscle protein that has been suggested to function as a sensor of sarcomeric stress and strain, but the mechanisms by which it does so are unresolved. To gain insight into its mechanosensory function we manipulated single titin molecules with high-resolution optical tweezers. Discrete, step-wise transitions, with rates faster than canonical Ig domain unfolding occurred during stretch at forces as low as 5 pN. Multiple mechanisms and molecular regions (PEVK, proximal tandem-Ig, N2A) are likely to be involved. The pattern of transitions is sensitive to the history of contractile events. Monte-Carlo simulations of our experimental results predicted that structural transitions begin before the complete extension of the PEVK domain. High-resolution atomic force microscopy (AFM) supported this prediction. Addition of glutamate-rich PEVK domain fragments competitively inhibited the viscoelastic response in both single titin molecules and muscle fibers, indicating that PEVK domain interactions contribute significantly to sarcomere mechanics. Thus, under non-equilibrium conditions across the physiological force range, titin extends by a complex pattern of history-dependent discrete conformational transitions, which, by dynamically exposing ligand-binding sites, could set the stage for the biochemical sensing of the mechanical status of the sarcomere.