title: Low-force transitions in single titin molecules reflect a memory of contractile history.
abstract: 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.
authors.affiliation: Department of Biophysics and Radiation Biology, Semmelweis University, Tűzoltó u. 37-47, Budapest, H1094 Hungary.
authors.forename: Zsolt
authors.initials: Z
authors.lastname: Mártonfalvi
authors.forename: Pasquale
authors.initials: P
authors.lastname: Bianco
authors.forename: Marco
authors.initials: M
authors.lastname: Linari
authors.forename: Marco
authors.initials: M
authors.lastname: Caremani
authors.forename: Attila
authors.initials: A
authors.lastname: Nagy
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Lombardi
authors.forename: Miklós
authors.initials: M
authors.lastname: Kellermayer
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D064211:Connectin / Q000737:chemistry / Q000502:physiology*; D018625:Microscopy, Atomic Force; D009119:Muscle Contraction; D020836:Protein Structure, Quaternary; D017434:Protein Structure, Tertiary; D011817:Rabbits
chemical_list: D064211:Connectin
keywords: Atomic force microscopy; Mechanosensor; Monte-Carlo simulation; Optical tweezers; PEVK domain; Single-molecule mechanics; Titin