title: Titin domains progressively unfolded by force are homogenously distributed along the molecule.
abstract: Titin is a giant filamentous protein of the muscle sarcomere in which stretch induces the unfolding of its globular domains. However, the mechanisms of how domains are progressively selected for unfolding and which domains eventually unfold have for long been elusive. Based on force-clamp optical tweezers experiments we report here that, in a paradoxical violation of mechanically driven activation kinetics, neither the global domain unfolding rate, nor the folded-state lifetime distributions of full-length titin are sensitive to force. This paradox is reconciled by a gradient of mechanical stability so that domains are gradually selected for unfolding as the magnitude of the force field increases. Atomic force microscopic screening of extended titin molecules revealed that the unfolded domains are distributed homogenously along the entire length of titin, and this homogeneity is maintained with increasing overstretch. Although the unfolding of domains with progressively increasing mechanical stability makes titin a variable viscosity damper, the spatially randomized variation of domain stability ensures that the induced structural changes are not localized but are distributed along the molecule's length. Titin may thereby provide complex safety mechanims for protecting the sarcomere against structural disintegration under excessive mechanical conditions.
authors.affiliation: Department of Biophysics and Radiation Biology, Semmelweis University, Budapest, Hungary.
authors.forename: Pasquale
authors.initials: P
authors.lastname: Bianco
authors.affiliation: Department of Biophysics and Radiation Biology, Semmelweis University, Budapest, Hungary.
authors.forename: Zsolt
authors.initials: Z
authors.lastname: Mártonfalvi
authors.affiliation: Department of Biophysics and Radiation Biology, Semmelweis University, Budapest, Hungary.
authors.forename: Katalin
authors.initials: K
authors.lastname: Naftz
authors.affiliation: Department of Biophysics and Radiation Biology, Semmelweis University, Budapest, Hungary.
authors.forename: Dorina
authors.initials: D
authors.lastname: Kőszegi
authors.affiliation: Department of Biophysics and Radiation Biology, Semmelweis University, Budapest, Hungary; MTA-SE Molecular Biophysics Research Group, Semmelweis University, Budapest, Hungary. Electronic address: kellermayer.miklos@med.semmelweis-univ.hu.
authors.forename: Miklós
authors.initials: M
authors.lastname: Kellermayer
mesh_terms: D000818:Animals; D020240:Apraxia, Ideomotor; D064211:Connectin / Q000737:chemistry*; D018625:Microscopy, Atomic Force; D018482:Muscle, Skeletal; D052898:Optical Tweezers; D058767:Protein Unfolding; D011817:Rabbits; D014783:Viscosity
chemical_list: D064211:Connectin