Mechanosensitive Structural States of Titin

Abstract

Titin, a giant, multi-domain filamentous protein has been suggested to act as a sensor of sarcomeric stress and strain. The exact mechanisms of this putative mechanosensing function are, however, yet unknown. To gain an insight into the mechanosensitive structural states we have manipulated titin with high-resolution optical tweezers and imaged structural states of extended molecules with atomic force microscopy. Discrete, stepwise transitions can be resolved in titin during stretch at forces as low as 5 pN. Multiple mechanisms and molecular regions, such as the unfolding of globular domains and the extension of unique sequences, contribute to a pattern of transitions which is sensitive to the history of contractile events. Globular domains are apparently selected for unfolding according to a spatially dispersed gradient of mechanical stabilities which emerges as a safety mechanim for protecting the sarcomere against structural disintegration under excessive mechanical conditions. A C-terminally located region corresponding to the titin kinase unfolds systematically under overstretching forces suggesting that this domain may indeed be mechanically activated in intrasarcomeric conditions. Mechanically unfolded weak domains may dynamically reorganize towards the molten-globule state, thereby yielding an extra contractility that aids saromere mechanics. Altogether, titin displays a complex pattern of history-dependent, force-driven structural transitions which, by dynamically exposing ligand-binding sites, may set the stage for the sensing of the sarcomeric mechanical status.