Mechanics of collective unfolding

Matthieu Caruel (1, 2) , Jean-Marc Allain (3) , Lev Truskinovsky (3)

1 MSME - Laboratoire de Modélisation et Simulation Multi Echelle
2 M3DISIM - Mathematical and Mechanical Modeling with Data Interaction in Simulations for Medicine
3 LMS - Laboratoire de mécanique des solides

Mechanically induced unfolding of passive crosslinkers is a fundamental biological phenomenon encountered across the scales from individual macro-molecules to cytoskeletal actin networks. In this paper we study a conceptual model of athermal load-induced unfolding and use a minimalistic setting allowing one to emphasize the role of long-range interactions while maintaining full analytical transparency. Our model can be viewed as a description of a parallel bundle of N bistable units confined between two shared rigid backbones that are loaded through a series spring. We show that the ground states in this model correspond to synchronized, single phase configurations where all individual units are either folded or unfolded. We then study the fine structure of the wiggly energy landscape along the reaction coordinate linking the two coherent states and describing the optimal mechanism of cooperative unfolding. Quite remarkably, our study shows the fundamental difference in the size and structure of the folding-unfolding energy barriers in the hard (fixed displacements) and soft (fixed forces) loading devices which persists in the continuum limit. We argue that both, the synchronization and the non-equivalence of the mechanical responses in hard and soft devices, have their origin in the dominance of long-range interactions. We then apply our minimal model to skeletal muscles where the power-stroke in acto-myosin crossbridges can be interpreted as passive folding. A quantitative analysis of the muscle model shows that the relative rigidity of myosin backbone provides the long-range interaction mechanism allowing the system to effectively synchronize the power-stroke in individual crossbridges even in the presence of thermal fluctuations. In view of the prototypical nature of the proposed model, our general conclusions pertain to a variety of other biological systems where elastic interactions are mediated by effective backbones.

Mots clés: Long-range interactions; Statistical mechanics

Mots clés: Muscle

Mots clés: Motor proteins

Mots clés: Protein folding

Domaines: Mécanique des solides [physics.class-ph]; Biomécanique [physics.med-ph]; Matériaux

Matthieu Caruel, Jean-Marc Allain, Lev Truskinovsky. Mechanics of collective unfolding. Journal of the Mechanics and Physics of Solids, 2015, 76, pp.237 - 259. ⟨10.1016/j.jmps.2014.11.010⟩. ⟨hal-01100823v2⟩