Vimentin intermediate filament assembly is a reversible process
Résumé
Networks of intermediate filaments (IF) need to constantly reorganize to fulfill their functions at different locations within the cell. IF assembly results from end-to-end annealing, which is commonly assumed to be irreversible. By contrast, the mechanisms involved in IF disassembly are far less understood. IF fragmentation has however been observed in many cell types, and it has been suggested that it could be associated with post-translational modifications. In this article, we investigate the contribution of filament fragmentation in the assembly dynamics of type III vimentin IF using a combination of in vitro reconstitution, fluorescence imaging, and theoretical modeling. We first show that vimentin assembly at low concentrations results in an equilibrium between filament annealing and fragmentation at time ≥ 24 h. At higher concentration, entanglements kinetically trap the system out of equilibrium, and we show that this trapping is reversible upon dilution. Taking into account both fragmentation and entanglement, we estimated that the mean bond breaking time was ∼18 hours Finally, we provide direct evidence through dual color imaging that filament fragmentation and annealing coexist during assembly. By showing that IF fragmentation can occur without cofactors or post-translational-modifications, our study provides a physical understanding of the IF length regulation. Significance Statement Vimentin intermediate filaments are a key component of the cytoskeleton and are involved in many cellular functions, such as the regulation of cell shape, migration and division. These functions require cytoskeletal filaments to simultaneously assemble and disassemble throughout the life of the cell. While the mechanisms of intermediate filament assembly have been widely studied, the minimal ingredients underpinning their disassembly are not understood. Here, we demonstrate that vimentin constantly disassembles through filament breakage without the assistance of any other protein or post-translational modification, contrary to common wisdom. Our findings suggest that the dynamic cytoskeletal steady-states observed in cells could be largely shaped by simple physical effects linked to the reversible association of vimentin subunits, combined with dramatic kinetic trapping effects that hinder network reorganization as soon as the filaments become too dense and too long.
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