Continuous self-repair protects vimentin intermediate filaments from fragmentation

Abstract

Intermediate filaments are key regulators of cell mechanics. Vimentin, a type of intermediate filament expressed in mesenchymal cells and involved in migration, forms a dense network in the cytoplasm that is constantly remodeling through filament transport, elongation/shortening, and subunit exchange. While it is known that filament elongation involves end-to-end annealing, the reverse process of filament shortening by fragmentation remains unclear. Here, we use a combination of in vitro reconstitution, probed by fluorescence imaging and AFM, with theoretical modeling to uncover the molecular mechanism involved in filament breakage. We first show that vimentin filaments are composed of two populations of subunits, half of which are exchangeable and half immobile. We also show that the exchangeable subunits are tetramers. Furthermore, we reveal a mechanism of continuous filament self-repair, where a soluble pool of vimentin tetramers in equilibrium with the filaments is essential to maintain filament integrity. Filaments break due to local fluctuations in the number of tetramers per cross-section, induced by the constant subunit exchange. We determine that a filament tends to break if approximately four tetramers are removed from the same filament cross-section. Finally, we analyze the dynamics of association/dissociation and fragmentation to estimate the binding energy of a tetramer to a complete versus a partially disassembled filament. Our results provide a comprehensive description of vimentin turnover and reveal the link between subunit exchange and fragmentation. SIGNIFICANCE STATEMENTIntermediate filaments such as vimentin are key contributors to cell mechanics. These filaments are continuously assembled and disassembled by the cell as it grows and moves. While the assembly process is relatively well characterized, here we investigate their much less understood disassembly. We find that the filaments continuously exchange their constitutive proteins with the surrounding solution throughout their length and when too many subunits are removed, the filament breaks. This process sharply differs from the length regulation mechanisms known to occur in other types of filaments involved in the regulation of cell mechanics. Importantly, we also identify two distinct populations of subunits within the filaments, which raises new questions about vimentin intermediate filament structure.