Effect of Divalent Cations on the Structure and Mechanics of Vimentin Intermediate Filaments

Divalent cations behave as effective cross-linkers of intermediate filaments (IFs) such as vimentin IF (VIF). These interactions have been mostly attributed to their multivalency. However, ion-protein interactions often depend on the ion species, and these effects have not been widely studied in IFs. Here, we investigate the effects of two biologically important divalent cations, Zn²⁺ and Ca²⁺, on VIF network structure and mechanics in vitro. We find that the network structure is unperturbed at micromolar Zn²⁺ concentrations, but strong bundle formation is observed at a concentration of 100 μM. Microrheological measurements show that network stiffness increases with cation concentration. However, bundling of filaments softens the network. This trend also holds for VIF networks formed in the presence of Ca²⁺, but remarkably, a concentration of Ca²⁺ that is two orders higher is needed to achieve the same effect as with Zn²⁺, which suggests the importance of salt-protein interactions as described by the Hofmeister effect. Furthermore, we find evidence of competitive binding between the two divalent ion species. Hence, specific interactions between VIFs and divalent cations are likely to be an important mechanism by which cells can control their cytoplasmic mechanics.