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.

中文翻译：

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二价阳离子对波形蛋白中间丝结构和力学的影响

二价阳离子充当中间长丝 (IF) 的有效交联剂，例如波形蛋白 IF (VIF)。这些相互作用主要归因于它们的多价性。然而，离子-蛋白质相互作用通常取决于离子种类，这些影响尚未在 IF 中得到广泛研究。在这里，我们研究了两种生物学上重要的二价阳离子 Zn2+ 和 Ca2+ 对体外 VIF 网络结构和力学的影响。我们发现网络结构在微摩尔 Zn2+ 浓度下不受干扰，但在 100 μM 浓度下观察到强烈的束形成。微流变测量表明网络刚度随着阳离子浓度的增加而增加。然而，细丝的捆绑会软化网络。这种趋势也适用于在 Ca2+ 存在下形成的 VIF 网络，但值得注意的是，要达到与 Zn2+ 相同的效果，需要高两个数量级的 Ca2+ 浓度，这表明 Hofmeister 效应所描述的盐 - 蛋白质相互作用的重要性。此外，我们发现了两种二价离子物种之间竞争性结合的证据。因此，VIF 和二价阳离子之间的特定相互作用可能是细胞控制其细胞质力学的重要机制。