Cations Modulate Actin Bundle Mechanics, Assembly Dynamics, and Structure

Abstract

Actin bundles are key factors in the mechanical support and dynamic reorganization of the cytoskeleton. High concentrations of multivalent counterions promote bundle formation through electrostatic attraction between actin filaments that are negatively charged polyelectrolytes. In this study, we evaluate how physiologically relevant divalent cations affect the mechanical, dynamic, and structural properties of actin bundles. Using a combination of total internal reflection fluorescence microscopy, transmission electron microscopy, and dynamic light scattering, we demonstrate that divalent cations modulate bundle stiffness, length distribution, and lateral growth. Molecular dynamics simulations of an all-atom model of the actin bundle reveal specific actin residues coordinate cation-binding sites that promote the bundle formation. Our work suggests that specific cation interactions may play a fundamental role in the assembly, structure, and mechanical properties of actin bundles.

Counterion crossbridges enable robust multiscale elasticity in actin networks

“…Bundles of actin filaments are inherently stiffer than single filaments, such that a network of counterion-crossbridged filaments should have distinct mechanical properties from a network of single filaments [10]. For example, Mg 2+ concentrations of 10-50 mM have been reported to produce actin bundles with persistence lengths l p up to ∼4× larger than that of single actin filaments (l p ≈ 10 μm) [9,[11][12][13][14]. Further, theoretical studies have shown that the bending stiffness of a bundle is impacted by both the number of filaments comprising the bundle as well as the density of connections the bundle has with its neighbors, suggesting that both bundling as well as crosslinking of bundles may play an important role in the mechanics of counterion-crossbridged networks [5].…”

Regulation of Streptococcus pneumoniae FtsZ assembly by divalent cations: paradoxical effects of Ca2+ on the nucleation and bundling of FtsZ polymers

“…Thus, we suggest a model for the mechanism by which Ca 2+ affects the assembly of FtsZ. The neutralization of the charges on the surface of the filaments by the counter ion-Ca 2+ could also be responsible for the bundling as suggested for the actin polymers [56,57]. 11).…”

“…During the initial stages of the assembly, the conformational change caused by Ca 2+ binding to FtsZ affects either the activation of monomers or the nucleation of the polymers by hindering the monomer-monomer association, as suggested above (Fig. It has been suggested that higher concentrations of divalent cations lead to the 'overcharge' on the filaments which perturbs the bundling [56,57]. The bundling of FtsZ filaments caused by Ca 2+ could be an independent effect where the conformational changes induced in FtsZ on binding to Ca 2+ , expose the residues or regions required for the lateral association [40].…”

“…The neutralization of the charges on the surface of the filaments by the counter ion-Ca 2+ could also be responsible for the bundling as suggested for the actin polymers [56,57]. It has been suggested that higher concentrations of divalent cations lead to the 'overcharge' on the filaments which perturbs the bundling [56,57]. Since there is no inhibition of the assembly of FtsZ at higher concentrations of Ca 2+ , we speculate that the prevention of bundling can be due to a high positive charge of Ca 2+ bound to the filaments, perturbing the lateral interactions between them.…”

“…The bundling of FtsZ filaments caused by Ca 2+ could be an independent effect where the conformational changes induced in FtsZ on binding to Ca 2+ , expose the residues or regions required for the lateral association [40]. The neutralization of the charges on the surface of the filaments by the counter ion-Ca 2+ could also be responsible for the bundling as suggested for the actin polymers [56,57]. It has been suggested that higher concentrations of divalent cations lead to the 'overcharge' on the filaments which perturbs the bundling [56,57].…”