Condensation Agents Determine the Temperature–Pressure Stability of F‐Actin Bundles

Abstract

Biological cells provide a large variety of rodlike filaments, including filamentous actin (F-actin), which can form meshworks and bundles. One key question remaining in the characterization of such network structures revolves around the temperature and pressure stabilities of these architectures as a way to understand why cells actively use proteins for forming them. The packing properties of F-actin in fascin- and Mg(2+) -induced bundles are compared, and significantly different pressure-temperature stabilities are observed because of marked differences in their nature of interaction, solvation, and packing efficiency. Moreover, differences are observed in their morphologies and disintegration scenarios. The pressure-induced dissociation of the actin bundles is reminiscent of a single unbinding transition as observed in other soft elastic manifolds.

On the Origin of Microtubules’ High-Pressure Sensitivity

“…This can be explained by the weakening effect of pressure on electrostatic interactions (referred to as the electrostrictive effect), because dissociation of ion pairs results in rehydration of the ions, which is accompanied by formation of a compact hydration layer and thus reduction in the overall volume (53). Similar pressure sensitivity was recently observed for Mg 2þ -induced actin filament bundles (36). In contrast, the charge density of the tetravalent cation spermine seems to be high enough to overcome the electrostrictive effect and maintain the electrostatic condensation of the microtubules up to 150-165 MPa, where pressure causes complete disintegration of taxol-stabilized microtubules.…”

“…In contrast, actin filaments, another important component of the eukaryotic cytoskeleton, have been shown to be stable up to $150-200 MPa (34,35). Recently, our laboratory showed that the pressure sensitivity of actin filaments originates from the limited pressure stability of the monomeric building block, which is hardly stable under abyssal conditions, and that actin-binding proteins such as cross-linkers and nucleators can significantly improve their pressure resistance (35)(36)(37). Here, we systematically studied the pressure stability of microtubules at different structural levels and for distinct dynamic states using high-pressure Fourier-transform infrared (FTIR) spectroscopy and Synchrotron small-angle x-ray scattering (SAXS).…”

Highly Dispersed Fullerenols Hamper Osteoclast Ruffled Border Formation by Perturbing Ca2+ Bundles

“…The first order peaks of q a = 0.886 and q c = 0.576, and the higher order peaks of q b = 1.338 and q d = 1.165 are obvious diffraction peaks of actin bundles. The q a : q b and q c : q d ratios are similar to 1:√3 and 1:2, respectively, indicating that Ca 2+ and fascin bundles are hexagonal packings of F‐actins . The peak intensity weakens in Ca 2+ bundles treated with fullerenols but shows little change in fascin bundles, hinting at a lower amount of B‐actin.…”

“…ABPs such as fascin, Arp2/3 complex, and fimbrin are responsible for actin bundling. However, positively charged cations (e.g., Ca 2+ , Mg 2+ ) and polyamines (e.g., alkaline amino acid‐rich polypeptides) can also induce bundle formation in vitro and are present in vivo during processes such as bone resorption, neuron repair, and sperm activation . Fullerenols induce distinct alterations for the three typical bundles.…”

“…The direct and ultimate driving forces (the bundlers) for actin bundling can be categorized into three types: actin‐binding proteins (ABPs, e.g., fascin, espin, fimbrin, etc. ), metal cations (e.g., Ca 2+ , Mg 2+ , etc. ), and the polycation‐like polypeptide .…”