Exploring the Stability Limits of Actin and its Suprastructures

Abstract

Actin is the main component of the microfilament system in eukaryotic cells and can be found in distinct morphological states. G-actin is able to assemble into highly organized, supramolecular cellular structures, filamentous (F) and bundled (B) actin. To evaluate the structure and establish the stability of G-, F- and B- actin over a wide range of temperatures and pressures, FTIR spectroscopy in combination with differential scanning and pressure perturbation calorimetry, small-angle X-ray scattering, laser confocal scanning and transmission electron microscopy was used. On the one hand, our analysis was designed to provide new insights into the stabilizing forces of actin self-assembly and to reveal the stability of the actin polymorphs, including conditions encountered in extreme environments. On the other hand, our data help understand the limited pressure stability of actin self-assembly observed in vivo. G-actin is not only the least temperature-stable, but also the least pressure-stable actin species. At abyssal conditions, where temperatures as low as 1-4°C and pressures up to 1 kbar are reached, G-actin is hardly stable anymore. The supramolecular assemblies of actin are stable enough to withstand the extreme conditions usually encountered on Earth, however. Beyond ∼3-4 kbar, filamentous structures disassemble, and beyond ∼4 kbar, complete dissociation of F-actin structures is observed. Between about 1 and 2 kbar, some disordering of actin assemblies commences already, in agreement with in vivo observations. The limited pressure stability of the monomeric building block seems to be responsible for the suppression of actin assembly in the kbar pressure range.