(346ab) Mechanism of Inorganic Phosphate Release from Actin Subunits

The diverse functionalities of cells such as their ability to move, divide, and respond to environmental perturbations are governed by actin filaments. The nucleotide bound to actin subunits in the filament undergoes a series of reactions called ATP hydrolysis and inorganic phosphate (Pi) release. It is well known that the rate of ATP hydrolysis reaction is four orders of magnitude faster in actin filaments compared to monomeric actin (g-actin) and a lot of studies have focused on unraveling the key steps driving this reaction. However, the Pi release reaction is not well studied and a critical understanding of the structural changes favoring this reaction is still lacking. In this regard, here, we investigate the mechanism of Pi release reaction and correlate it with actin subunit conformational changes. Specifically, we adopt Transition Tempered Metadynamics (TTMetaD), an enhanced sampling simulation method, to efficiently sample the underlying free-energy landscape and identify the minimum free energy path for the release of inorganic phosphate. Several actin-binding proteins preferentially interact to actin subunits with a specific nucleotide state and regulate the formation of various types of higher-order actin filament structures such as crosslinked networks, bundled filaments, branched networks, and so on. A comprehensive knowledge of the mechanism of the phosphate release reaction would not only help in understanding the binding of these regulatory proteins but also in the development of rigorous bottom-up coarse-grained models, which can be used to simulate actin filament networks in an efficient manner.