(501i) Mechanism of Inorganic Phosphate Release Reaction in Actin Subunits

Actin filaments and their dynamic response to environmental perturbations impart cells with varied functionalities such as their ability to migrate, divide, shape change, transport cargo, etc. The ATP bound within actin subunits in a filament undergoes a series of reactions called ATP hydrolysis and inorganic phosphate (Pi) release. These reactions play a crucial role in dynamically remodeling the higher-order structures of actin filaments by modulating their interactions with other binding proteins. Although a lot of studies have focused on unraveling the key steps driving the ATP hydrolysis reaction, a lot is not known about the mechanism of the Pi release. In this regard, here, we investigate the mechanism of Pi release reaction and correlate it with actin subunit conformational fluctuations. To do so, we adopt Transition Tempered Metadynamics (TTMetaD), an enhanced free energy sampling technique, to efficiently sample the underlying free-energy landscape and identify the minimum free energy path driving the release of inorganic phosphate. Comprehensive knowledge of the mechanism of phosphate release from actin subunits would help in elucidating the potential binding modes of several regulatory proteins, which play important roles in the remodeling of actin networks. Furthermore, it would enable the development of rigorous bottom-up coarse-grained models of actin filaments that can be used to simulate actin networks efficiently.