Structural Biology of Actin: Interplay Between Molecular Assembly, Conformational Polymorphism and ATPase

Actin was first identified as a major muscle protein with two key activities: polymerization and ATPase. Although the significance of these activities in muscle contraction was unclear, subsequent cell biological studies revealed that actin is abundant also in non-muscle cells, where it drives dynamic remodeling through ATP-dependent treadmilling. Biochemical studies revealed that the polymerization coupled with ATP hydrolysis produces two distinct F-actin states: a stable ADP-P_i state and an unstable ADP state. The transition between the ADP-P_i and ADP states (i.e., P_i release) likely generates the free energy that drives treadmilling.In this chapter, we introduce our structural biological studies of these states and the mechanisms underlying their formation. Our F-actin model showed that polymerization induces a rotation between its two rigid domains, shifting actin from a twisted G-form to a flat F-form. We also resolved the cryo-EM structure of cofilin-decorated F-actin, in which actin adopts a distinct C-form. Analysis of PDB data classified actin structures into four conformations: G-, F-, C-, and O-forms, each linked to specific functions-G-form for nucleotide exchange, F-form for ATP hydrolysis, and C-form for filament severing. High-resolution F-form structures further elucidated the ATP hydrolysis pathway and the basis for the stability of the ADP-P_i state.Despite these advances, key questions remain. Although the global structure of F-form actin is identical across nucleotide states, its properties differ: ATP/ADP-P_i states are stable and cofilin-resistant, whereas the ADP state is prone to depolymerization and cofilin-mediated severing. We suggest that each state should be characterized by the distinct nature of conformational fluctuations from F-form back to G-form.