Phalloidin stabilizes the F-actin pointed end by bridging terminal subunits, whereas DNase I binding to the penultimate subunit displaces the ultimate subunit, causing filament disassembly.
Cryo-EM structures elucidate the molecular mechanisms by which phalloidin stabilizes and DNase I disassembles the pointed end of actin filaments.
Actin filament turnover involves subunits binding to and dissociating from the filament ends, with the pointed end being the primary site of filament disassembly. Several molecules modulate filament turnover, but the underlying mechanisms remain incompletely understood. Here, we present three cryo-EM structures of the F-actin pointed end in the presence and absence of phalloidin or DNase I. The two terminal subunits at the undecorated pointed end adopt a twisted conformation. Phalloidin can still bind and bridge these subunits, inducing a conformational shift to a flattened, F-actin-like state. This explains how phalloidin prevents depolymerization at the pointed end. Interestingly, two DNase I molecules simultaneously bind to the phalloidin-stabilized pointed end. In the absence of phalloidin, DNase I binding would disrupt the terminal actin subunit packing, resulting in filament disassembly. Our findings uncover molecular principles of pointed end regulation and provide structural insights into the kinetic asymmetry between the actin filament ends.
Sanders et al. (2024) studied this question. Phalloidin and DNase I vs. Undecorated F-actin was evaluated on Cryo-EM structure and conformational changes of the F-actin pointed end. Phalloidin stabilizes the F-actin pointed end by bridging terminal subunits, whereas DNase I binding to the penultimate subunit displaces the ultimate subunit, causing filament disassembly.