Structural Dynamics of F-Actin: II. Cooperativity in Structural Transitions

A large body of biochemical evidence suggests that the F-actin filament can have internal cooperativity. We have observed large cooperative effects on the low-resolution structure of actin filaments under three very different conditions. First, when G-Ca²⁺-actin is polymerized by both Mg²⁺and KCl, filaments may be found in two different populations, with two discrete positions seen for subdomain 2. When G- Ca²⁺actin is polymerized by only Mg²⁺, a single F-Mg²⁺-actin population is seen. The structural data suggest that an entire filament exists with subdomain 2 in one state or the other when there is a heterogenous mixture of Mg²⁺and Ca²⁺-actin. Second, when actin filaments are nucleated from gelsolin there is a conformational change that can be observed throughout the filament that is consistent with a large shift in the actin C terminus. There must be a large cooperative propagation of this effect throughout the filament from the nucleation piont. Third, we have used phalloidin to stabilize F-actin in which two C-terminal residues have been proteolytically removed by trypsin. It has been shown biochemically that this stabilization occurs at substoichometric amounts of phalloidin. Phalloidin, at either a 1:1 or a 1:20 molar ratio with actin, restores the connectivity between the long-pitch helical strands. F-actin's internal cooperativity will have large implicationsin vivo, particularly in muscle.