Figure 8.

Model figure illustrating the function of mutant actin constructs and the role of Y53 phosphorylation during dendritic spine maturation and LTP formation. A, Working model for the effects of the Y53E- and Y53A-actin expression on actin filament turnover. In a cellular context, there is a net addition of monomers at the plus end and net dissociation at the pointed end, leading to filament elongation occurring mainly at the plus end. The constant incorporation and dissociation of monomers at filament ends results in a “treadmilling” effect. Y53E-actin creates points with weaker actin–actin binding along the filament. When filaments break at these points, the filaments become shorter and the increase in numbers of both plus and minus ends enhances the turnover rate. Y53A-actin creates higher affinity binding with adjacent actin monomers, thus slowing down the depolymerization rate at minus ends. This “pointed end capping” increases filament length. B, During spine morphogenesis, actin filaments are maintained highly dynamic by Y53 phosphorylation (spine on left). High-frequency synapse activation induces reorganization, polymerization, and stabilization of actin filaments. We propose that actin phosphorylation serves as a mechanism to break actin filaments into shorter filaments to facilitate the reorganization of the actin cytoskeleton (spine in the middle). In turn, dephosphorylation is required for the stabilization of actin filaments during LTP maintenance or during dendritic spine maturation (spine on right).