Practice question

Filament remodeling during migration requires rapid creation of new ends for turnover. Cofilin ADF family accomplishes this through severing. Preferential binding to ADP F actin saturated regions induces over twist, changing crossover length from 36 to 27 nanometer, reducing bending rigidity and generating strain at boundaries between decorated and undecorated segments. Molecular dynamics simulations show increased fragmentation probability at boundaries. In vitro viscometry demonstrates precipitous drop in viscosity upon addition of low nanomolar cofilin, fluorescence microscopy shows exponential increase in filament number, generating free barbed ends that elongate if capped protein uncapped and pointed ends that depolymerize quickly. Severing synergizes with Aip1 and coronin to disassemble arrays. Tropomyosin protects filaments by competing for overlapping binding site, formin FH2 stays processively at barbed end resisting severing, vinculin anchors to adhesions not severing. Phosphoregulation through LIM kinase phosphorylating Ser3 blocks actin binding, phosphatase slingshot reactivates during chemotaxis driving network turnover and protrusion.