Action of cofilin on ADP-Pi-containing actin filaments

Abstract

In cells, the disassembly of actin filaments is tightly controlled by molecular mechanisms where cofilin plays a central role. Cofilin binds to actin filaments cooperatively, forming clusters that lead to the severing of the filaments. Since cofilin binds preferentially filaments that have hydrolysed ATP and released inorganic phosphate (Pi), most studies have focused on the action of cofilin on ADP-actin filaments. Here, we ask how cofilin binds and severs filaments that still contain a large fraction of ADP{middle dot}Pi-actin. To answer this, we use microfluidics and fluorescence microscopy to monitor single filaments exposed to cofilin in controlled conditions, allowing us to disentangle and to quantify the different reactions. We find that the step limiting cluster nucleation is the binding of 2 cofilins on a minimal group of 3-4 contiguous ADP-actin subunits. Cofilin clusters then expand mostly in the direction of the pointed end of the filament, by easily binding to the ADP-actin neighbor of an ADP{middle dot}Pi-actin subunit, whose Pi release it accelerates. Severing at the ADP{middle dot}Pi-actin/cofilin cluster boundary is 30-fold slower than for an ADP-actin boundary. Globally, it seems that cofilin senses and modifies the nucleotide state of the actin subunits that are in its direct vicinity only. Our results provide a more comprehensive understanding of the action of cofilin, taking into account the nucleotide-state complexity of actin filaments.