A biochemical pacemaker system for robust actin growth

Abstract

Eukaryotic cells determine their shape and organize their interior through a dynamic actin cytoskeleton that forms a variety of architectures, each crucial for a unique cellular function. Common to all actin structures is their growth from soluble subunits that assemble into filaments. The assembly of functional actin networks requires control over the speed at which filaments grow. How this can be realized at the high but variable concentrations of soluble actin subunits found in cells is not known. Biochemical reconstitution has singled out the concentration of soluble actin subunits as the key factor that directly controls the speed of actin growth in a linear manner. At the same time, however, this creates a paradox: The soluble actin concentration differs strongly not only between organisms, but also among distinct cell types and likely even in distinct locations within a single cell. Our current view of actin assembly posits that these large differences will result in dramatically different actin growth rates. We presently do not understand how cells deal with changes in the concentration of polymerizable actin to control filament growth. Here we develop new methods to visualize actin growth at single filament resolution over concentrations previously simply inaccessible. This puts us in the unique position to reconstitute actin assembly over the full physiological range of free subunit levels for the first time. Using this novel approach, we discover that under cell-like conditions, actin growth is not controlled by the concentration of soluble subunits. Instead, we identify a key reaction in the actin elongation cycle the release of the monomer-binding protein profilin from the filament end- as the kinetic bottleneck that limits the speed of filament growth. We show that this kinetic limit we discovered in vitro confers robustness to actin growth in the cytoplasm of mammalian cells. The fundamental finding is that this mechanism buffers the speed of actin filament growth against changes in the soluble subunit concentration, an essential requirement for the control of actin dynamics in cells. This opens a myriad of new opportunities to understand the regulation of actin dynamics and we explore the most immediate consequences. We show that the kinetic limit to actin growth imposed by profilin is not fixed but actively modulated by actin polymerases: Formin proteins, previously thought to simply increase the concentration of actin subunits at the actin filament end, catalytically promote the release of profilin from actin. This transforms our mechanistic understanding of these essential actin polymerases. In general terms, we show how the collective action of formin and profilin constitutes a molecular pacemaker system that creates robust, but tunable rates of actin growth.