How cofilin severs an actin filament

Abstract

The actin regulatory protein, cofilin, promotes actin assembly dynamics by severing filaments and increasing the number of ends from which subunits add and dissociate. Recent studies provide biophysical descriptions of cooperative filament interactions in energetic, mechanical and structural terms. A one-dimensional Ising model with nearest-neighbor interactions permits thermodynamic analysis of cooperative binding and indicates that one or a few cofilin molecules can sever a filament. Binding and cooperative interactions are entropically driven. A significant fraction of the binding free energy results from the linked dissociation of filament-associated ions (polyelectrolyte effect), which modulate filament structure, stability and mechanics. The remaining binding free energy and essentially all of the cooperative free energy arise from the enhanced conformational dynamics of the cofilactin complex. Filament mechanics are modulated by cofilin such that cofilin-saturated filaments are approximately 10- to 20-fold more compliant in bending and twisting than bare filaments. Cofilin activity is well described by models in which discontinuities in topology, mechanics and conformational dynamics generate stress concentration and promote fracture at junctions of bare and decorated segments, analogous to the grain boundary fracture of crystalline materials and the thermally driven formation of shear transformation zones in colloidal glass.

Fig. 1

a Schematic of one-dimensional Ising model of cofilin binding to an actin filament. Individual filament subunits are depicted as squares of an infinite, one-dimensional (1-D) lattice. A subunit with bound cofilin is indicated with a filled circle. K_a is the association equilibrium binding constant, and ω is the unitless cooperativity factor. The overall binding constants are given by K_a (isolated, non-contiguous bound cofilin), K_aω (singly-contiguous bound cofilin) and K_aω² (doubly contiguous bound cofilin). b Cooperative cofilin binding to actin filaments. The lines represent the best fit of the data for human cofilin-1 binding to rabbit muscle actin (filled circles) to the Hill equation (dotted line) or binding to a 1-D lattice with nearest neighbor interactions as depicted in a. The figure is adapted from De La Cruz (2005). c Cofilin cluster size distribution. The probability of bound cofilin being in a given cluster size. Each line represents the distribution at the color-specified binding density

Fig. 2

Conformational dynamics of actin filaments. a Schematic representation of actin filament thermal motions. Figure is adapted from Prochniewicz et al. (2005). b Models used to analyze material properties of filaments. Top to bottom Schematic of a linear array of cylindrical subunits connected by a flexible elastic linker that is used when modeling filament torsional dynamics. An actin filament (yellow) modeled with an elliptical cross-section of a 2.7-nm minor radius, a 4.5-nm major radius and 37-nm crossover lengths, and a cofilactin filament (red) modeled with an elliptical cross-section of a 2.7-nm minor radius, a 6.7-nm major radius and 27-nm crossover lengths. These geometric models were used to calculate the second moment of inertia (I) and apparent elastic modulus (E) from the filament flexural rigidity. An actin filament modeled as a homogenous isotropic elliptical cylinder has a second moment of inertia (I) of 120 nm⁴. Cofilin binding increases the filament radius and has a value of 240 nm⁴ for I. Models are presented with and without overlays of the corresponding reconstructions based on cryoelectron microscopy (McGough et al. 1997). Parts of the figure are adapted from McCullough et al. (2008)

Fig. 3

Overlay of actin and cofilactin filament shape configurations. Digital images of actin (yellow) and cofilactin filament (red) segments of identical length undergoing thermal fluctuations were overlaid and oriented with one end starting from the same point of origin and the other end on the same axis. The straight pink rod represents a rigid filament. Image is adapted from McCullough et al. (2008)

Fig. 4

Correlation of filament assembly dynamics activity with cofilin binding modes. a Cofilin concentration-dependence of the populated bound state equilibrium distributions. The lines represent the fractional distribution of actin filament sites as predicted from the equilibrium binding constants obtained from the nearest-neighbor cooperative binding model (De La Cruz, ; Cao et al. 2006). Note that the sites are plotted as a function of the total cofilin concentration, and not of free cofilin concentration; the actin concentration used in the calculations is approximately 1/K _a ω. b Same as a, except plotted as a function of the filament binding density. c Correlation of cofilin activity with the existence of bare and cofilin-decorated boundaries (i.e. junctions) on a filament. Boundaries were calculated as the sum of singly contiguous sites plus twice the sum of non-contiguous bound cofilin. The filled squares represent the change in phase transition temperature as measured by differential scanning calorimetry (Bobkov et al. 2006). The filled circles represent the cofilin-dependence of net subunit dissociation from filament pointed ends presented by Yeoh et al. (2002). Both sets of data have been normalized to scale the y-axes for presentation. These measurements were made under buffer conditions similar to those used to measure cooperative binding parameters and with the identical cofilin and actin isoforms. d Overlay of cofilin activity with formation of non-contiguous (isolated) bound cofilin