Processive capping by formin suggests a force-driven mechanism of actin polymerization

Abstract

Regulation of actin polymerization is essential for cell functioning. Here, we predict a novel phenomenon-the force-driven polymerization of actin filaments mediated by proteins of the formin family. Formins localize to the barbed ends of actin filaments, but, in contrast to the standard capping proteins, allow for actin polymerization in the barbed direction. First, we show that the mechanism of such "leaky capping" can be understood in terms of the elasticity of the formin molecules. Second, we demonstrate that if a pulling force acts on the filament end via the leaky cap, the elastic stresses can drive actin polymerization. We estimate that a moderate pulling force of approximately 3.4 pN is sufficient to reduce the critical actin concentration required for barbed end polymerization by an order of magnitude. Furthermore, the pulling force increases the polymerization rate. The suggested mechanism of force-driven polymerization could be a key element in a variety of cellular mechanosensing devices.

Figure 1.

Stages of leaky capping driven by pulling force and elasticity of formin dimer. Actin subunits are represented by gray discs. The formin dimer is shown as a bar consisting of blue and pink halves (hemidimers). (a) Actin polymerization in the absence of pulling force. (a1) Formin dimer is in a nondeformed state; it is bound to the protruding but not to the recessed actin subunit, thereby leaving a vacancy for a new actin monomer to insert. (a2) An actin monomer inserts into the existing vacancy and binds to the formin hemidimer and the recessed actin subunit. The two binding events are energetically favorable and accompanied by release of energies ɛ_AB and ɛ_AF, respectively. At the same time, the formin dimer and, probably, the terminal actin subunits involved in interaction with formin undergo deformation, resulting in a relative shift of the formin hemidimers by distance Δz ≈ 2.75 nm, characterizing the helix periodicity of an actin filament (Lorenz et al., 1993). This results in accumulation of elastic energy ɛ_EL and development of elastic force f _EL (red arrows) tending to restore the initial relative position of the formin subunits. (a3) Elastically driven detachment of formin from the recessed actin subunit accompanied by relaxation of the elastic energy, ɛ_EL → 0, and elastic force, f _EL → 0, at the expense of the actin-formin binding energy. This results in creation of a new vacancy for the next actin monomer. (b) Actin polymerization in the presence of pulling force (blue arrows). Insertion of the new actin monomer (b2) and detachment of formin from the recessed subunit (b3) are facilitated by the pulling force, which reduces the energy of each of these stages by f_pull · Δz. As a result, the critical actin concentration required for polymerization at the barbed end is reduced dramatically compared with the critical concentration in the absence of the force.