No abstract

Formins promote processive elongation of actin filaments for cytokinetic contractile rings and other cellular structures. In vivo, these structures are exposed to tension, but the effect of tension on these processes was unknown. Here we used single-molecule imaging to investigate the effects of tension on actin polymerization mediated by yeast formin Bni1p. Small forces on the filaments dramatically slowed formin-mediated polymerization in the absence of profilin, but resulted in faster polymerization in the presence of profilin. We propose that force shifts the conformational equilibrium of the end of a filament associated with formin homology 2 domains toward the closed state that precludes polymerization, but that profilin-actin associated with formin homology 1 domains reverses this effect. Thus, physical forces strongly influence actin assembly by formin Bni1p.A host of proteins regulate the actin cytoskeleton by controlling filament nucleation, elongation, capping, branching, and bundling. Members of the formin family of proteins nucleate new filaments and remain processively attached to barbed ends while promoting the elongation of unbranched filaments (1, 2). Formin homology (FH)2 domains form a donut-shaped head-to-tail homodimer that encircles the fast-growing barbed end of actin filaments and promotes nucleation and polymerization (1, 3). When an actin monomer binds to the barbed end of a filament, one FH2 domain steps onto the new subunit, allowing the formin to remain attached to the filament through thousands of cycles of subunit addition ( Fig. 1A) (2, 4). The FH2-bound end of the filament binds incoming actin monomers when in an "open" conformation but not in the "closed" conformation. As a consequence, FH2 domains slow barbed-end elongation compared with free barbed ends, a phenomenon termed "gating" (1, 5). Despite gating, FH2 domains can promote rapid filament elongation when coupled to FH1 domains (6), which are located N-terminal to the FH2 domain (7). Multiple polyproline tracks in FH1 domains bind the small actin-binding protein profilin, which mediates association of several profilin-actin complexes in close proximity to the end of a filament. Diffusive motions of the FH1 domain transfer actin rapidly to the filament barbed end (5), allowing elongation at rates faster than addition of subunits from the bulk phase.Actin filaments are subject to tension in cells, yet the influence of tension on formin-mediated polymerization was unknown, and theories predicted different outcomes in the absence and presence of profilin. Kozlov and colleagues proposed that the elasticity of the FH2 domain and the formin-barbed end binding energy govern the polymerization rate (8, 9). Their simulations suggested that tension increases the rate of polymerization by lowering the activation barrier for subunit addition and energetically favoring FH2 domain stepping onto the incoming subunit (8). Vavylonis et al. (10) postulated that force-induced stretching of FH1 domains slows the transfer of profilin-actin t...

Cellular viability requires tight regulation of actin cytoskeletal dynamics. Distinct families of nucleation-promoting factors enable the rapid assembly of filament nuclei that elongate and are incorporated into diverse and specialized actin-based structures. In addition to promoting filament nucleation, the formin family of proteins directs the elongation of unbranched actin filaments. Processive association of formins with growing filament ends is achieved through continuous barbed end binding of the highly conserved, dimeric formin homology (FH) 2 domain. In cooperation with the FH1 domain and C-terminal tail region, FH2 dimers mediate actin subunit addition at speeds that can dramatically exceed the rate of spontaneous assembly. Here, I review recent biophysical, structural, and computational studies that have provided insight into the mechanisms of formin-mediated actin assembly and dynamics.