Computational modeling highlights the role of the disordered Formin Homology 1 domain in profilin‐actin transfer

Abstract

Formins accelerate actin polymerization, assumed to occur through flexible FH1 domain mediated transfer of profilin-actin to the barbed end. To study FH1 properties and address sequence effects including varying length/distribution of profilin-binding proline-rich motifs, we performed all-atom simulations of mouse mDia1, mDia2; budding yeast Bni1, Bnr1; fission yeast Cdc12, For3, and Fus1 FH1s. We find FH1 has flexible regions between high propensity polyproline helix regions. A coarse-grained model retaining sequence-specificity, assuming rigid polyproline segments, describes their size. Multiple bound profilins or profilin-actin complexes expand mDia1-FH1, which may be important in cells. Simulations of the barbed end bound to Bni1-FH1-FH2 dimer show the leading FH1 can better transfer profilin or profilin-actin, having decreasing probability with increasing distance from FH2.

Competition for delivery of profilin–actin to barbed ends limits the rate of formin-mediated actin filament elongation

“…Our experiments with double-tract variants of Bni1p indicated that neighboring polyproline tracts do not compete to bind profilin-actin ( Figure 6D). This is consistent with published crystallographic and molecular modeling studies, which demonstrated that FH1 domains can accommodate simultaneous binding of multiple profilin-actin complexes to neighboring tracts (18,19). Instead, competition arises when multiple tracts compete to deliver profilin-actin to the barbed end ( Figure 6E).…”

“…What determines the number of polyproline tracts encoded by a formin's FH1 domain? This number ranges among formin isoforms from as few as 1 to as many as 14 (18). Because the probability of simultaneous binding of multiple profilin-actin complexes increases with the number of polyproline tracts, competition for delivery should strongly attenuate the elongation rates mediated by formins with large numbers of tracts.…”

“…Binding of the protein profilin to actin monomers enables formins to speed elongation beyond the basal rate mediated by their FH2 domains (8,13). Profilin-actin complexes bind to polyproline tracts located in the otherwise unstructured formin FH1 domain (18,19), which is positioned directly N-terminal to the FH2 domain ( Figure 1). Diffusion of the flexible FH1 domain facilitates rapid delivery and transfer of bound profilin-actin complexes to the barbed end for incorporation via FH2 stepping (13,20) (Figure 1).…”

“…The presence of multiple polyproline tracts in formin FH1 domains gives rise to an array of parallel pathways for formin-mediated delivery of profilin-actin to the barbed end ( Figure 1). The number of polyproline tracts encoded in formin FH1 domain sequences varies widely, from as few as 1 to as many as 14 (18). It has been observed that formins whose FH1 domains contain a large number of polyproline tracts mediate faster elongation than do formins with fewer tracts (8).…”

Geometrical constraints greatly hinder formin mDia1 activity

“…It has recently been reported that different surface-anchoring schemes, putting different constraints on formin rotation may affect its ability to elongate filaments (Yu et al, 2017) . Here, for FH1-side anchored formins, the FH1 domain can be considered as a flexible linker with a contour length of~40 nm, connecting the formin to the surface (Horan et al, 2018) . In contrast, when formins are anchored by their FH2 side, the DAD domain, which lies downstream of FH2 and is composed of a short alpha-helix followed by an unstructured region, can be considered as a linker of~10 nm in contour length (Kühn and Geyer, 2014) .…”