Biphasic Effect of Profilin Impacts the Formin mDia1 Force-Sensing Mechanism in Actin Polymerization

Abstract

Formins are force-sensing proteins that regulate actin polymerization dynamics. Here, we applied stretching tension to individual actin filaments under the regulation of formin mDia1 to investigate the mechanical responses in actin polymerization dynamics. We found that the elongation of an actin filament was accelerated to a greater degree by stretching tension for ADP-G-actin than that for ATP-G-actin. An apparent decrease in the critical concentration of G-actin was observed, especially in ADP-G-actin. These results on two types of G-actin were reproduced by a simple kinetic model, assuming the rapid equilibrium between pre- and posttranslocated states of the formin homology domain two dimer. In addition, profilin concentration dramatically altered the force-dependent acceleration of actin filament elongation, which ranged from twofold to an all-or-none response. Even under conditions in which actin depolymerization occurred, applications of a several-piconewton stretching tension triggered rapid actin filament elongation. This extremely high force-sensing mechanism of mDia1 and profilin could be explained by the force-dependent coordination of the biphasic effect of profilin; i.e., an acceleration effect masked by a depolymerization effect became dominant under stretching tension, negating the latter to rapidly enhance the elongation rate. Our findings demonstrate that the biphasic effect of profilin is controlled by mechanical force, thus expanding the function of mDia1 as a mechanosensitive regulator of actin polymerization.

Geometrical constraints greatly hinder formin mDia1 activity

“…In all these configurations, individual formins behaved similarly in terms of elongation rate (figure 1B) and processivity ( figure 1C). These results are in agreement with previous reports for glass-anchored formins (Cao et al, 2018;Jégou et al, 2013;Kubota et al, 2017) . In the lipid-anchored formin configuration, the microfluidics flow dragged the filament-elongating formins to the edge of the lipid pattern, where their activity was unaffected.…”

“…In cases where the leading filament buckled between the leading and trailing formin attachment points, the buckling force was estimated to be lower than 0.15 pN (see Methods). For trailing formins, this force adds up to the minimal viscous pulling force applied to the trailing filament, and is expected to accelerate elongation, not slow it down (Jégou et al, 2013;Kubota et al, 2017;Yu et al, 2017) . We therefore concluded that the applied forces are negligible and likely play no role in the slowing down of formin elongation rates.…”

“…The mechanosensitivity of formins is now well established. Applying a pulling force on a formin bound to an actin filament led to an increase in barbed end elongation rate for mDia1 (Jégou et al, 2013;Kubota et al, 2017;Yu et al, 2017Yu et al, , 2018 , mDia2 (Cao et al, 2018; and Bni1p (Courtemanche et al, 2013) , while the opposite effect was observed for Cdc12 and for Bni1p in absence of profilin (Courtemanche et al, 2013) . Recently, mDia1 and mDia2 formin processivity was shown to depend on the efficiency of FH1 domains to bind, with the help of profilin, to the actin filament barbed end and increase the lifetime of the FH2 domains interaction with the barbed end (Cao et al, 2018) .…”

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

“…is, most likely, the configuration after addition of a new profilin‐actin subunit and prior to the stepping forward of the lagging FH2 domain of the dimer, in preparation to accept the next subunit . Specifically, this corresponds to the addition of a new subunit in the ‘stair‐stepping model’ .…”

Cell Biology: Capturing Formin’s Mechano-Inhibition

“…Reduction of Cdc12-mediated actin polymerization by myosin pulling was an important assumption to prevent node clump formation in the first formulation of the SCPR model [5] (later models incorporating additional mechanisms, such as actin filament cross-linking, reproduced ring assembly without relying on such mechanosensing [13]). However, recent experimental studies testing formin mechanosensitivity [14–16] showed extensional forces on the FH1 domains promote, rather than inhibit, profilin-regulated actin polymerization by mouse formin mDia1 [15,16] and budding yeast formin Bni1 [14]. …”