Profilin Interaction with Actin Filament Barbed End Controls Dynamic Instability, Capping, Branching, and Motility

Abstract

Cell motility and actin homeostasis depend on the control of polarized growth of actin filaments. Profilin, an abundant regulator of actin dynamics, supports filament assembly at barbed ends by binding G-actin. Here, we demonstrate how, by binding and destabilizing filament barbed ends at physiological concentrations, profilin also controls motility, cell migration, and actin homeostasis. Profilin enhances filament length fluctuations. Profilin competes with Capping Protein at barbed ends, which generates a lower amount of profilin-actin than expected if barbed ends were tightly capped. Profilin competes with barbed end polymerases, such as formins and VopF, and inhibits filament branching by WASP-Arp2/3 complex by competition for filament barbed ends, accounting for its as-yet-unknown effects on motility and metastatic cell migration observed in this concentration range. In conclusion, profilin is a major coordinator of polarized growth of actin filaments, controlled by competition between barbed end cappers, trackers, destabilizers, and filament branching machineries.

Graphical abstract

Figure 1

Profilin Promotes Mild Dynamic Instability of Actin Filaments (A) Effect of profilin on the rate of barbed end growth as a function of MgATP-G-actin, monitored using the pyrene fluorescence assay. See Supplemental Experimental Procedures for details. (B) Effect of profilin on the rate of barbed end growth of single filaments initiated from immobilized spectrin-actin seeds (open symbols). Lines: numerical simulations, using the kinetic parameters in Table S1. Inset: individual filaments observed in microfluidics-assisted microscopy. (C and E) Length of individual filaments versus time, without (black) or with (red) 30 μM profilin were measured (C) or simulated numerically (E) with the same parameters as in Figure 1B. Actin concentrations are adjusted to obtain mean elongation rates of −1.6, −0.25, and 0.8 subunits/s (C) and −2, 0.3 and 2 subunits/s (E). Curves are shifted vertically for readability. (D and F) 1D diffusion coefficient D, from experimental (D) or numerical (F) data, for mean growth rates around null (vertical gray line) with or without profilin as indicated. Error bars are SDs.

Figure 2

Nucleotide Dependence of the Effect of Profilin (A) Effect of profilin on the rate of filament elongation in ATP, AMPPNP, and ADP. Conditions as in Figure 1A, with 3 μM G-actin. Dark green curve: in ADP, no G-actin. (B) Single filament experiments at 5 μM ADP-actin. Data points from Jegou et al. (2011). The solid line is computed using parameters from Table S1, assuming that the barbed end on-rate constant for profilin-ADP-actin is the same as for ADP-actin (2.6 μM⁻¹ s⁻¹). Error bars are standard deviations. (C) Effect of profilin on barbed end assembly in ADP (C) and AMPPNP (D). Blue curves, no profilin; red curves, 100 μM profilin.

Figure 3

Profilin Competes with CP at Barbed Ends (A) Time course of the fraction of individual filaments that get capped for the first time, in the presence of 1 nM CP and 0, 20, or 60 μM profilin, and G-actin at concentrations ensuring a growth rate of 10 subunits/s (see Supplemental Experimental Procedures). Inset: Pseudo first-order rate constant for CP binding versus profilin, representing binding of profilin to ATP-bound barbed ends (K_d = 29.25 ± 1.75) μM). (B) F-actin assembled at steady state (2.3 μM F-actin, 2% pyrenyl labeled) in the absence and the presence of CP, and profilin as indicated. Inset: concentration of actin monomers at steady state versus CP, in the absence (blue sigmoidal curve) or the presence of 5 μM profilin (red curve). Data derived from main frame. Orange triangles, gelsolin in place of CP. (C) Profilin-actin complex in the supernatants of F-actin assembled at 20 μM in the presence of either 100 nM CP (blue symbols) or gelsolin at a 1:300 ratio to actin (red symbols). Closed and open symbols represent values derived from pyrene fluorescence and SDS-PAGE (top panel), respectively. (D) Unassembled actin in supernatants of F-actin assembled in the presence of thymosin β4 or profilin, and CP or gelsolin. Note that in contrast to profilin, thymosin β4 sequesters actin identically when filaments are capped by CP or by gelsolin. Top panel: SDS-PAGE of the samples. (E) Distribution of F-actin, profilin-actin, and free profilin in a medium containing 50 μM total actin and 50 μM total profilin, with various states of barbed ends.

Figure 4

Competition between Profilin and Formin or VopF for Barbed Ends (A) Profilin inhibits FH1-FH2 association to barbed ends: Filaments elongated from coverslip-immobilized spectrin-actin seeds and exposing free barbed ends (B) were first incubated with 10 nM FH1-FH2 (F) and 0 μM (black), 25 μM (red), 75 μM (blue), or 100 μM profilin (magenta) (and no actin) for various periods of time (Figure S2A), then immediately exposed to a flow of profilin-actin. The fraction of filaments in BF (fast growth) and B (slow growth) states was measured (Shekhar et al., 2015). Symbols: data points for 5, 10, 20, and 30 s exposure time. Lines: exponential fits. Inset: Pseudo first-order rate constant for formin binding versus profilin, representing the saturation of ATP-bound barbed ends by profilin (K_d = 36.52 ± 4.1 μM). At least 150 filaments were observed for each combination of exposure time and profilin concentration. (B) Profilin first assists, then inhibits formin-based rapid elongation at high profilin concentration. Filaments initiated from immobilized spectrin-actin seeds with free (red) or formin-bound (black) barbed ends are exposed to a flow containing profilin only (open symbols) or profilin with 0.5 μM G-actin (closed symbols). Barbed end elongation rates are monitored, N = 40–50 filaments. Error bars are SEM. (C) Profilin and formin antagonize in controlling the steady-state of actin assembly. Amount of F-actin at steady state (2 μM actin) as a function of profilin in the absence and the presence of 44 nM FH1-FH2. Inset: effect of FH1-FH2 on F-actin at 80 μM profilin. (D) Profilin and VopF antagonize in controlling the steady-state of actin assembly. Amount of F-actin at steady state in the absence and the presence of VopF (red, 50 nM; orange, 70 nM) and profilin. Inset: Effect of VopF on F-actin at 52 μM profilin.

Figure 5

Profilin Inhibits Filament Branching by Arp2/3 Complex and Resulting Actin-Based Motility (A) Double fluorescence (Alexa 594-actin and Alexa 488-Arp2/3) images of N-WASP-coated beads propelling in the reconstituted motility assay in the presence of profilin. (B) Branching density ratio derived from integrated fluorescence intensity of Arp2/3 and actin along the comet tail. (C) Effect of 60 μM profilin on barbed end growth initiated by 0.3 nM spectrin-actin seeds in the presence of 3 μM MgATP-G-actin and 0.16 μM VCA, in the absence and the presence of 46 nM Arp2/3 complex. Controls (no profilin) in dimmer colors. Dashed lines are calculated using a model (see Supplemental Experimental Procedures) in which filament branching is unaffected by profilin and filaments grow at the standard growth rate (dim red) and at a rate 55% lowered by profilin (bright red). (D) Images of filaments branched with VCA and Arp2/3 complex at different concentrations of profilin after 1,000 s. Red dots, branch junctions. (E) Destabilization of filaments by profilin is relieved by VCA-Arp2/3. Assembled F-actin in the presence of profilin and 50 nM VCA, with or without 30 nM Arp2/3. Inset: Increase in F-actin upon addition of Arp2/3 complex (30 nM) to F-actin pre-assembled with VCA and without (blue) or with (red) 60 μM profilin.

Figure 6

Filaments Branch upon Interaction of VCA-Arp2/3 Complex with Barbed Ends (A) Sketch of morphologies of branched filaments initiated from a single nucleus and undergoing side branching only (left) or barbed end branching only (right). Rainbow color coding is used to indicate the time course of assembly. (B) Time-lapse images of single filaments growing and branching with 0.9 μM actin (10% Alexa 488 labeled), 50 nM VCA, and 12.5 nM Arp2/3. See also Movie S1. (C) Distribution of branching distances d₀, d₁, d₂, d₃ between consecutive branching points. See text for details: Inset: average values of d₀, d₁, d₂, d₃. See also Movie S2. (D) Left: Time-lapse images of side branching of filaments. Pre-assembled green filaments branching in the presence of red G-actin, VCA, and Arp2/3. Right: Frequency of side branching derived from the linear time dependence of side-branching events. See also Movie S3.

Figure 7

Barbed End Trackers Formin, VopF, and VASP and Destabilizer Profilin Compete with Filament Branching at Barbed Ends (A) VopF inhibits filament branching by VCA and Arp2/3. Conditions as in Figure 5C. Spectrin-actin seeds 0.3 nM; actin (2 μM, 10% pyrenyl labeled), 3 μM profilin, 160 nM VCA, 40 nM Arp2/3 in the presence and the absence of VopF. (B) Time-lapse images of filaments branching in the presence of 0.9 μM actin and 3 μM profilin, without or with 200 nM VopF, 15 nM FH1-FH2 mDia1, or 80 μM profilin. Red dots, branched junctions. See also Movie S4. (C) Number of side-branching events as a function of t² in the presence of VopF, FH1-FH2, and profilin (data from B). (D) Time-lapse phase contrast images of ActA-coated beads in a reconstituted motility assay (7 μM F-actin, 2 μM profilin, 100 nM Arp2/3, 3.5 μM ADF, and 200 nM gelsolin) in the absence (top) or the presence (bottom) of 100 nM VASP. See also Movie S5.