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. 2009 Apr 14;19(7):537-45.
doi: 10.1016/j.cub.2009.02.060.

Cofilin dissociates Arp2/3 complex and branches from actin filaments

Chikio Chan  1 , Christopher C Beltzner, Thomas D Pollard
Affiliations

Cofilin dissociates Arp2/3 complex and branches from actin filaments

Chikio Chan et al. Curr Biol. .

Abstract

Background: Actin-based cellular motility requires spatially and temporally coordinated remodeling of a network of branched actin filaments. This study investigates how cofilin and Arp2/3 complex, two main players in the dendritic nucleation model, interact to produce sharp spatial transitions between densely branched filaments and long, unbranched filaments.

Results: We found that cofilin binding reduces both the affinity of actin filaments for Arp2/3 complex and the stability of branches. We used fluorescence spectroscopy to measure the kinetics of cofilin association with filaments and the resulting dissociation of Arp2/3 complex and TIRF microscopy to visualize filament severing and the loss of actin filament branches. Cofilin severs filaments optimally when few actin subunits are occupied but dissociates branches rapidly only at higher occupancies. Effective debranching is nevertheless achieved, as a result of cooperative binding and reduced affinity of Arp2/3 complex for the filament, at cofilin concentrations below those required for direct competition.

Conclusions: Cofilin rapidly dissociates Arp2/3 complex and branches by direct competition for binding sites on the actin filament and by propagation of structural changes in the actin filament that reduce affinity for Arp2/3 complex.

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Figures

Figure 1
Figure 1. Binding of Cofilin to Pyrenyl ADP and ADP-Pi Filaments
Time courses of normalized fluorescence change upon mixing cofilin with pyrenyl-actin filaments. Actin filaments were mixed with cofilin at time zero to give either (A) 1 µM actin or (B) 5 µM actin with the following concentrations of cofilin: circles, 1 µM; triangles, 2 µM; squares, 3 µM; long dash, 4 µM; short dash, 5 µM cofilin. (A and B) Conditions for ADP-actin filaments: 1 or 5 µM pyrenyl-ATP-actin was polymerized at 22°C for 1 hr in 10 mM imidazole (pH 7.0), 50 mM KCl, 2 mM Tris-HCl, 1 mM MgCl2, 1 mM EGTA, 0.2 mM ATP, 0.5 mM DTT, 1 mM NaN3. (C and D) Conditions for ADP-Pi actin filaments: 1 µM (C) or 5 µM (D) pyrenyl-ATP-actin was polymerized at 22°C for 1 hras in (A) and (B) but with 15.4 mM H2KPO4, 9.6 mM HK2PO4, and 20 mM KCl rather than 50 mM KCl.
Figure 2
Figure 2. Dissociation of Pyrenyl-Arp2/3 Complex from Actin Filaments by Cofilin
Time courses of normalized fluorescence changes upon mixing reactants (A) Comparison of fluorescence changes of actin bound pyrenyl-Arp2/3 complex ± cofilin and pyrenyl-actin filaments + cofilin: gray circles, 5 µM cofilin mixed with 5 µM pyrenyl actin filaments; black circles, 600 nM pyrenyl-Arp2/3 complex bound to 5 µM unlabeled actin filaments mixed with 5 mM cofilin; black line, 600 nM pyrenyl-Arp2/3 complex bound to 5 µM unlabeled actin filaments mixed with buffer. Conditions: Pyrene or unlabeled actin monomers were incubated in KMEI-F buffer to generate ADP-actin filaments. Pyrenyl-Arp2/3 complex in 10 mM imidazole (pH 7.0), 50 mM KCl, 1 mM MgCl2, 1 mM EGTA, 0.2 mM ATP (1 × KMEI) was incubated overnight with black ADP-actin filaments at 4°C. (B) ADP-Pi filaments were generated as in Figures 1C and 1D. Pyrenyl-Arp2/3 complex in 10 mM imidazole pH 7.0, 1 mM MgCl2, 1mM EGTA, 15.4 mM H2KPO4, 9.6 mM HK2PO4, 20 mM KCl, and 0.2 mM ATP (1 × MEI-phosphate buffer) was incubated overnight with unlabeled ADP-Pi filaments at 4°C. (C) The dependence of observed rate constant for binding of cofilin (filled symbols) to and dissociation of pyrenyl-Arp2/3 complex (open symbols) from 5 µM ADP-actin (circles) and 5 µM ADP-Pi filaments (triangles) on cofilin concentrations.
Figure 3
Figure 3. Cofilin Reduces the Affinity of Arp2/3 Complex for ADP-Actin Filaments
(A) Dependence of equilibrium fluorescence of 600 nM pyrenyl-Arp2/3 complex on the concentration of ADP-actin filaments and on saturation of the filaments with cofilin: circles, no cofilin; triangles, 10% saturation; inverted triangles, 20% saturation; rhombuses, 30% saturation; squares, 45% saturation. Conditions: 600 nM Arp2/3 complex, 10 mM imidazole (pH 7), 2 mM Tris-HCl, 1 mM MgCl2, 1 mM EGTA, 0.2 mM ATP, 0.5 mM DTT, 1 mM NaN3, 50 mM KCl incubated over night at 4°C with a range of concentrations of ADP-actin filaments and cofilin. Fluorescence was measured at room temperature. (B) Dependence of the normalized equilibrium fluorescence of pyrenyl-Arp2/3 complex with 5 µM ADP-actin filaments on the fractional saturation of the filaments with cofilin. Filled circles: Experimental observations with 600 nM pyrenyl Arp2/3 complex, 5 µM ADP-actin filaments, and a range of cofilin concentrations incubated in KEMI-F buffer for 24 hr at 4°C. The fraction of actin-bound Arp2/3 complex was calculated from the increase in fluorescence relative to the initial value. The dashed line is a theoretical calculation of fraction bound Arp2/3 complex assuming direct competition between Arp2/3 complex and cofilin for actin filaments. Here cofilin competes solely by limiting the number of 3-subunit binding sites for Arp2/3 complex by a factor of (1 – c)1.94 as described in Equation 5 and Figure S1. Arp2/3 complex has a constant affinity (KDR = 0.67 µM) for filaments whereas cofilin associates with increasing affinity (Kd = 8.2 µM to 0.11 µM), as calculated from the fractional occupancy and free cofilin concentration in Equation 7. The solid line is another theoretical calculation of fraction bound Arp2/3 complex assuming both competition for binding sites (Equation 5) and a cofilin-induced structural propagation in the filament that increases the affinity of cofilin (Kd = 8.2 µM to 0.11 µM) and decreases the affinity of Arp2/3 complex (KDR = 0.67 µM to 23 µM, calculated from Equation 6) for actin filaments as filaments change from zero to full cofilin occupancy.
Figure 4
Figure 4. Real-Time Observations of Cofilin Severing Actin Filaments and Dissociating Branches by Fluorescence Microscopy
(A and B) Time series of total internal reflection fluorescence micrographs showing the formation of actin filament branches and dissociation of branches by cofilin. Conditions: 10 mM imidazole (pH 7.0), 50 mM KCl, 1 mM MgCl2, 1 mM EGTA, 100 mM DTT, 0.2 mM ATP, 50 mM CaCl2, 15 mM glucose, 20 mg/ml catalase, 100 mg/ml glucose oxidase, and 0.5% methylcellulose (15 cP at 2%). (A) Time course of actin filament branching and elongation by 2 mM Mg-ATP-actin monomers (40% Oregon green), 20 nM Arp2/3 complex, and 200 nM VCA imaged every 10 s. (B) Time course after addition of 1 µM cofilin imaged every second. (C) Time course of the loss of branches after adding a range of concentrations of cofilin. (D) Dependence of the severing and debranching rates on the cofilin occupancy of the actin filaments during the first 30 s after adding cofilin. Rates are the total number of observed severing and debranching events divided by the time of observation (30 s).
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