High-speed depolymerization at actin filament ends jointly catalysed by Twinfilin and Srv2/CAP | Nature Cell Biology
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High-speed depolymerization at actin filament ends jointly catalysed by Twinfilin and Srv2/CAP

Nature Cell Biology volume 17, pages 1504–1511 (2015)Cite this article

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Abstract

Purified actin filaments depolymerize slowly, and cytosolic conditions strongly favour actin assembly over disassembly, which has left our understanding of how actin filaments are rapidly turned over in vivo incomplete1,2. One mechanism for driving filament disassembly is severing by factors such as Cofilin. However, even after severing, pointed-end depolymerization remains slow and unable to fully account for observed rates of actin filament turnover in vivo. Here we describe a mechanism by which Twinfilin and Cyclase-associated protein work in concert to accelerate depolymerization of actin filaments by 3-fold and 17-fold at their barbed and pointed ends, respectively. This mechanism occurs even under assembly conditions, allowing reconstitution and direct visualization of individual filaments undergoing tunable, accelerated treadmilling. Further, we use specific mutations to demonstrate that this activity is critical for Twinfilin function in vivo. These findings fill a major gap in our knowledge of cellular disassembly mechanisms, and suggest that depolymerization and severing may be deployed separately or together to control the dynamics and architecture of distinct actin networks.

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Figure 1: Twinfilin and Srv2 accelerate depolymerization from actin filament ends.
Figure 2: Reconstitution and direct visualization of actin filament treadmilling under assembly-promoting conditions.
Figure 3: Mechanism and domain requirements for Twinfilin depolymerization activity.
Figure 4: The depolymerization activity of Twinfilin is important for its cellular function.
Figure 5: Twinfilin interactions with F-actin and N-Srv2 visualized by multi-wavelength TIRF and electron microscopy.

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Acknowledgements

We are extremely grateful to D. Breitsprecher, M. Cataldo, Z. Dogic, J. Eskin, S. Guo, J. Henty-Ridilla, S. Jansen, J. Moseley and A. Rodal for helpful comments on the manuscript. We thank A. Goodman for extensive help with the cellular concentration experiments. This work was supported by grants from the NIH (GM063691) and the National Science Foundation (DMR-MRSEC-0820429) to B.L.G.

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  1. Department of Biology, Rosenstiel Basic Medical Science Research Center, Brandeis University, Waltham, Massachusetts 02454, USA

    Adam B. Johnston, Agnieszka Collins & Bruce L. Goode

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  1. Adam B. Johnston
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Contributions

A.B.J. and B.L.G. designed experiments. A.C. performed electron microscopy experiments. A.B.J. performed all other experiments and analyses. A.B.J. and B.L.G. wrote the paper.

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Correspondence to Bruce L. Goode.

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The authors declare no competing financial interests.

Integrated supplementary information

Supplementary Figure 1 Twinfilin and Cofilin activities in bulk and TIRF F-actin disassembly assays.

(A) F-actin depolymerization assays induced by Vitamin-D Binding Protein. All reactions contained 2 μM F-actin (10% pyrene-labeled) and 100 nM Capping Protein, and further include 1 μM Twf1 and/or 50 nM Cof1, as indicated. Data shown are representative of experiments repeated 4 times. (B) Filament depolymerization rates (subunits s−1) induced by 0.5 μM Twf1 in the presence of 0.5 μM N-Srv2 or Srv2, measured from TIRF reactions. Rates from each condition were measured from one representative experiment (n = 20 filaments each), giving mean depolymerization rates (left to right) of 3.252 and 4.018 subunits s−1 at the barbed end, and 3.517 and 3.579 subunits s−1 at the pointed end. Error bars, SEM. All pairings, ns. Two-tailed Student’s t-test. Statistics source data can be found in Supplementary Table 1. (C) Total F-actin fluorescence signal over time in TIRF reactions containing F-actin (10% OG-labeled, 0.5% biotin-actin) ±10 nM Cof1, ±35 mM phosphate (Pi). Data are average curves from two fields of view from a single representative experiment, which was repeated twice. (D) TIRF analysis of tethered filaments (10% OG-labeled, 0.5% biotin-actin) disassembling in the presence of 0.5 μM Twf1 and 0.5 μM N-Srv2, in the additional presence (purple, top) and absence (blue, bottom) of 35 mM free Pi, which drives filaments into the ADP + Pi state. Images correspond to fluorescence quantification curves shown in Fig. 2a. Representative portions of fields of view are shown.

Supplementary Figure 2 Domain requirements for Twinfilin depolymerization activity and combined effects with Cofilin.

(A) Time lapse imaging of tethered filaments disassembling in reactions containing 100 nM Capping Protein, and 10 nM Cof1, 0.5 μM Twf1, 0.5 μM Srv2, as indicated. Images correspond to fluorescence quantification curves shown in Fig. 3a. (B,C) F-actin depolymerization assays induced by Vitamin-D Binding Protein. Final concentrations: 2 μM F-actin (10% pyrene-labeled), 100 nM Capping Protein, 1 μM N-Srv2, and 1 μM of indicated Twf1 construct. Data shown are representative of experiments repeated 3 times. (D) Barbed end depolymerization rates (subunits s−1) induced by Twf1 and Twf1-10, measured from TIRF reactions. Rates for each condition (left to right) were pooled from two representative experiments (n = 29, 29, and 30 filaments), giving mean depolymerization rates of 1.455, 3.911 and 4.120 subunits s−1 at the barbed end. Error bars, SEM. p < 0.0001 for rate of Twf1 or Twf1-10 versus buffer. One-way ANOVA analysis with Tukey post-hoc test. Statistics source data can be found in Supplementary Table 1.

Supplementary Figure 3 Twinfilin induces structural changes in actin filaments.

Electron micrographs of negatively stained actin filaments alone and decorated with Cof1, Twf1, N-Srv2, or Twf1 + N-Srv2. Right, higher magnification views. These images accompany Fig. 3e, f. Decoration by Cof1 induced a helical twist and decreased the helical crossover distance, as previously reported27. In the presence of Twf1 and N-Srv2, filaments appeared heavily decorated, and fragile, as many fragmented on hitting on the grid. Images are representative from experiments repeated 2–3 times.

Supplementary Figure 4 Dual-colour TIRF analysis of SNAP-549-Twf1.

(A) Oligomeric state of SNAP-549-Twf1 determined by step photobleaching analysis. Color-coded symbols represent theoretical expected ratios at the observed labeling efficiency of the protein (0.58 dye per SNAP-Twf1 monomer) for a given oligomeric state. Bars represent the fraction of measured events. Data are from two independent experiments (n = 75 photobleaching events each). The bars show the fraction of events photobleached in one, two, or three steps, averaged from the two experiments. Error bars, SEM. (B) Stepwise photobleaching of surface-attached SNAP-549-biotin-Twf1. Representative examples of one-step (blue) and two-step (red) photobleaching events for SNAP-549-Twf1. Top, a subset of time points from TIRF imaging; bottom, graphs of fluorescence intensity versus time for the same molecules. (C,D) Representative images of SNAP-549-Twf1 (yellow) interacting with the sides and barbed ends of actin filaments (10% OG-labeled, 0.5% biotin-actin, magenta), both in the absence (C) and presence (D) of N-Srv2. Yellow arrows indicate SNAP-549-Twf1 puncta which interact with filaments. These images accompany Fig. 5c, and are representative of localization observed in at least 10 reactions for each condition.

Supplementary Figure 5 Interactions of Twinfilin with N-Srv2 and G-actin.

(A) Full Coomassie-stained gel of the supernatant depletion assays shown in Fig. 5f measuring binding of Cof1, Twf1, and/or G-actin (2 μM each) to GST (lanes 2–7) or GST-N-Srv2 (lanes 8–13) immobilized on beads (lane 1: no beads). Supernatants contained: Cof1, Twf1, ADP-G-actin (lane 1); Cof1, ADP-G-actin (2, 8); Twf1 (3, 9); ADP-G-actin (4, 10); Twf1, ADP-G-actin (5, 11); ATP-G-actin (6, 12); Twf1, ATP-G-actin (7, 13). Molecular weights (in kDa) of the protein standard are indicated on the left. Cof1 is not visible due to running off the gel. (B–D) Electron micrographs of N-Srv2 hexamers alone (B), bound to Twf1 (C), and bound to Twf1 and G-actin (D). These images accompany Fig. 5g, and are representative from experiments repeated 2–3 times.

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Supplementary Table 1

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TIRF analysis of actin filament shortening in the presence and absence of Twf1 and N-Srv2.

Movie shows tethered OG-actin filaments undergoing disassembly in either control buffer (top) or 0.5 μM Twf1 and 0.5 μM N-Srv2 (bottom). Blue arrows, barbed ends; yellow arrows, pointed ends. Time stamp in min:s. Movie has been sped up 60-fold over real time. (AVI 435 kb)

TIRF analysis of an actin filament capped at its barbed end shortening in the presence and absence of Twf1 and N-Srv2.

Movie shows tethered OG-actin filaments undergoing disassembly in either control buffer (top) or 0.5 μM Twf1 and 0.5 μM N-Srv2 (bottom). Both reactions contain 100 nM Capping Protein. Blue arrows, barbed ends; yellow arrows, pointed ends. Time stamp in min:s. Movie has been sped up 60-fold over real time. (AVI 647 kb)

Visualization of rapid actin filament treadmilling by dual-color TIRF analysis.

Initially, tethered filaments (magenta) were grown from 1 μM actin (5% DY647-labeled), then at the start of the movie, the solution was replaced with 1 μM actin (10% OG-labeled), 5 μM Profilin, 0.5 μM Twf1, and 0.5 μM N-Srv2. New filament growth appears in cyan. This movie corresponds to Fig. 2b, c. Time stamp in min:s. Movie has been sped up 90-fold over real time. (AVI 5774 kb)

Alternating runs of polymerization and depolymerization at a filament barbed end.

This movie shows a tethered DY647-labeled filament (5% labeled, magenta) in the presence of 1 μM actin (10% OG-labeled; cyan), 5 μM Profilin, 1.5 μM Twf1, and 1.5 μM N-Srv2. The pointed end (yellow arrow) undergoes continuous depolymerization while the barbed end stochastically alternates between phases of polymerization (cyan arrow) and depolymerization (magenta arrow). Bracket at left indicates initial length of filament. This movie corresponds to Fig. 2f. Time stamp in min:s. Movie has been sped up 60-fold over real time. (AVI 1144 kb)

TIRF analysis of the combined effects of Twf1, Srv2, and Cof1 on actin filament disassembly.

This movie shows whole fields of view for TIRF reactions containing preformed tethered actin filaments (10% OG-labeled). At time zero, disassembly was induced by flowing in Twf1 and Srv2 (left), Cof1 and Srv2 (center), and Twf1, Cof1, and Srv2 (right). Final concentrations: 0.5 μM Twf1, 0.5 μM N-Srv2, and 10 nM Cof1. All reactions contained 100 nM Capping Protein. This movie corresponds to the light blue, orange, and red curves in Fig. 3a. Time stamp in min:s. Movie has been sped up 60-fold over real time. (AVI 3066 kb)

Severing and end-shortening activities working in concert to disassemble a single actin filament.

Close-up view of a single tethered actin filament (10% OG-labeled) being disassembled by a combination of 0.5 μM Twf1, 0.5 μM N-Srv2, and 10 nM Cof1 in the presence of 100 nM Capping Protein. This corresponds to the red curve in Fig. 3a and is a magnification from the rightmost field of Supplementary Video 5 . Severing events, red arrows; barbed ends, blue arrows; pointed ends, yellow arrows. Time stamp in min:s. Movie has been sped up 60-fold over real time. (AVI 493 kb)

Processive association of SNAP-549-Twf1 with the barbed end of a filament.

A SNAP-549-Twf1 molecule (yellow) passively absorbed to a microscope slide remains bound to the barbed end of a shortening actin filament (10% OG-labeled, magenta). Reactions contain 100 nM SNAP-549-Twf1 and 1 μM N-Srv2. This movie corresponds to Fig. 5d. Time stamp in min:s. Movie has been sped up 120-fold over real time. (AVI 1285 kb)

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Johnston, A., Collins, A. & Goode, B. High-speed depolymerization at actin filament ends jointly catalysed by Twinfilin and Srv2/CAP. Nat Cell Biol 17, 1504–1511 (2015). https://doi.org/10.1038/ncb3252

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