Molecular mechanism of filament branching by WASP- Arp2/3 complex: possible models and role of the WH2 domain of WASP proteins

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1 Molecular mechanism of filament branching by WASP- Arp2/3 complex: possible models and role of the WH2 domain of WASP proteins 1. Filament branching in vivo and in reconstituted motility assays in vitro In living cells, formation of branched filament arrays is used in various motile processes to produce protrusive, compressive and propulsive forces. Pioneering studies have shown that Arp2/3 complex, which sits at the branched junction, is responsible for filament branching driving lamellipodial extension 1. Various proteins of the WASP family (comprising WASP, N- WASP, WAVE and WASH regulatory complexes, WHAMM and JMY 2 act in a stimulus- responsive fashion, once activated at membranes, to catalyze filament branching. They all use Arp2/3 complex. Reconstituted motility assays have demonstrated that as part of the branching cycle, Arp2/3 first binds to N- WASP coated beads 3. At each catalytic cycle of filament branching, WASP- Arp2/3 complex interacts with a growing filament and Arp2/3 complex is delivered at the branch junction. As filaments grow, the repeated cycle perpetuates the fractal pattern of branched filaments. Evidence for this process has also been obtained in vivo 4, 5. Arp2/3 molecules get incorporated into the branched array at the same rate at which actin is assembled 3, accounting for in vivo observations 1, 6. In electron microscopy studies, newly assembled filaments in the actin tail have been observed with barbed ends attached to Listeria 7, or to ActA- coated beads 8 or to baculovirus 9, consistent with the tethered ratchet model for actin- based motility 10, Biochemical approaches of filament branching by WASP family proteins with Arp2/3 complex The C- terminal constitutively active catalytic domain of all WASP proteins, often called VCA, harbors a WH2 domain (V, sometimes two WH2 domains in tandem repeat) followed by a connector (C ) region and an short acidic (A) sequence. The VCA domain of all WASP family members branches filaments in bulk solution using Arp2/3 complex. The respective roles of the three regions V, C and A, in the branching process have been extensively studied. Branching requires at least V and C, but is most effective with VCA. Hence the WH2 domain (V) is essential for branching. VCA and CA bind Arp2/3 complex with identical affinities (KD = 0.1 µm) and promote the same conformational change of Arp2/3 complex 12, 13. The conformational change in VCA- Arp2/3 complex 14 is linked to a large increase in affinity of Arp2 subunit for ATP 15. Although sufficient for binding to Arp2/3 complex and inducing the same structural change as VCA, CA by itself does not activate Arp2/3 and simply acts as a sequesterer of Arp2/3 complex by competing out VCA 12, 16. Altogether, these results indicate that 1) the structural change promoted by binding of VCA to Arp2/3 complex is insufficient for branching; 2) the WH2 domain is essential for branching without being involved in the binding of Arp2/3 complex. Moreover, the branching efficiency greatly varies from one WASP protein to the other,

2 while all VCAs bind Arp2/3 with very similar affinities 17, 18. The WH2 domain thus plays a crucial role in specifying branching efficiency, independently of Arp2/3 binding 19. Covalent crosslinking experiments suggest that the CA region interacts with several of the seven subunits of Arp2/3 complex, namely Arp2, Arp3, p40 (ARPC1) and p21 (ARPC3) 18, The amphipathic α- helix of the C region remarkably binds the hydrophobic pocket at the barbed face of Arp2, in a similar way to WH2 domains 23, 24. In the presence of G- actin only, VCA binds G- actin via its WH2 domain (KD = 1 µm) in a way that does not affect the binding strength of Arp2/3 to the CA moiety. The VCA- actin complex then participates in barbed- end assembly like profilin- actin 15, 25-27, i.e. VCA dissociates from the barbed end following addition of the VCA- bound actin subunit. The VCA- enhanced ATP binding to Arp2 is not affected by binding of G- actin to the WH2 domain of VCA 15. Fluorescence and size exclusion chromatography studies indicate that both 1:1 VCA- Arp2/3 complex and 1:1:1 actin- VCA- Arp2/3 complexes form in solution in vitro 15, 26, 28, Proposed models for filament branching There is general agreement that filament branching is an autocatalytic reaction, i.e. binding of VCA- Arp2/3 complex to a mother filament is required to initiate a daughter filament that grows at a 70 angle from the growing mother filament 30, 31. The Arp2/3 complex is embedded in the branch junction. Release of WASP from the branch junction terminates the catalytic branching cycle restoring the free enzyme (WASP) and allows daughter branch growth 32. Two different models have been proposed so far: the side- branching model and barbed- end branching model. The proposed side- branching model 33 (for review) posits that WASP proteins first activate the Arp2/3 complex by moving Arp2 by about 30 Å from its position in the isolated state of the complex 34 to a position in which it makes a short pitch connection with Arp3, thus mimicking two consecutive F- actin subunits. The low resolution EM structures of 1:1 WASP- Arp2/3 or VCA- Arp2/3 complex can accommodate this hypothesis 14. The complex of WASP proteins with Arp2/3 then associates with the side of a filament, in the presence of G- actin. The adjacent WH2 domain adjacent to CA, which is essential for branching, binds a G- actin molecule and places it as the third subunit of the daughter filament 35, the first one being Arp3, the second one Arp2. This reaction nucleates the growth of the daughter filament. 3D- reconstructions of the branched junction support this structural organization 35. Notably, this model implicitly posits that the ternary complex of VCA with G- actin and Arp2/3 forms in the presence of filaments, as well as in the presence of G- actin only.

3 The side- branching model faces some internal difficulties. SAXS studies of Arp2/3 in complex with VCA and actin and 3D- reconstructions of the branch junction provide conflicting views of the positions of Arp2 and Arp3 in the daughter filament 23, 35, 36. 3D- reconstructions of VCA or N- WASP in complex with Arp2/3 show that the bound WASP protein occludes the actin filament side- binding site and indicate that VCA is bound to the pointed end of the Arp2- Arp3 heterodimer 14. To resolve these difficulties, it was recently proposed that a second VCA molecule binds Arp2/3 complex to achieve optimum branching, a view supported by the more efficient filament branching of GST- dimerized constructs of VCA 37. This second site, also seen in X- ray crystallographic studies, has a ten- to 100- fold lower affinity 29, 36, 38, which explains why only 1:1 VCA- Arp2/3 and 1:1:1 G- actin- VCA- Arp2/3 complexes have been detected by size- exclusion chromatography 23, 26. Efficient polymerization in branched filaments is monitored in the presence of nm VCA, conditions under which binding of two VCA molecules to Arp2/3 complex may not be significant. Efficient propulsion of N- WASP coated beads takes place at a density of immobilized N- WASP lower than close contact, precluding dimerization. Similarly, the WAVE regulatory complex 39 contains only one WAVE molecule. Whether appropriate dimerization of the WRC allowing binding of two WAVE molecules to a single Arp2/3 complex occurs in vivo is not known. Single molecule imaging of the branching process show that daughter filament nucleation is triggered by the dissociation of a single VCA molecule, initially bound to Arp2/3, from the branch junction 32. An additional difficulty with the side- branching model is that in the physiological context, G- actin is essentially bound to profilin, which is two orders of magnitude more abundant than WH2 domains of activated WASP proteins and has a one order of magnitude higher affinity. Under these conditions, binding of WH2 to G- actin is problematic, while formation of dendritic arrays 40, 41 appears stimulated. The barbed- end branching model was proposed based on evidences for equal length of mother and daughter filaments in EM images and TIRF microscopy video recordings, and for thermodynamic antagonism between branching machineries and proteins targeting barbed ends, including cappers, formins, WH2 domain polymerases and profilin 31, 42, 43. No structural organization of the branched junction at barbed ends was proposed. In physiological solutions containing F- actin and profilin- actin, the barbed end of the mother filament appears as a plausible binding site for the WH2 in the WASP- Arp2/3 complex. In this putative scenario 43, the WH2 domain of VCA would target the terminal actin subunit at the barbed end of a mother filament while the adjacent CA moiety of VCA bound to Arp2/3 complex would simultaneously be positioned at this barbed end to initiate growth of the daughter branch at 70 angle from the mother filament. In this model, the binding of VCA to Arp2/3 complex does not occlude its association to the filament. The CA- Arp2/3 moiety may contribute in transient stabilization of the WH2 domain at the barbed end. Growth of both the mother and daughter filaments may occur from profilin- actin with no constraint, following dissociation of VCA.

4 Remarkably, in the barbed- end branching model, the WH2 domain of WASP proteins shares a functional similarity with other WH2 domain barbed end trackers. Corroborating reports indicate that the WH2 of WASP captures barbed ends at least in absence of Arp2/3 complex 44, the WH2 of WAVE1 inhibits barbed end growth at nanomolar concentrations 19, and N- WASP remains processively bound to filament barbed ends 45. These observations add to other evidences for the multifunctionality of WH2 domains and the binding of Spire, VASP and VopF to filament barbed ends via their WH2 domains 46 (for review). In the barbed- end branching model, profilin is expected to compete with the WH2 domain of WASP proteins at barbed ends to inhibit branching, at high concentrations (>10 µm profilin), which accounts for in vitro observations 43, 47. Finally, insertion of the WH2 domain in the filament to find its binding site, as gelsolin, Spire, INF2 or Cordon- Bleu do to sever filaments 48 (for review), may lead to side branching by WASP- Arp2/3. This alternate model includes both barbed- end and side- branching reactions logically, in a way that leads to an identical structural organization of the branch junction. In conclusion, within this model, WASP family proteins appear as new members of the barbed end tracking machineries that control site- directed actin- based motility. The barbed- end branching model satisfactorily accounts for the perpetuation of the polarized, site- directed assembly of the dendritic actin array. Formation of branches that point away from the lamellipodium tip does not happen in this model. Testing the side- and barbed- end branching models requires biochemical and structural data using various mutated Arp2/3 complexes and engineered VCA domains harboring mutations in the WH2 moiety. Structural modeling studies are also required to examine the possibility that the WH2 domain of VCA binds the barbed end of a mother filament to initiate branching. 4. Possible technical issues linked to preferential observation of barbed- end or side- branching of filaments in TIRF microscopy In a number of studies performed on coverslip- immobilized single filaments, only side- branching reactions have been recorded from the side of adsorbed filaments subsequently exposed to G- actin, VCA and Arp2/3 complex Using a different protocol, by initializing branched filament assembly in a test tube before placing the solution in a chamber for TIRF microscopy observation or on a grid for EM observation, both barbed- end branching and side branching events were recorded 53, 31. More recently, barbed- end branching was monitored on single filaments using a protocol that avoids immobilization on the coverslip, thus preserving the freedom of filament barbed ends 43. It has been suggested that differences in coverslip passivation or in filament

5 immobilization, causing more or less restricted motion of filament barbed ends could affect the detection of barbed end branching. So far, the low frequency of side branching has been measured in only two studies 43, 52. The frequency of barbed end branching was measured in only one paper 43. More detailed investigations are required to establish a consensus view of the mechanism of filament branching, a pivotal reaction in a plethora of motile processes. References 1. Svitkina, T.M. & Borisy, G.G. Arp2/3 complex and actin depolymerizing factor/cofilin in dendritic organization and treadmilling of actin filament array in lamellipodia. J Cell Biol 145, (1999). 2. Rotty, J.D., Wu, C. & Bear, J.E. New insights into the regulation and cellular functions of the ARP2/3 complex. Nat Rev Mol Cell Biol 14, 7-12 (2013). 3. Wiesner, S. et al. A biomimetic motility assay provides insight into the mechanism of actin- based motility. J Cell Biol 160, (2003). 4. Millius, A., Watanabe, N. & Weiner, O.D. Diffusion, capture and recycling of SCAR/WAVE and Arp2/3 complexes observed in cells by single- molecule imaging. J Cell Sci 125, (2012). 5. Weisswange, I., Newsome, T.P., Schleich, S. & Way, M. The rate of N- WASP exchange limits the extent of ARP2/3- complex- dependent actin- based motility. Nature 458, (2009). 6. Iwasa, J.H. & Mullins, R.D. Spatial and temporal relationships between actin- filament nucleation, capping, and disassembly. Curr Biol 17, (2007). 7. Zhukarev, V., Ashton, F., Sanger, J.M., Sanger, J.W. & Shuman, H. Organization and structure of actin filament bundles in Listeria- infected cells. Cell Motil Cytoskeleton 30, (1995). 8. Cameron, L.A., Svitkina, T.M., Vignjevic, D., Theriot, J.A. & Borisy, G.G. Dendritic organization of actin comet tails. Curr Biol 11, (2001). 9. Mueller, J. et al. Electron tomography and simulation of baculovirus actin comet tails support a tethered filament model of pathogen propulsion. PLoS Biol 12, e (2014). 10. Demoulin, D., Carlier, M.F., Bibette, J. & Baudry, J. Power transduction of actin filaments ratcheting in vitro against a load. Proc Natl Acad Sci U S A 111, (2014). 11. Mogilner, A. & Oster, G. Force generation by actin polymerization II: the elastic ratchet and tethered filaments. Biophys J 84, (2003). 12. Hufner, K. et al. The verprolin- like central (vc) region of Wiskott- Aldrich syndrome protein induces Arp2/3 complex- dependent actin nucleation. J Biol Chem 276, (2001). 13. Rodnick- Smith, M., Luan, Q., Liu, S.L. & Nolen, B.J. Role and structural mechanism of WASP- triggered conformational changes in branched actin filament nucleation by Arp2/3 complex. Proc Natl Acad Sci U S A (2016). 14. Xu, X.P. et al. Three- dimensional reconstructions of Arp2/3 complex with bound nucleation promoting factors. EMBO J 31, (2012).

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