Structural and biochemical characterization of two binding sites for nucleation-promoting factor WASp-VCA on Arp2/3 complex. Proc Natl Acad Sci U S A. 2011;108:E463-E471. [PMID: 21676862 DOI: 10.1073/pnas.1100125108]

Activation of nucleation promoting factors for directional actin filament elongation: allosteric regulation and multimerization on the membrane

Nucleation promoting factors (NPFs) activate the Arp2/3 complex to produce branched actin filaments. Branched actin filaments are observed in most organelles, and specific NPFs, such as WASP, N-WASP, WAVEs, WASH, and WHAMM, exist for each organelle. Interestingly, Arp2/3 and NPFs are both inactive by themselves, and thus require activation. The exposure of the Arp2/3 activating region, the VCA fragment, is recognized to be a key event in the activation of the NPFs. Together, small GTPase binding, phosphorylation, SH3 binding, and membrane binding promote VCA exposure synergistically. The increase in the local concentration of NPF by multimerization is thought to occur with the combination of such activators, to maximally activate the NPF and confine the region of actin polymerization. The mechanism of uni-directional filament extension beneath the membrane also is discussed.

Pathway of actin filament branch formation by Arp2/3 complex revealed by single-molecule imaging

Actin filament nucleation by actin-related protein (Arp) 2/3 complex is a critical process in cell motility and endocytosis, yet key aspects of its mechanism are unknown due to a lack of real-time observations of Arp2/3 complex through the nucleation process. Triggered by the verprolin homology, central, and acidic (VCA) region of proteins in the Wiskott-Aldrich syndrome protein (WASp) family, Arp2/3 complex produces new (daughter) filaments as branches from the sides of preexisting (mother) filaments. We visualized individual fluorescently labeled Arp2/3 complexes dynamically interacting with and producing branches on growing actin filaments in vitro. Branch formation was strikingly inefficient, even in the presence of VCA: only ~1% of filament-bound Arp2/3 complexes yielded a daughter filament. VCA acted at multiple steps, increasing both the association rate of Arp2/3 complexes with mother filament and the fraction of filament-bound complexes that nucleated a daughter. The results lead to a quantitative kinetic mechanism for branched actin assembly, revealing the steps that can be stimulated by additional cellular factors.

In vitro studies of actin filament and network dynamics

Now that many genomes have been sequenced, a central concern of cell biology is to understand how the proteins they encode work together to create living matter. In vitro studies form an essential part of this program because understanding cellular functions of biological molecules often requires isolating them and reconstituting their activities. In particular, many elements of the actin cytoskeleton were first discovered by biochemical methods and their cellular functions deduced from in vitro experiments. We highlight recent advances that have come from in vitro studies, beginning with studies of actin filaments, and ending with multi-component reconstitutions of complex actin-based processes, including force-generation and cell spreading. We describe both scientific results and the technical innovations that made them possible.

Structure of the formin-interaction domain of the actin nucleation-promoting factor Bud6

Formin proteins and their associated factors cooperate to assemble unbranched actin filaments in diverse cellular structures. The Saccharomyces cerevisiae formin Bni1 and its associated nucleation-promoting factor (NPF) Bud6 generate actin cables and mediate polarized cell growth. Bud6 binds to both the tail of the formin and G-actin, thereby recruiting monomeric actin to the formin to create a nucleation seed. Here, we structurally and functionally dissect the nucleation-promoting C-terminal region of Bud6 into a Bni1-binding "core" domain and a G-actin binding "flank" domain. The ∼2-Å resolution crystal structure of the Bud6 core domain reveals an elongated dimeric rod with a unique fold resembling a triple-helical coiled-coil. Binding and actin-assembly assays show that conserved residues on the surface of this domain mediate binding to Bni1 and are required for NPF activity. We find that the Bni1 dimer binds two Bud6 dimers and that the Bud6 flank binds a single G-actin molecule. These findings suggest a model in which a Bni1/Bud6 complex with a 2:4 subunit stoichiometry assembles a nucleation seed with Bud6 coordinating up to four actin subunits.

Insertions within the actin core of actin-related protein 3 (Arp3) modulate branching nucleation by Arp2/3 complex

The Arp2/3 (actin-related protein 2/3) complex nucleates branched actin filaments involved in multiple cellular functions, including endocytosis and cellular motility. Two subunits (Arp2 and Arp3) in this seven-subunit assembly are closely related to actin and upon activation of the complex form a "cryptic dimer" that stably mimics an actin dimer to nucleate a new filament. Both Arps contain a shared actin core structure, and each Arp contains multiple insertions of unknown function at conserved positions within the core. Here we characterize three key insertions within the actin core of Arp3 and show that each one plays a distinct role in modulating Arp2/3 function. The β4/β5 insert mediates interactions of Arp2/3 complex with actin filaments and "dampers" the nucleation activity of the complex. The Arp3 hydrophobic plug plays an important role in maintaining the integrity of the complex but is not absolutely required for formation of the daughter filament nucleus. Deletion of the αK/β15 insert did not constitutively activate the complex, as previously hypothesized. Instead, it abolished in vitro nucleation activity and caused defects in endocytic actin patch assembly in fission yeast, indicating a role for the αK/β15 insert in the activated state of the complex. Biochemical characterization of each mutant revealed steps in the nucleation pathway influenced by each Arp3-specific insert to provide new insights into the structural basis of activation of the complex.

Clathrin potentiates vaccinia-induced actin polymerization to facilitate viral spread

During their egress, newly assembled vaccinia virus particles fuse with the plasma membrane and enhance their spread by inducing Arp2/3-dependent actin polymerization. Investigating the events surrounding vaccinia virus fusion, we discovered that vaccinia transiently recruits clathrin in a manner dependent on the clathrin adaptor AP-2. The recruitment of clathrin to vaccinia dramatically enhances the ability of the virus to induce actin-based motility. We demonstrate that clathrin promotes clustering of the virus actin tail nucleator A36 and host N-WASP, which activates actin nucleation through the Arp2/3 complex. Increased clustering enhances N-WASP stability, leading to more efficient actin tail initiation and sustained actin polymerization. Our observations uncover an unexpected role for clathrin during virus spread and have important implications for the regulation of actin polymerization.

SLAC, a complex between Sla1 and Las17, regulates actin polymerization during clathrin-mediated endocytosis

During clathrin-mediated endocytosis, branched actin polymerization nucleated by the Arp2/3 complex provides force needed to drive vesicle internalization. Las17 (yeast WASp) is the strongest activator of the Arp2/3 complex in yeast cells; it is not autoinhibited and arrives to endocytic sites 20 s before actin polymerization begins. It is unclear how Las17 is kept inactive for 20 s at endocytic sites, thus restricting actin polymerization to late stages of endocytosis. In this paper, we demonstrate that Las17 is part of a large and biochemically stable complex with Sla1, a clathrin adaptor that inhibits Las17 activity. The interaction is direct, multivalent, and strong, and was mapped to novel Las17 polyproline motifs that are simultaneously class I and class II. In vitro pyrene-actin polymerization assays established that Sla1 inhibition of Las17 activity depends on the class I/II Las17 polyproline motifs and is based on competition between Sla1 and monomeric actin for binding to Las17. Furthermore, live-cell imaging showed the interaction with Sla1 is important for normal Las17 recruitment to endocytic sites, inhibition during the initial 20 s, and efficient endocytosis. These results advance our understanding of the regulation of actin polymerization in endocytosis.

Interactions of isolated C-terminal fragments of neural Wiskott-Aldrich syndrome protein (N-WASP) with actin and Arp2/3 complex

Wiskott-Aldrich syndrome proteins (WASP) are a family of proteins that all catalyze actin filament branching with the Arp2/3 complex in a variety of actin-based motile processes. The constitutively active C-terminal domain, called VCA, harbors one or more WASP homology 2 (WH2) domains that bind G-actin, whereas the CA extension binds the Arp2/3 complex. The VCA·actin·Arp2/3 entity associates with a mother filament to form a branched junction from which a daughter filament is initiated. The number and function of WH2-bound actin(s) in the branching process are not known, and the stoichiometry of the VCA·actin·Arp2/3 complex is debated. We have expressed the tandem WH2 repeats of N-WASP, either alone (V) or associated with the C (VC) and CA (VCA) extensions. We analyzed the structure of actin in complex with V, VC, and VCA using protein crystallography and hydrodynamic and spectrofluorimetric methods. The partial crystal structure of the VC·actin 1:1 complex shows two actins in the asymmetric unit with extensive actin-actin contacts. In solution, each of the two WH2 domains in V, VC, and VCA binds G-actin in 1:2 complexes that participate in barbed end assembly. V, VC, and VCA enhance barbed end depolymerization like profilin but neither nucleate nor sever filaments, in contrast with other WH2 repeats. VCA binds the Arp2/3 complex in a 1:1 complex even in the presence of a large excess of VCA. VCA·Arp2/3 binds one actin in a latrunculin A-sensitive fashion, in a 1:1:1 complex, indicating that binding of the second actin to VCA is weakened in the ternary complex.

Stoichiometry of Nck-dependent actin polymerization in living cells

Regulation of actin dynamics through the Nck/N-WASp (neural Wiskott-Aldrich syndrome protein)/Arp2/3 pathway is essential for organogenesis, cell invasiveness, and pathogen infection. Although many of the proteins involved in this pathway are known, the detailed mechanism by which it functions remains undetermined. To examine the signaling mechanism, we used a two-pronged strategy involving computational modeling and quantitative experimentation. We developed predictions for Nck-dependent actin polymerization using the Virtual Cell software system. In addition, we used antibody-induced aggregation of membrane-targeted Nck SH3 domains to test these predictions and to determine how the number of molecules in Nck aggregates and the density of aggregates affected localized actin polymerization in living cells. Our results indicate that the density of Nck molecules in aggregates is a critical determinant of actin polymerization. Furthermore, results from both computational simulations and experimentation support a model in which the Nck/N-WASp/Arp2/3 stoichiometry is 4:2:1. These results provide new insight into activities involving localized actin polymerization, including tumor cell invasion, microbial pathogenesis, and T cell activation.

Actin filament attachments for sustained motility in vitro are maintained by filament bundling

We reconstructed cellular motility in vitro from individual proteins to investigate how actin filaments are organized at the leading edge. Using total internal reflection fluorescence microscopy of actin filaments, we tested how profilin, Arp2/3, and capping protein (CP) function together to propel thin glass nanofibers or beads coated with N-WASP WCA domains. Thin nanofibers produced wide comet tails that showed more structural variation in actin filament organization than did bead substrates. During sustained motility, physiological concentrations of Mg(2+) generated actin filament bundles that processively attached to the nanofiber. Reduction of total Mg(2+) abolished particle motility and actin attachment to the particle surface without affecting actin polymerization, Arp2/3 nucleation, or filament capping. Analysis of similar motility of microspheres showed that loss of filament bundling did not affect actin shell formation or symmetry breaking but eliminated sustained attachments between the comet tail and the particle surface. Addition of Mg(2+), Lys-Lys(2+), or fascin restored both comet tail attachment and sustained particle motility in low Mg(2+) buffers. TIRF microscopic analysis of filaments captured by WCA-coated beads in the absence of Arp2/3, profilin, and CP showed that filament bundling by polycation or fascin addition increased barbed end capture by WCA domains. We propose a model in which CP directs barbed ends toward the leading edge and polycation-induced filament bundling sustains processive barbed end attachment to the leading edge.

Actin filament curvature biases branching direction

Mechanical cues affect many important biological processes in metazoan cells, such as migration, proliferation, and differentiation. Such cues are thought to be detected by specialized mechanosensing molecules linked to the cytoskeleton, an intracellular network of protein filaments that provide mechanical rigidity to the cell and drive cellular shape change. The most abundant such filament, actin, forms branched networks nucleated by the actin-related protein (Arp) 2/3 complex that support or induce membrane protrusions and display adaptive behavior in response to compressive forces. Here we show that filamentous actin serves in a mechanosensitive capacity itself, by biasing the location of actin branch nucleation in response to filament bending. Using an in vitro assay to measure branching from curved sections of immobilized actin filaments, we observed preferential branch formation by the Arp2/3 complex on the convex face of the curved filament. To explain this behavior, we propose a fluctuation gating model in which filament binding or branch nucleation by Arp2/3 occur only when a sufficiently large, transient, local curvature fluctuation causes a favorable conformational change in the filament, and we show with Monte Carlo simulations that this model can quantitatively account for our experimental data. We also show how the branching bias can reinforce actin networks in response to compressive forces. These results demonstrate how filament curvature can alter the interaction of cytoskeletal filaments with regulatory proteins, suggesting that direct mechanotransduction by actin may serve as a general mechanism for organizing the cytoskeleton in response to force.

Three-dimensional reconstructions of Arp2/3 complex with bound nucleation promoting factors

Arp2/3 complex initiates the growth of branched actin-filament networks by inducing actin polymerization from the sides of pre-existing filaments. Nucleation promoting factors (NPFs) are essential for the branching reaction through interactions with the Arp2/3 complex prior to branch formation. The modes by which NPFs bind Arp2/3 complex and associated conformational changes have remained elusive. Here, we used electron microscopy to determine three-dimensional structures at ~2 nm resolution of Arp2/3 complex with three different bound NPFs: N-WASp, Scar-VCA and cortactin. All of these structures adopt a conformation with the two actin-related proteins in an actin-filament-like dimer and the NPF bound to the pointed end. Distance constraints derived by fluorescence resonance energy transfer independently verified the NPF location. Furthermore, all bound NPFs partially occlude the actin-filament binding site, suggesting that additional local structural rearrangements are required in the pathway of Arp2/3 complex activation to allow branch formation.

Arp2/3 complex is bound and activated by two WASP proteins

Actin related protein 2/actin related protein 3 (Arp2/3) complex nucleates new actin filaments in eukaryotic cells in response to signals from proteins in the Wiskott-Aldrich syndrome protein (WASP) family. The conserved VCA domain of WASP proteins activates Arp2/3 complex by inducing conformational changes and delivering the first actin monomer of the daughter filament. Previous models of activation have invoked a single VCA acting at a single site on Arp2/3 complex. Here we show that activation most likely involves engagement of two distinct sites on Arp2/3 complex by two VCA molecules, each delivering an actin monomer. One site is on Arp3 and the second is on ARPC1 and Arp2. The VCAs at these sites have distinct roles in activation. Our findings reconcile apparently conflicting literature on VCA activation of Arp2/3 complex and lead to a new model for this process.