Scaling behaviour in steady-state contracting actomyosin networks

Contractile actomyosin network flows are crucial for many cellular processes including cell division and motility, morphogenesis and transport. How local remodeling of actin architecture tunes stress production and dissipation and regulates large-scale network flows remains poorly understood. Here, we generate contracting actomyosin networks with rapid turnover in vitro, by encapsulating cytoplasmic Xenopus egg extracts into cell-sized 'water-in-oil' droplets. Within minutes, the networks reach a dynamic steady-state with continuous inward flow. The networks exhibit homogeneous, density-independent contraction for a wide range of physiological conditions, implying that the myosin-generated stress driving contraction and the effective network viscosity have similar density dependence. We further find that the contraction rate is roughly proportional to the network turnover rate, but this relation breaks down in the presence of excessive crosslinking or branching. Our findings suggest that cells use diverse biochemical mechanisms to generate robust, yet tunable, actin flows by regulating two parameters: turnover rate and network geometry.

Tropomyosin isoforms differentially tune actin filament length and disassembly

Cellular actin networks exhibit diverse filamentous architectures and turnover dynamics, but how these differences are specified remains poorly understood. Here, we used multicolor total internal reflection fluorescence microscopy to ask how decoration of actin filaments by five biologically prominent Tropomyosin (TPM) isoforms influences disassembly induced by Cofilin alone, or by the collaborative effects of Cofilin, Coronin, and AIP1 (CCA). TPM decoration restricted Cofilin binding to pointed ends, while not interfering with Coronin binding to filament sides. Different isoforms of TPM provided variable levels of protection against disassembly, with the strongest protection by Tpm3.1 and the weakest by Tpm1.6. In biomimetic assays in which filaments were simultaneously assembled by formins and disassembled by CCA, these TPM isoform-specific effects persisted, giving rise to filaments with different lengths and treadmilling behavior. Together, our data reveal that TPM isoforms have quantitatively distinct abilities to tune actin filament length and turnover.

A novel mode of capping protein-regulation by twinfilin

Cellular actin assembly is controlled at the barbed ends of actin filaments, where capping protein (CP) limits polymerization. Twinfilin is a conserved in vivo binding partner of CP, yet the significance of this interaction has remained a mystery. Here, we discover that the C-terminal tail of Twinfilin harbors a CP-interacting (CPI) motif, identifying it as a novel CPI-motif protein. Twinfilin and the CPI-motif protein CARMIL have overlapping binding sites on CP. Further, Twinfilin binds competitively with CARMIL to CP, protecting CP from barbed-end displacement by CARMIL. Twinfilin also accelerates dissociation of the CP inhibitor V-1, restoring CP to an active capping state. Knockdowns of Twinfilin and CP each cause similar defects in cell morphology, and elevated Twinfilin expression rescues defects caused by CARMIL hyperactivity. Together, these observations define Twinfilin as the first ‘pro-capping’ ligand of CP and lead us to propose important revisions to our understanding of the CP regulatory cycle.

Plant and animal cells are supported by skeleton-like structures that can grow and shrink beneath the cell membrane, pushing and pulling on the edges of the cell. This scaffolding network – known as the cytoskeleton – contains long strands, or filaments, made from many identical copies of a protein called actin. The shape of the actin proteins allows them to slot together, end-to-end, and allows the strands to grow and shrink on-demand. When the strands are the correct length, the cell caps the growing ends with a protein known as Capping Protein. This helps to stabilize the cell’s skeleton, preventing the strands from getting any longer, or any shorter.

Proteins that interfere with the activity of Capping Protein allow the actin strands to grow or shrink. Some, like a protein called V-1, attach to Capping Protein and get in the way so that it cannot sit on the ends of the actin strands. Others, like CARMIL, bind to Capping Protein and change its shape, making it more likely to fall off the strands. So far, no one had found a partner that helps Capping Protein limit the growth of the actin cytoskeleton.

A protein called Twinfilin often appears alongside Capping Protein, but the two proteins seemed to have no influence on each other, and had what appeared to be different roles. Whilst Capping Protein blocks growth and stabilizes actin strands, Twinfilin speeds up their disassembly at their ends. But Johnston, Hilton et al. now reveal that the two proteins actually work together. Twinfilin helps Capping Protein resist the effects of CARMIL and V-1, and Capping Protein puts Twinfilin at the end of the strand. Thus, when Capping Protein is finally removed by CARMIL, Twinfilin carries on with disassembling the actin strands.

The tail of the Twinfilin protein looks like part of the CARMIL protein, suggesting that they might interact with Capping Protein in the same way. Attaching a fluorescent tag to the Twinfilin tail revealed that the two proteins compete to attach to the same part of the Capping Protein. When mouse cells produced extra Twinfilin, it blocked the effects of CARMIL, helping to grow the actin strands. V-1 attaches to Capping Protein in a different place, but Twinfilin was also able to interfere with its activity. When Twinfilin attached to the CARMIL binding site, it did not directly block V-1 binding, but it made the protein more likely to fall off.

Understanding how the actin cytoskeleton moves is a key question in cell biology, but it also has applications in medicine. Twinfilin plays a role in the spread of certain blood cancer cells, and in the formation of elaborate structures in the inner ear that help us hear. Understanding how Twinfilin and Capping Protein interact could open paths to new therapies for a range of medical conditions.

Integrated control of formin-mediated actin assembly by a stationary inhibitor and a mobile activator

This study shows that in vivo actin nucleation by the yeast formin Bnr1 is controlled through the coordinated effects of two distinct regulators, a stationary inhibitor (the F-BAR protein Hof1) and a mobile activator (Bud6), establishing a positive feedback loop for precise spatial and temporal control of actin assembly.

Formins are essential actin assembly factors whose activities are controlled by a diverse array of binding partners. Until now, most formin ligands have been studied on an individual basis, leaving open the question of how multiple inputs are integrated to regulate formins in vivo. Here, we show that the F-BAR domain of Saccharomyces cerevisiae Hof1 interacts with the FH2 domain of the formin Bnr1 and blocks actin nucleation. Electron microscopy of the Hof1–Bnr1 complex reveals a novel dumbbell-shaped structure, with the tips of the F-BAR holding two FH2 dimers apart. Deletion of Hof1’s F-BAR domain in vivo results in disorganized actin cables and secretory defects. The formin-binding protein Bud6 strongly alleviates Hof1 inhibition in vitro, and bud6Δ suppresses hof1Δ defects in vivo. Whereas Hof1 stably resides at the bud neck, we show that Bud6 is delivered to the neck on secretory vesicles. We propose that Hof1 and Bud6 functions are intertwined as a stationary inhibitor and a mobile activator, respectively.

Species-Specific Functions of Twinfilin in Actin Filament Depolymerization

Twinfilin is a highly conserved member of the actin depolymerization factor homology (ADF-H) protein superfamily, which also includes ADF/Cofilin, Abp1/Drebrin, GMF, and Coactosin. Twinfilin has a unique molecular architecture consisting of two ADF-H domains joined by a linker and followed by a C-terminal tail. Yeast Twinfilin, in conjunction with yeast cyclase-associated protein (Srv2/CAP), increases the rate of depolymerization at both the barbed and pointed ends of actin filaments. However, it has remained unclear whether these activities extend to Twinfilin homologs in other species. To address this, we purified the three mouse Twinfilin isoforms (mTwf1, mTwf2a, mTwf2b) and mouse CAP1, and used total internal reflection fluorescence microscopy assays to study their effects on filament disassembly. Our results show that all three mouse Twinfilin isoforms accelerate barbed end depolymerization similar to yeast Twinfilin, suggesting that this activity is evolutionarily conserved. In striking contrast, mouse Twinfilin isoforms and CAP1 failed to induce rapid pointed end depolymerization. Using chimeras, we show that the yeast-specific pointed end depolymerization activity is specified by the C-terminal ADF-H domain of yeast Twinfilin. In addition, Tropomyosin decoration of filaments failed to impede depolymerization by yeast and mouse Twinfilin and Srv2/CAP, but inhibited Cofilin severing. Together, our results indicate that Twinfilin has conserved functions in regulating barbed end dynamics, although its ability to drive rapid pointed end depolymerization appears to be species-specific. We discuss the implications of this work, including that pointed end depolymerization may be catalyzed by different ADF-H family members in different species.

GMF as an Actin Network Remodeling Factor

Glia maturation factor (GMF) has recently been established as a regulator of the actin cytoskeleton with a unique role in remodeling actin network architecture. Conserved from yeast to mammals, GMF is one of five members of the ADF-H family of actin regulatory proteins, which includes ADF/cofilin, Abp1/Drebrin, Twinfilin, and Coactosin. GMF does not bind actin, but instead binds the Arp2/3 complex with high affinity. Through this association, GMF catalyzes the debranching of actin filament networks and inhibits actin nucleation by Arp2/3 complex. Here, we discuss GMF's emerging role in controlling actin filament spatial organization and dynamics underlying cell motility, endocytosis, and other biological processes. Further, we attempt to reconcile these functions with its earlier characterization as a cell differentiation factor.

Structural basis of actin monomer re-charging by cyclase-associated protein

Actin polymerization powers key cellular processes, including motility, morphogenesis, and endocytosis. The actin turnover cycle depends critically on "re-charging" of ADP-actin monomers with ATP, but whether this reaction requires dedicated proteins in cells, and the underlying mechanism, have remained elusive. Here we report that nucleotide exchange catalyzed by the ubiquitous cytoskeletal regulator cyclase-associated protein (CAP) is critical for actin-based processes in vivo. We determine the structure of the CAP-actin complex, which reveals that nucleotide exchange occurs in a compact, sandwich-like complex formed between the dimeric actin-binding domain of CAP and two ADP-actin monomers. In the crystal structure, the C-terminal tail of CAP associates with the nucleotide-sensing region of actin, and this interaction is required for rapid re-charging of actin by both yeast and mammalian CAPs. These data uncover the conserved structural basis and biological role of protein-catalyzed re-charging of actin monomers.

Rapid production of pure recombinant actin isoforms in Pichia pastoris

Actins are major eukaryotic cytoskeletal proteins, and they are involved in many important cell functions, including cell division, cell polarity, wound healing and muscle contraction. Despite obvious drawbacks, muscle actin, which is easily purified, is used extensively for biochemical studies of the non-muscle actin cytoskeleton. Here, we report a rapid and cost-effective method to purify heterologous actins expressed in the yeast Pichia pastoris. Actin is expressed as a fusion with the actin-binding protein thymosin β4 and purified by means of an affinity tag introduced in the fusion. Following cleavage of thymosin β4 and the affinity tag, highly purified functional full-length actin is liberated. We purify actins from Saccharomycescerevisiae and Schizosaccharomycespombe, and the β- and γ-isoforms of human actin. We also report a modification of the method that facilitates expression and purification of arginylated actin, a form of actin thought to regulate dendritic actin networks in mammalian cells. The methods we describe can be performed in all laboratories equipped for molecular biology, and should greatly facilitate biochemical and cell biological studies of the actin cytoskeleton.

Summary:Here, we describe a method to purify recombinant actin to homogeneity by expression in Pichia pastoris. The purified actin is polymerisation competent and should facilitate biochemical and cell biological studies of the actin cytoskeleton.

Profilin Directly Promotes Microtubule Growth through Residues Mutated in Amyotrophic Lateral Sclerosis

Profilin is an abundant actin monomer-binding protein with critical actin regulatory roles in vivo [1, 2]. However, profilin also influences microtubule dynamics in cells, which may be mediated in part through its interactions with formins that in turn bind microtubules [3, 4]. Specific residues on human profilin-1 (PFN1) are mutated in patients with amyotrophic lateral sclerosis (ALS) [5, 6]. However, the observation that some ALS-linked PFN1 mutants fail to alter cellular actin organization or dynamics [5-8] or in vitro actin-monomer affinity [9] has been perplexing, given that profilin is best understood as an actin regulator. Here, we investigated direct effects of profilin on microtubule dynamics and whether ALS-linked mutations in PFN1 disrupt such functions. We found that human, fly, and yeast profilin homologs all directly enhance microtubule growth rate by several-fold in vitro. Microtubule stimulatory effects were unaffected by mutations in the canonical actin- or poly-proline-binding sites of profilin. Instead, microtubule activities depended on specific surface residues on profilin mutated in ALS patients. Furthermore, microtubule effects were attenuated by increasing concentrations of actin monomers, suggesting competition between actin and microtubules for binding profilin. Consistent with these biochemical observations, a 2-fold increase in the expression level of wild-type PFN1, but not the ALS-linked PFN1 mutants, increased microtubule growth rates in cells. Together, these results demonstrate that profilin directly enhances the growth rate of microtubules. They further suggest that ALS-linked mutations in PFN1 may perturb cellular microtubule dynamics and/or the coordination between the actin and microtubule cytoskeletons, leading to motor neuron degeneration.

Adenomatous polyposis coli nucleates actin assembly to drive cell migration and microtubule-induced focal adhesion turnover

Cell motility depends on tight coordination between the microtubule (MT) and actin cytoskeletons, but the mechanisms underlying this MT-actin cross talk have remained poorly understood. Here, we show that the tumor suppressor protein adenomatous polyposis coli (APC), which is a known MT-associated protein, directly nucleates actin assembly to promote directed cell migration. By changing only two residues in APC, we generated a separation-of-function mutant, APC (m4), that abolishes actin nucleation activity without affecting MT interactions. Expression of full-length APC carrying the m4 mutation (APC (m4)) rescued cellular defects in MT organization, MT dynamics, and mitochondrial distribution caused by depletion of endogenous APC but failed to restore cell migration. Wild-type APC and APC (m4) localized to focal adhesions (FAs), and APC (m4) was defective in promoting actin assembly at FAs to facilitate MT-induced FA turnover. These results provide the first direct evidence for APC-mediated actin assembly in vivo and establish a role for APC in coordinating MTs and actin at FAs to direct cell migration.

Structural Basis of Arp2/3 Complex Inhibition by GMF, Coronin, and Arpin

The evolutionarily conserved Arp2/3 complex plays a central role in nucleating the branched actin filament arrays that drive cell migration, endocytosis, and other processes. To better understand Arp2/3 complex regulation, we used single-particle electron microscopy to compare the structures of Arp2/3 complex bound to three different inhibitory ligands: glia maturation factor (GMF), Coronin, and Arpin. Although the three inhibitors have distinct binding sites on Arp2/3 complex, they each induced an "open" nucleation-inactive conformation. Coronin promoted a standard (previously described) open conformation of Arp2/3 complex, with the N-terminal β-propeller domain of Coronin positioned near the p35/ARPC2 subunit of Arp2/3 complex. GMF induced two distinct open conformations of Arp2/3 complex, which correlated with the two suggested binding sites for GMF. Furthermore, GMF synergized with Coronin in inhibiting actin nucleation by Arp2/3 complex. Arpin, which uses VCA-related acidic (A) motifs to interact with the Arp2/3 complex, induced the standard open conformation, and two new masses appeared at positions near Arp2 and Arp3. Furthermore, Arpin showed additive inhibitory effects on Arp2/3 complex with Coronin and GMF. Together, these data suggest that Arp2/3 complex conformation is highly polymorphic and that its activities can be controlled combinatorially by different inhibitory ligands.

Tropomyosin and Profilin Cooperate to Promote Formin-Mediated Actin Nucleation and Drive Yeast Actin Cable Assembly

Tropomyosins comprise a large family of actin-binding proteins with critical roles in diverse actin-based processes [1], but our understanding of how they mechanistically contribute to actin filament dynamics has been limited. We addressed this question in S. cerevisiae, where tropomyosins (Tpm1 and Tpm2), profilin (Pfy1), and formins (Bni1 and Bnr1) are required for the assembly of an array of actin cables that facilitate polarized vesicle delivery and daughter cell growth. Formins drive cable formation by promoting actin nucleation and by accelerating actin filament elongation together with profilin [2]. In contrast, how tropomyosins contribute mechanistically to cable formation has been unclear, but genetic studies demonstrate that Tpm1 plays a more important role than Tpm2 [3, 4]. Here, we found that loss of TPM1 in strains lacking BNR1, but not BNI1, leads to severe defects in cable formation, polarized secretion, and cell growth, suggesting that TPM1 function is required for proper Bni1-mediated cable assembly. Furthermore, in vitro total internal reflection fluorescence (TIRF) microscopy demonstrated that Tpm1 strongly enhances Bni1-mediated, but not Bnr1-mediated, actin nucleation without affecting filament elongation rate, whereas Tpm2 has no effects on Bni1 or Bnr1. Tpm1 stimulation of Bni1-mediated nucleation also requires profilin and its interactions with both G-actin and formins. Together, these results demonstrate that yeast Tpm1 works in concert with profilin to promote formin-dependent nucleation of actin cables, thus expanding our understanding of how specific tropomyosin isoforms influence actin dynamics.

Accelerated actin filament polymerization from microtubule plus ends

Microtubules (MTs) govern actin network remodeling in a wide range of biological processes, yet the mechanisms underlying this cytoskeletal cross-talk have remained obscure. We used single-molecule fluorescence microscopy to show that the MT plus-end-associated protein CLIP-170 binds tightly to formins to accelerate actin filament elongation. Furthermore, we observed mDia1 dimers and CLIP-170 dimers cotracking growing filament ends for several minutes. CLIP-170-mDia1 complexes promoted actin polymerization ~18 times faster than free-barbed-end growth while simultaneously enhancing protection from capping proteins. We used a MT-actin dynamics co-reconstitution system to observe CLIP-170-mDia1 complexes being recruited to growing MT ends by EB1. The complexes triggered rapid growth of actin filaments that remained attached to the MT surface. These activities of CLIP-170 were required in primary neurons for normal dendritic morphology. Thus, our results reveal a cellular mechanism whereby growing MT plus ends direct rapid actin assembly.

Design Principles of Length Control of Cytoskeletal Structures

Cells contain elaborate and interconnected networks of protein polymers, which make up the cytoskeleton. The cytoskeleton governs the internal positioning and movement of vesicles and organelles and controls dynamic changes in cell polarity, shape, and movement. Many of these processes require tight control of the size and shape of cytoskeletal structures, which is achieved despite rapid turnover of their molecular components. Here we review mechanisms by which cells control the size of filamentous cytoskeletal structures, from the point of view of simple quantitative models that take into account stochastic dynamics of their assembly and disassembly. Significantly, these models make experimentally testable predictions that distinguish different mechanisms of length control. Although the primary focus of this review is on cytoskeletal structures, we believe that the broader principles and mechanisms discussed herein will apply to a range of other subcellular structures whose sizes are tightly controlled and are linked to their functions.

TIRF microscopy analysis of human Cof1, Cof2, and ADF effects on actin filament severing and turnover

Dynamic remodeling and turnover of cellular actin networks requires actin filament severing by actin-depolymerizing factor (ADF)/Cofilin proteins. Mammals express three different ADF/Cofilins (Cof1, Cof2, and ADF), and genetic studies suggest that in vivo they perform both overlapping and unique functions. To gain mechanistic insights into their different roles, we directly compared their G-actin and F-actin binding affinities, and quantified the actin filament severing activities of human Cof1, Cof2, and ADF using in vitro total internal reflection fluorescence microscopy. All three ADF/Cofilins had similar affinities for G-actin and F-actin. However, Cof2 and ADF severed filaments much more efficiently than Cof1 at both lower and higher concentrations and using either muscle or platelet actin. Furthermore, Cof2 and ADF were more effective than Cof1 in promoting "enhanced disassembly" when combined with actin disassembly co-factors Coronin-1B and actin-interacting protein 1 (AIP1), and these differences were observed on both preformed and actively growing filaments. To probe the mechanism underlying these differences, we used multi-wavelength total internal reflection fluorescence microscopy to directly observe Cy3-Cof1 and Cy3-Cof2 interacting with actin filaments in real time during severing. Cof1 and Cof2 each bound to filaments with similar kinetics, yet Cof2 induced severing much more rapidly than Cof1, decreasing the time interval between initial binding on a filament and severing at the same location. These differences in ADF/Cofilin activities and mechanisms may be used in cells to tune filament turnover rates, which can vary widely for different actin structures.

Common formin-regulating sequences in Smy1 and Bud14 are required for the control of actin cable assembly in vivo

Formins comprise a large family of proteins with diverse roles in remodeling the actin cytoskeleton. However, the spatiotemporal mechanisms used by cells to control formin activities are only beginning to be understood. Here we dissected Smy1, which has dual roles in regulating formins and myosin. Using mutagenesis, we identified specific sequences in Smy1 critical for its in vitro inhibitory effects on the FH2 domain of the formin Bnr1. By integrating smy1 alleles targeting those sequences, we genetically uncoupled Smy1's functions in regulating formins and myosin. Quantitative imaging analysis further demonstrated that the ability of Smy1 to directly control Bnr1 activity is crucial in vivo for proper actin cable length, shape, and velocity and, in turn, efficient secretory vesicle transport. A Smy1-like sequence motif was also identified in a different Bnr1 regulator, Bud14, and found to be essential for Bud14 functions in regulating actin cable architecture and function in vivo. Together these observations reveal unanticipated mechanistic ties between two distinct formin regulators. Further, they emphasize the importance of tightly controlling formin activities in vivo to generate specialized geometries and dynamics of actin structures tailored to their physiological roles.

Combinatorial genetic analysis of a network of actin disassembly-promoting factors

The patterning of actin cytoskeleton structures in vivo is a product of spatially and temporally regulated polymer assembly balanced by polymer disassembly. While in recent years our understanding of actin assembly mechanisms has grown immensely, our knowledge of actin disassembly machinery and mechanisms has remained comparatively sparse. Saccharomyces cerevisiae is an ideal system to tackle this problem, both because of its amenabilities to genetic manipulation and live‐cell imaging and because only a single gene encodes each of the core disassembly factors: cofilin (COF1), Srv2/CAP (SRV2), Aip1 (AIP1), GMF (GMF1/AIM7), coronin (CRN1), and twinfilin (TWF1). Among these six factors, only the functions of cofilin are essential and have been well defined. Here, we investigated the functions of the nonessential actin disassembly factors by performing genetic and live‐cell imaging analyses on a combinatorial set of isogenic single, double, triple, and quadruple mutants in S. cerevisiae. Our results show that each disassembly factor makes an important contribution to cell viability, actin organization, and endocytosis. Further, our data reveal new relationships among these factors, providing insights into how they work together to orchestrate actin turnover. Finally, we observe specific combinations of mutations that are lethal, e.g., srv2Δ aip1Δ and srv2Δ crn1Δ twf1Δ, demonstrating that while cofilin is essential, it is not sufficient in vivo, and that combinations of the other disassembly factors perform vital functions. © 2015 The Authors. Cytoskeleton Published by Wiley Periodicals, Inc.

Structural basis for mutation-induced destabilization of profilin 1 in ALS

Mutations in profilin 1 (PFN1) are associated with amyotrophic lateral sclerosis (ALS); however, the pathological mechanism of PFN1 in this fatal disease is unknown. We demonstrate that ALS-linked mutations severely destabilize the native conformation of PFN1 in vitro and cause accelerated turnover of the PFN1 protein in cells. This mutation-induced destabilization can account for the high propensity of ALS-linked variants to aggregate and also provides rationale for their reported loss-of-function phenotypes in cell-based assays. The source of this destabilization is illuminated by the X-ray crystal structures of several PFN1 proteins, revealing an expanded cavity near the protein core of the destabilized M114T variant. In contrast, the E117G mutation only modestly perturbs the structure and stability of PFN1, an observation that reconciles the occurrence of this mutation in the control population. These findings suggest that a destabilized form of PFN1 underlies PFN1-mediated ALS pathogenesis.

Structure of a Bud6/Actin Complex Reveals a Novel WH2-like Actin Monomer Recruitment Motif

In budding yeast, the actin-binding protein Bud6 cooperates with formins Bni1 and Bnr1 to catalyze the assembly of actin filaments. The nucleation-enhancing activity of Bud6 requires both a "core" domain that binds to the formin and a "flank" domain that binds monomeric actin. Here, we describe the structure of the Bud6 flank domain in complex with actin. Two helices in Bud6(flank) interact with actin; one binds in a groove at the barbed end of the actin monomer in a manner closely resembling the helix of WH2 domains, a motif found in many actin nucleation factors. The second helix rises along the face of actin. Mutational analysis verifies the importance of these Bud6-actin contacts for nucleation-enhancing activity. The Bud6 binding site on actin overlaps with that of the formin FH2 domain and is also incompatible with inter-subunit contacts in F-actin, suggesting that Bud6 interacts only transiently with actin monomers during filament nucleation.

Single-molecule imaging of a three-component ordered actin disassembly mechanism

The mechanisms by which cells destabilize and rapidly disassemble filamentous actin networks have remained elusive; however, Coronin, Cofilin and AIP1 have been implicated in this process. Here using multi-wavelength single-molecule fluorescence imaging, we show that mammalian Cor1B, Cof1 and AIP1 work in concert through a temporally ordered pathway to induce highly efficient severing and disassembly of actin filaments. Cor1B binds to filaments first, and dramatically accelerates the subsequent binding of Cof1, leading to heavily decorated, stabilized filaments. Cof1 in turn recruits AIP1, which rapidly triggers severing and remains bound to the newly generated barbed ends. New growth at barbed ends generated by severing was blocked specifically in the presence of all three proteins. This activity enabled us to reconstitute and directly visualize single actin filaments being rapidly polymerized by formins at their barbed ends while simultanteously being stochastically severed and capped along their lengths, and disassembled from their pointed ends.

The roles of Coronin, Cofilin and AIP1 in promoting actin disassembly have not been well understood. Here using single-molecule fluorescence imaging, Jansen et al. show that the three proteins act together in a coordinated, temporal pathway to induce rapid severing and disassembly of actin filaments.

Actin and endocytosis in budding yeast

Endocytosis, the process whereby the plasma membrane invaginates to form vesicles, is essential for bringing many substances into the cell and for membrane turnover. The mechanism driving clathrin-mediated endocytosis (CME) involves > 50 different protein components assembling at a single location on the plasma membrane in a temporally ordered and hierarchal pathway. These proteins perform precisely choreographed steps that promote receptor recognition and clustering, membrane remodeling, and force-generating actin-filament assembly and turnover to drive membrane invagination and vesicle scission. Many critical aspects of the CME mechanism are conserved from yeast to mammals and were first elucidated in yeast, demonstrating that it is a powerful system for studying endocytosis. In this review, we describe our current mechanistic understanding of each step in the process of yeast CME, and the essential roles played by actin polymerization at these sites, while providing a historical perspective of how the landscape has changed since the preceding version of the YeastBook was published 17 years ago (1997). Finally, we discuss the key unresolved issues and where future studies might be headed.

A novel role for WAVE1 in controlling actin network growth rate and architecture

A novel functional role for WAVE1 is found that is lacking in N-WASP and WAVE2. Through its unique WH2 domain, WAVE1 dramatically reduces the rate of actin filament elongation independently of its interactions with the Arp2/3 complex. These findings help explain how cells build actin networks with distinct geometries and growth rates.

Branched actin filament networks in cells are assembled through the combined activities of Arp2/3 complex and different WASP/WAVE proteins. Here we used TIRF and electron microscopy to directly compare for the first time the assembly kinetics and architectures of actin filament networks produced by Arp2/3 complex and dimerized VCA regions of WAVE1, WAVE2, or N-WASP. WAVE1 produced strikingly different networks from WAVE2 or N-WASP, which comprised unexpectedly short filaments. Further analysis showed that the WAVE1-specific activity stemmed from an inhibitory effect on filament elongation both in the presence and absence of Arp2/3 complex, which was observed even at low stoichiometries of WAVE1 to actin monomers, precluding an effect from monomer sequestration. Using a series of VCA chimeras, we mapped the elongation inhibitory effects of WAVE1 to its WH2 (“V”) domain. Further, mutating a single conserved lysine residue potently disrupted WAVE1's inhibitory effects. Taken together, our results show that WAVE1 has unique activities independent of Arp2/3 complex that can govern both the growth rates and architectures of actin filament networks. Such activities may underlie previously observed differences between the cellular functions of WAVE1 and WAVE2.

GMF promotes leading-edge dynamics and collective cell migration in vivo

Lamellipodia are dynamic actin-rich cellular extensions that drive advancement of the leading edge during cell migration. Lamellipodia undergo periodic extension and retraction cycles, but the molecular mechanisms underlying these dynamics and their role in cell migration have remained obscure. We show that glia-maturation factor (GMF), which is an Arp2/3 complex inhibitor and actin filament debranching factor, regulates lamellipodial protrusion dynamics in living cells. In cultured S2R(+) cells, GMF silencing resulted in an increase in the width of lamellipodial actin filament arrays. Importantly, live-cell imaging of mutant Drosophila egg chambers revealed that the dynamics of actin-rich protrusions in migrating border cells is diminished in the absence of GMF. Consequently, velocity of border cell clusters undergoing guided migration was reduced in GMF mutant flies. Furthermore, genetic studies demonstrated that GMF cooperates with the Drosophila homolog of Aip1 (flare) in promoting disassembly of Arp2/3-nucleated actin filament networks and driving border cell migration. These data suggest that GMF functions in vivo to promote the disassembly of Arp2/3-nucleated actin filament arrays, making an important contribution to cell migration within a 3D tissue environment.

Structure and mechanism of mouse cyclase-associated protein (CAP1) in regulating actin dynamics

Srv2/CAP is a conserved actin-binding protein with important roles in driving cellular actin dynamics in diverse animal, fungal, and plant species. However, there have been conflicting reports about whether the activities of Srv2/CAP are conserved, particularly between yeast and mammalian homologs. Yeast Srv2 has two distinct functions in actin turnover: its hexameric N-terminal-half enhances cofilin-mediated severing of filaments, while its C-terminal-half catalyzes dissociation of cofilin from ADP-actin monomers and stimulates nucleotide exchange. Here, we dissected the structure and function of mouse CAP1 to better understand its mechanistic relationship to yeast Srv2. Although CAP1 has a shorter N-terminal oligomerization sequence compared with Srv2, we find that the N-terminal-half of CAP1 (N-CAP1) forms hexameric structures with six protrusions, similar to N-Srv2. Further, N-CAP1 autonomously binds to F-actin and decorates the sides and ends of filaments, altering F-actin structure and enhancing cofilin-mediated severing. These activities depend on conserved surface residues on the helical-folded domain. Moreover, N-CAP1 enhances yeast cofilin-mediated severing, and conversely, yeast N-Srv2 enhances human cofilin-mediated severing, highlighting the mechanistic conservation between yeast and mammals. Further, we demonstrate that the C-terminal actin-binding β-sheet domain of CAP1 is sufficient to catalyze nucleotide-exchange of ADP-actin monomers, while in the presence of cofilin this activity additionally requires the WH2 domain. Thus, the structures, activities, and mechanisms of mouse and yeast Srv2/CAP homologs are remarkably well conserved, suggesting that the same activities and mechanisms underlie many of the diverse actin-based functions ascribed to Srv2/CAP homologs in different organisms.