Cortactin Phosphorylation by Casein Kinase 2 Regulates Actin-Related Protein 2/3 Complex Activity, Invadopodia Function, and Tumor Cell Invasion.

Malregulation of the actin cytoskeleton enhances tumor cell motility and invasion. The actin-binding protein cortactin facilitates branched actin network formation through activation of the actin-related protein (Arp) 2/3 complex. Increased cortactin expression due to gene amplification is observed in head and neck squamous cell carcinoma (HNSCC) and other cancers, corresponding with elevated tumor progression and poor patient outcome.

CPI motif interaction is necessary for capping protein function in cells.

Capping protein (CP) has critical roles in actin assembly in vivo and in vitro. CP binds with high affinity to the barbed end of actin filaments, blocking the addition and loss of actin subunits. Heretofore, models for actin assembly in cells generally assumed that CP is constitutively active, diffusing freely to find and cap barbed ends.

The Drosophila planar polarity gene multiple wing hairs directly regulates the actin cytoskeleton.

The evolutionarily conserved frizzled/starry night (fz/stan) pathway regulates planar cell polarity (PCP) in vertebrates and invertebrates. This pathway has been extensively studied in the Drosophila wing, where it is manifested by an array of distally pointing cuticular hairs. Using in vivo imaging we found that, early in hair growth, cells have multiple actin bundles and hairs that subsequently fuse into a single growing hair.

Capping protein regulators fine-tune actin assembly dynamics.

Capping protein (CP) binds the fast growing barbed end of the actin filament and regulates actin assembly by blocking the addition and loss of actin subunits. Recent studies provide new insights into how CP and barbed-end capping are regulated. Filament elongation factors, such as formins and ENA/VASP (enabled/vasodilator-stimulated phosphoprotein), indirectly regulate CP by competing with CP for binding to the barbed end, whereas other molecules, including V-1 and phospholipids, directly bind to CP and sterically block its interaction with the filament.

Dynamin2 organizes lamellipodial actin networks to orchestrate lamellar actomyosin.

Actin networks in migrating cells exist as several interdependent structures: sheet-like networks of branched actin filaments in lamellipodia; arrays of bundled actin filaments co-assembled with myosin II in lamellae; and actin filaments that engage focal adhesions. How these dynamic networks are integrated and coordinated to maintain a coherent actin cytoskeleton in migrating cells is not known. We show that the large GTPase dynamin2 is enriched in the distal lamellipod where it regulates lamellipodial actin networks as they form and flow in U2-OS cells.

IQGAP1 interactome analysis by in vitro reconstitution and live cell 3-color FRET microscopy.

IQGAP1 stimulates branched actin filament nucleation by activating N-WASP, which then activates the Arp2/3 complex. N-WASP can be activated by other factors, including GTP-bound Cdc42 or Rac1, which also bind IQGAP1. Here we report the use of purified proteins for in vitro binding and actin polymerization assays, and Förster (or fluorescence) resonance energy transfer (FRET) microscopy of cultured cells to illuminate functional interactions among IQGAP1, N-WASP, actin, and either Cdc42 or Rac1.

Dynamin: expanding its scope to the cytoskeleton.

The large GTPase dynamin is well known for its actions on budded cellular membranes to generate vesicles, most often, clathrin-coated endocytic vesicles. The scope of cellular processes in which dynamin-mediated vesicle formation occurs, has expanded to include secretory vesicle formation at the Golgi, from other endosomes and nonclathrin structures, such as caveolae, as well as membrane remodeling during exocytosis and vesicle fusion. An intriguing new facet of dynamin's sphere of influence is the cytoskeleton.

Psidin, a conserved protein that regulates protrusion dynamics and cell migration.

Dynamic assembly and disassembly of actin filaments is a major driving force for cell movements. Border cells in the Drosophila ovary provide a simple and genetically tractable model to study the mechanisms regulating cell migration. To identify new genes that regulate cell movement in vivo, we screened lethal mutations on chromosome 3R for defects in border cell migration and identified two alleles of the gene psidin (psid).

Multiplexing MIM.

Signaling circuits often coordinate cellular membranes and actin filaments at distinct sites to direct cell behavior. In this issue of Developmental Cell, Bershteyn et al. outline how the molecular scaffold protein, MIM, which bends membranes and binds actin filaments, is at the middle of one such circuit to regulate ciliogenesis.

Dynamin2 GTPase and cortactin remodel actin filaments.

The large GTPase dynamin, best known for its activities that remodel membranes during endocytosis, also regulates F-actin-rich structures, including podosomes, phagocytic cups, actin comet tails, subcortical ruffles, and stress fibers. The mechanisms by which dynamin regulates actin filaments are not known, but an emerging view is that dynamin influences F-actin via its interactions with proteins that interact directly or indirectly with actin filaments. We show here that dynamin2 GTPase activity remodels actin filaments in vitro via a mechanism that depends on the binding partner and F-actin-binding protein, cortactin.

Coronin 1B antagonizes cortactin and remodels Arp2/3-containing actin branches in lamellipodia.

The dendritic actin network generated by the Arp2/3 complex in lamellipodia underlies formation of protrusions, directional sensing, and migration. While the generation of this network is well studied, the mechanisms regulating network disassembly are poorly understood. We report that Coronin 1B disassembles Arp2/3-containing actin filament branches by inducing Arp2/3 dissociation.

Ena/VASP proteins capture actin filament barbed ends.

Ena/VASP (vasodialator-stimulated protein) proteins regulate many actin-dependent events, including formation of protrusive structures, fibroblast migration, neurite extension, cell-cell adhesion, and Listeria pathogenesis. In vitro, Ena/VASP activities on actin are complex and varied. They promote actin assembly, protect filaments from cappers, bundle filaments, and inhibit filament branching.

Cordon-bleu: a new taste in actin nucleation.

Polymerization of actin filaments powers many dynamic cellular events and is directed by a small group of actin nucleators. In this issue, Ahuja et al. (2007) identify and characterize a new actin nucleator called cordon-bleu (Cobl) that may drive morphogenesis of the central nervous system during development.

Coronin 1B coordinates Arp2/3 complex and cofilin activities at the leading edge.

Actin filament formation and turnover within the treadmilling actin filament array at the leading edge of migrating cells are interdependent and coupled, but the mechanisms coordinating these two activities are not understood. We report that Coronin 1B interacts simultaneously with Arp2/3 complex and Slingshot (SSH1L) phosphatase, two regulators of actin filament formation and turnover, respectively. Coronin 1B inhibits filament nucleation by Arp2/3 complex and this inhibition is attenuated by phosphorylation of Coronin 1B at Serine 2, a site targeted by SSH1L.

IQGAP1 regulates cell motility by linking growth factor signaling to actin assembly.

IQGAP1 has been implicated as a regulator of cell motility because its overexpression or underexpression stimulates or inhibits cell migration, respectively, but the underlying mechanisms are not well understood. Here, we present evidence that IQGAP1 stimulates branched actin filament assembly, which provides the force for lamellipodial protrusion, and that this function of IQGAP1 is regulated by binding of type 2 fibroblast growth factor (FGF2) to a cognate receptor, FGFR1. Stimulation of serum-starved MDBK cells with FGF2 promoted IQGAP1-dependent lamellipodial protrusion and cell migration, and intracellular associations of IQGAP1 with FGFR1--and two other factors--the Arp2/3 complex and its activator N-WASP, that coordinately promote nucleation of branched actin filament networks.

Ena/VASP proteins enhance actin polymerization in the presence of barbed end capping proteins.

Ena/VASP proteins influence the organization of actin filament networks within lamellipodia and filopodia of migrating cells and in actin comet tails. The molecular mechanisms by which Ena/VASP proteins control actin dynamics are unknown. We investigated how Ena/VASP proteins regulate actin polymerization at actin filament barbed ends in vitro in the presence and absence of barbed end capping proteins.

Regulating actin dynamics at membranes: a focus on dynamin.

Dynamin, the large guanosine triphosphatase, is generally considered to have a key role in deforming membranes to create tubules or vesicles. Dynamin, particularly dynamin2 isoforms, also are localized with actin filaments, often at locations where cellular membranes undergo remodeling. Perturbing dynamin function interferes with endocytic traffic and actin function.

Dynamin2 and cortactin regulate actin assembly and filament organization.

The GTPase dynamin is required for endocytic vesicle formation. Dynamin has also been implicated in regulating the actin cytoskeleton, but the mechanism by which it does so is unclear. Through interactions via its proline-rich domain (PRD), dynamin binds several proteins, including cortactin, profilin, syndapin, and murine Abp1, that regulate the actin cytoskeleton.

Antagonism between Ena/VASP proteins and actin filament capping regulates fibroblast motility.

Cell motility requires lamellipodial protrusion, a process driven by actin polymerization. Ena/VASP proteins accumulate in protruding lamellipodia and promote the rapid actin-driven motility of the pathogen Listeria. In contrast, Ena/VASP negatively regulate cell translocation.

Coupling actin dynamics and membrane dynamics during endocytosis.

A convergence of cellular, genetic and biochemical studies supports the hypothesis that the actin cytoskeleton is coupled to endocytic processes, but the roles played by actin filaments during endocytosis are not yet clear. Recent studies have identified several proteins that may functionally link the endocytic machinery with actin filament dynamics. Three of these proteins, Abp1p, Pan1p and cortactin, are activators of actin assembly nucleated by the Arp2/3 complex, a key regulator of actin assembly in vivo.