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Correspondence to: Laura M. Machesky, Department of Biochemistry, University of Birmingham, Birmingham B15 2TT, UK. Tel: 44-121-414-2504., r.h.insall{at}bham.ac.uk (E-mail), 44-121-414-3982 (fax)
| Introduction |
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THE actin cytoskeleton of a eukaryotic cell is central to locomotion, phagocytosis, contractility, shape changes, cytokinesis and maintenance of polarity. The mechanisms through which actin coordinates these different activities have been fascinating for many years, but the pace of discovery has recently been especially rapid. This review will concentrate on how cells control actin polymerization to produce the force for motility and shape changes.
It is becoming clear that the Arp2/31 complex, a complex of seven proteins including the actin-related proteins Arp2 and Arp3, regulates the assembly of new actin filament networks at the leading edges of cells. Proteins of the WASP (Wiskott-Aldrich Syndrome protein) family bind directly to the Arp2/3 complex and stimulate its ability to promote the nucleation of new actin filaments. Upstream of WASP-family proteins, receptor tyrosine kinases, the Rho-family GTPase Cdc42, and likely G proteincoupled receptors, receive and transmit the signals leading to WASP-Arp2/3 complex-mediated actin nucleation. Together, these ideas and discoveries suggest complete signaling pathways from outside of the cell to actin polymerization-driven cell motility.
While the Arp2/3 complex and WASP family proteins are very important, any model of actin polymerization must include many other participants, in particular Ena/VASP, gelsolin, and capping protein. Members of the Ena/VASP family colocalize with the Arp2/3 complex at the leading edges of cells and catalyze the elongation of newly formed filaments. Capping protein and gelsolin regulate the growth of actin filaments by terminating elongation. They also mediate associations between actin and the plasma membrane, and may promote or permit filament elongation under the control of membrane phospholipids.
| Actin Polymerization in Cell-free Systems and the Role of Cdc42 |
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Recently, several groups have developed systems which allow reconstitution of actin polymerization in vitro (![]()
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The work described in the remainder of this review points to the likely essential components of a fully reconstituted actin-based motility system. These are the Arp2/3 complex, a system to recruit and activate Arp2/3, an Ena/VASP based catalyzer of elongation, and a capping/depolymerizing system to regulate dynamics.
| The Arp2/3 Complex and the WASP Family |
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Interest in the Arp2/3 complex has been intense since it proved essential for reconstitution of both in vitro actin-based motility and actin polymerization on the surface of the intracellular parasite Listeria monocytogenes (![]()
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One of the most important questions about the role of Arp2/3 complex in lamellipodia concerns how it becomes localized and activated at sites of new actin polymerization. A family of candidates has now been found, including WASP, its more widely expressed homologue N-WASP and a related protein group, the Scars. These proteins bind directly to the Arp2/3 complex and regulate its behavior in cells (![]()
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Members of the WASP family are composed of at least one domain and several motifs which connect them to upstream signaling and downstream cytoskeletal ligands. Two WASP proteins (WASP and N-WASP; ![]()
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Just COOH-terminal to the EVH1 domain in WASP and N-WASP is a CRIB motif, which confers interaction with small the GTPase Cdc42 (![]()
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| Control of Arp2/3 Activity by Scars, WASPs, and Signaling |
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Since WASP-family proteins bind to the Arp2/3 complex, the next question to ask was whether this interaction caused activation of the nucleation activity of the Arp2/3 complex. Indeed, Scar1 (![]()
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Previous studies suggested that WASP and N-WASP were actin depolymerizing proteins, and that their effects on the cytoskeleton were mediated by direct interactions with profilin or actin (![]()
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| Ena/VASP Proteins Are Important Regulators of Actin Assembly |
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In addition to Arp2/3 complexmediated actin nucleation, both mammalian cells and Listeria appear to use Ena/VASP proteins to promote actin filament assembly. The enabled gene was originally identified as a suppressor of Abl-dependent phenotypes in Drosophila axon guidance (![]()
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Mice with a VASP gene disruption exhibit defects in the cAMP- and cGMP-mediated inhibition of platelet aggregation (![]()
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| Gelsolin and Capping Protein |
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Gelsolin and capping protein have also been linked to cell signaling through polyphosphoinositides and small GTPases. Both proteins bind to the barbed ends of actin filaments and thus block filament elongation. As most barbed ends in living cells appear to be capped by these two proteins, removal of the cap in response to signals could trigger rapid, extensive actin polymerization. In human platelets, PIP2 synthesis induced by thrombin or Rac triggers the loss of gelsolin caps from barbed ends, and subsequent rapid actin polymerization (![]()
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Capping protein also appears to have a phosphoinositide-dependent role in dynamic actin remodelling. It dissociates from the barbed ands of actin filaments in response to PIP2 in vitro, promoting rapid polymerization under physiological conditions (![]()
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While it seems clear that PIP2 and other related phospholipids have an important role in the regulation of cytoskeletal proteins, we still have a lot to learn about the details. Lysophosphatidic acid (LPA) can mediate the dissociation of gelsolin from actin and modulate the severing activity of gelsolin family members (![]()
| Regulated Treadmilling Model for Actin Dynamics |
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The profusion of recent data suggest a regulated treadmilling model for actin dynamics in motility (Figure 3). This simplified model attempts to incorporate both new and old ideas in the field, including the attractive array treadmilling proposed by Svitkina and Borisy (![]()
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| Footnotes |
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1 Abbreviations used in this paper: Arp, actin-related protein; LPA, lysophosphatidic acid; PIP2, phosphatidylinositol 4,5 bisphosphate; RTK, receptor tyrosine kinase; WASP, Wiskott-Aldrich syndrome protein. ![]()
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