Review
How WASP-family proteins and the Arp2/3 complex convert intracellular signals into cytoskeletal structures

https://doi.org/10.1016/S0955-0674(99)00061-7Get rights and content

Abstract

In most cells, the structure of the actin cytoskeleton is regulated by Rho-family G proteins. Recent work has outlined a highly conserved signaling pathway from G protein activation to actin assembly. The key downstream components are WASP family proteins — adaptor molecules that bind multiple signaling and cytoskeletal proteins — and the Arp2/3 complex — a multi-functional protein complex that nucleates and crosslinks actin filaments.

Introduction

Under the microscope cytoskeletal structures, such as the mitotic spindle or the leading edge of a motile cell, convey an overwhelming sense of order and plan. How are such large and complex structures assembled? And how do cellular signaling pathways control this process? Recent work on regulation of actin filament assembly provides clues about how one type of intracellular signal is converted into a three-dimensional structure.

Directed motility of almost all motile eukaryotic cells requires the assembly of a large, specialized network of actin filaments at the forward or leading edge of the cell; polymerization of these actin filaments in this network drives membrane protrusion [1]. To change direction in response to external cues, such as chemoattractants, actin polymerization must somehow be controlled by cellular signaling pathways. At present, we can trace one pathway from activation of small Rho-family G-proteins to de novo nucleation and crosslinking of actin filaments (Figure 1). The following review outlines the work that has allowed this pathway to be described and the molecular details of how G-protein activation is converted into actin assembly is then discussed.

Section snippets

Rho-family G-proteins regulate actin assembly and organization

The Rho-family of small G-proteins includes Rho, Rac and Cdc42. Activation of these proteins in vivo alters the structure of the actin cytoskeleton. Microinjection of activated Rho induces the formation of actin stress fibers [2], activated Rac induces actin-dependent membrane ruffling [3] and activated Cdc42 induces protrusion of actin-rich microspikes from the cell surface [4]. Co-injection of fluorescently-labeled actin, along with each of the G-proteins, reveals that Rac and Cdc42 induce

WASP-family proteins are downstream effectors of Rho-family proteins

The WASP-family of proteins consists of WASP, N-WASP and SCAR (Figure 2a). The available data suggest that, in order to initiate actin polymerization, Cdc42 acts through WASP or N-WASP and Rac acts through SCAR. WASP was originally identified as a protein that was altered in patients with Wiskott–Aldrich Syndrome, a genetic immunodeficiency disorder characterized by reduced motility of lymphoid immune cells [8]. N-WASP is a related and more ubiquitous protein, first discovered in the brain [9].

The Arp2/3 complex is a downstream effector of WASP-family proteins

The Arp2/3 complex is a stable complex of seven-subunits — two actin-related proteins (Arp2 and Arp3) and five novel proteins (p40, p35, p19, p18, and p14). The Arp2/3 complex was first discovered by affinity chromatography on the actin-binding protein profilin in protozoans. It is abundant [14] and conserved [15] across eukaryotic phyla. One of the subunits, p40, is essential in S. cerevisiae. Individual knockouts of other subunits are conditionally lethal and produce a profoundly disrupted

The Arp2/3 complex is a multifunctional protein complex that nucleates actin filaments and crosslinks them into orthogonal networks

Actin filaments are polarized. In vitro monomers add preferentially to the fast growing (barbed) end of the filament and this is the major site of filament elongation in vivo. The Arp2/3 complex is the only known cellular factor that nucleates formation of actin filaments that grow from their fast-growing (barbed) ends. After nucleation, the Arp2/3 complex remains attached to the slow-growing (pointed) end of the filament [22••]. The Arp2/3 complex also binds to the sides of actin filaments and

A general model for regulated actin polymerization

The above results can be integrated into a simple model [22••] of regulated actin polymerization (Figure 1) that may apply generally to any cellular process in which de novo actin polymerization drives membrane protrusion (e.g. macropinocytosis or reorganization of synaptic butons).

The first step is localization and activation of the nucleation machinery. Prenylated, GTP-bound Cdc42 localizes to the membrane where it recruits and activates WASP. Following this, activated WASP recruits the

Coincidence and polarity

Polarity is essential for directed motion. But is cell polarity established by signaling systems, by the cytoskeleton, or by both? In chemotactic cells the activity of chemoattractant receptors is polarized, even in the absence of polymerized actin 41•, 42. The mechanism behind the ability of the signaling system to establish polarity is not well understood but is thought to involve production of two signals, one that locally enhances receptor activity and another that globally inhibits it [41]

Conclusion

With the discovery of the connection between Rho-family G proteins, WASP-family proteins and the Arp2/3 complex, we are now beginning to understand the signaling pathways that control actin assembly. It is clear, however, that much more work will be required before we understand precisely how cells establish polarity and the molecular details of how they convert signaling events into ordered three-dimensional structures.

References and recommended reading

Papers of particular interest, published within the annual period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

References (42)

  • D. Pantaloni et al.

    How profilin promotes actin filament assembly in the presence of thymosin b4

    Cell

    (1993)
  • L. Blanchoin et al.

    Interaction of actin monomers with Acanthamoeba Actophorin (ADF/Cofilin) and Profilin

    J Biol Chem

    (1998)
  • C.A. Parent et al.

    G protein signaling events are activated at the leading edge of chemotactic cells

    Cell

    (1998)
  • L.G. Tilney et al.

    Actin filaments elongate from their membrane-associated ends

    J Cell Biol

    (1981)
  • R. Kozma et al.

    The Ras-related protein Cdc42Hs and bradykinin promote formation of peripheral actin microspikes and filopodia in Swiss 3T3 fibroblasts

    Mol Cell Biol

    (1995)
  • L.M. Machesky et al.

    Role of actin polymerization and adhesion to extracellular matrix in Rac- and Rho-induced cytoskeletal reorganization

    J Cell Biol

    (1997)
  • W.E. Allen et al.

    A role for Cdc42 in macrophage chemotaxis

    J Cell Biol

    (1998)
  • S.H. Zigmond et al.

    Regulation of actin polymerization in cell-free systems by GTPgS and Cdc42

    J Cell Biol

    (1997)
  • H. Miki et al.

    N-WASP, a novel actin-depolymerizing protein, regulates the cortical cytoskeletal rearrangement in a PIP2-dependent manner downstream of tyrosine kinases

    EMBO J

    (1996)
  • J.E. Bear et al.

    SCAR, a WASP-related protein, isolated as a suppressor of receptor defects in late Dictyostelium development

    J Cell Biol

    (1998)
  • H. Miki et al.

    WAVE, a novel WASP-family protein involved in actin reorganization induced by Rac

    EMBO J

    (1998)
  • Cited by (142)

    • A numerical mechanical model integrating actin treadmilling and receptor recycling to explain selective disengagement of immune cells

      2019, Mathematical Biosciences
      Citation Excerpt :

      There are numerous proteins regulating actin network turnover. Important ones are Rho family GTPase Cdc42, WASp proteins, Arp2/3 complexes, and ADF/cofilin [34–41]. The first three are involved in actin polymerization and the last one regulates the depolymerization.

    • Defects in Rho GTPase Signaling to the Spine Actin Cytoskeleton in FMR1 Knockout Mice

      2017, Fragile X Syndrome: From Genetics to Targeted Treatment
    • N-WASP is required for B-cell-mediated autoimmunity in Wiskott-Aldrich syndrome

      2016, Blood
      Citation Excerpt :

      Neural WASP (N-WASP, encoded by the Wasl gene) is another member of the WASP family of proteins; it is ubiquitously expressed and shares 50% homology with WASP.6 Similar to WASP, N-WASP undergoes a conformational change upon activation that enables initiation of actin polymerization,7,8 thereby linking cellular activation to cytoskeletal modifications.9 Selective deletion of N-WASP in B lymphocytes of Was knockout (WKO) mice resulted in the aggravation of B-cell abnormalities, including a strong decrease of intracellular calcium flux and Bruton's tyrosine kinase (Btk) and Src homology 2-containing inositol 5′ phosphatase phosphorylation upon B-cell receptor (BCR) stimulation,10 further worsening of MZ B-cell depletion,11 and defective somatic hypermutation.12

    • Integrin Signaling: Cell Migration, Proliferation, and Survival

      2009, Handbook of Cell Signaling, Second Edition
    View all citing articles on Scopus
    View full text