158 Cellular actin assembly is tightly regulated. The study of pathogen motility has led to the identification of several cellular factors that are critical for controlling this process. Pathogens such as Listeria require Ena/VASP and Arp2/3 proteins to translate actin polymerization into movement. Recent work has extended these observations and uncovered some similarities and surprising differences in the way cells and pathogens utilize the actin cytoskeleton. Addresses Department of Biology, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA Correspondence: Frank B Gertler; e-mail: fgertler@mit.edu Current Opinion in Cell Biology 2001, 13:158–166 0955-0674/01/$ —see front matter © 2001 Elsevier Science Ltd. All rights reserved. Abbreviations APC antigen-presenting cell Ena Drosophila enabled EVH Ena/VASP homology EVL Ena/VASP-like Fyb/ S LAP Fyn-binding protein/SLP-76 associated protein Mena mammalian homolog of Ena N-WASP neural WASP PKA protein kinase A RII regulatory subunit of PKA SCAR suppresser of cAMP receptor mutation SH Src homology TCR T-cell receptor VAS P vasodilator-stimulated phosphoprotein WAS P Wiskott–Aldrich syndrome protein WAVE WASP/verprolin homologous protein WI P WASP-interacting protein Introduction Cells respond to environmental cues through signal trans- duction cascades and change their motile properties to suit the situation. Actin polymerization provides the initial dri- ving force for many types of cell motility [1]. As actin itself polymerizes under physiological salt conditions, cells must maintain a pool of non-polymerized actin and initiate poly- merization only in those parts of the cell where new filaments are required. This tight control of polymerization is achieved by a plethora of actin regulatory proteins, including monomer-binding, capping, severing and fila- ment side-binding proteins. One of the critical observations from the past five years of cell motility research is that the new filaments initiated by extracellular cues are often cre- ated by de novo nucleation events. In 1989, Tilney and colleagues noted that certain pathogens invade mammalian cells and utilize the host actin cytoskeleton to move within cells and spread to adja- cent cells [2]. This observation and the relative ease of genetic manipulations in these systems made such pathogens widely used model systems for actin-based motility. In fact, several cellular proteins were first implicated in actin assembly only after their role in the actin-driven rocketing of intracellular pathogens was documented. This review will focus on two groups of cellular factors involved in both cell and pathogen motility: Ena/VASP (Enabled/vasodilator-stimulated phosphoprotein) proteins and proteins that activate the Arp2/3 complex. Other cel- lular proteins such as ADF/cofilin and capping proteins are clearly required for cell and pathogen motility; however, their recruitment to pathogens is likely to be indirect and they have been reviewed elsewhere [3,4]. A brief summa- ry of the components discussed in this review is given in Table 1. It is easier to understand how these molecules function in the less-regulated context of pathogen motility, therefore their role in pathogen-driven actin assembly will be described first, followed by an examination of how these same molecules are used normally within cells to control actin assembly. Pathogens: molecular mimes that hijack cellular actin assembly Several bacterial and viral pathogens have evolved ways to subvert the host cell cytoskeleton to support their lifecy- cle. Intracellular pathogens circumvent the host’s humoral immune system by using actin-based motility to move within cells and to spread directly between cells. To this end, intracellular pathogens have evolved strategies to uti- lize the normal actin assembly machinery of the host cell. As pathogens are under strong selective pressure to propa- gate and spread, many of them have developed ways to bypass cellular systems that regulate actin assembly and therefore overcome natural barriers to virulence. The most widely studied examples of pathogen motility are Listeria monocytogenes, Shigella flexneri and the Vaccinia virus. In each case, the pathogen expresses a surface protein that recruits cellular factors and drives directed actin assembly. Listeria and Shigella require only one bacterial protein, ActA or IcsA/VirG respectively, for their actin-based intracellular motility (see Table 1; [5 • ]), whereas Vaccinia express at least two surface pro- teins, A36R and A34R, that are essential for motility [6]. Listeria monocytogenes ActA is a Listeria surface protein that has binding sites for both Ena/VASP proteins and the Arp2/3 complex in one molecule. The interaction with Ena/VASP proteins is essential for the acceleration of Listeria motility, as a dele- tion of the Ena/VASP-binding sites within ActA results in only slow residual intracellular movement and attenuated pathogenicity [7,8]. The remaining basal ActA function results from the induction of actin filament nucleation by the Arp2/3 complex [9,10]. The amino terminus of ActA Regulating cellular actin assembly James E Bear, Matthias Krause and Frank B Gertler