Putting on the Brakes A Negative Regulatory Function for Ena/VASP Proteins in Cell Migration

which binds to F-actin in vitro and may also be important Cell migration is essential throughout the life of multicelfor multimerization (Figure 1 and Lanier and Gertler, lular organisms, but is especially important during devel2000). opment. Two studies in this issue of Cell shed light on Ena/VASP proteins were first implicated directly in the molecular pathways coordinating cell motility and actin assembly in studies on the intracellular pathogenic development. Bear et al. (2000) and Bashaw et al. (2000) bacterium Listeria monocytogenes. Listeria has been have challenged current ideas regarding the role of the used extensively as a simplified model system for euEna/VASP family of proteins in cell motility. Integration karyotic cell motility. It requires only one bacterial protein, of signals when cells move is a very complex process. ActA, to recruit host cytoskeletal proteins and assemble For example, the mechanisms by which neurons navian actin tail to propel itself through the cytoplasm. In gate to their correct targets via attractive and repulsive infected cells, Ena/VASP proteins bind directly to ActA cues involve many receptors and require multiple deciand are recruited to the surface of the bacterium during sions. Ena/VASP proteins have been implicated in axon actin tail assembly (Niebuhr et al., 1997). Deletion of the guidance as well as fibroblast migration, platelet activaregion of ActA that binds to Ena/VASP proteins causes tion, and numerous other systems. Ena/VASP proteins reduced speed and a reduced percentage of moving appear to be multifunctional and bind to several targets, bacteria. This year, the minimal set of essential proteins complicating attempts to pinpoint their exact role in cell from the host that are required for actin-based bacterial motility. translocation has been identified (Loisel et al., 1999). The large number of genetic, biochemical, and cell Only three components in addition to actin are required: biological studies showing connections between Ena/ the actin nucleating Arp2/3 complex (for review see MaVASP proteins and cell motility preclude a simple linear chesky and Insall, 1999), capping protein, and the actin pathway from receptor to actin assembly or translocadepolymerizing protein ADF/cofilin (Loisel et al., 1999). tion. However, the two exciting studies featured in Cell In this reconstituted system, VASP enhanced translocathis month point to a number of new considerations and tion speed, but was not essential, in agreement with force us to revise our models for Ena/VASP function in data from live infected cells. cells. They also make potentially important distinctions The mechanism of action of Ena/VASP proteins in between cell translocation and cell motility. Although Listeria translocation has been somewhat controversial. these two terms have been used interchangeably by One widely held view is that the interaction of profilinsome authors, for this review I will define motility as any actin complexes with the proline-rich regions of Ena/ activity that includes actin-based protrusion or shape VASP serves to shuttle actin monomers to the surface change of the cell and translocation as directed migraof the bacterium where new filaments are nucleated. tion resulting in displacement of the entire cell. This fits well with studies showing clearly that profilin Evidence that Ena/VASP Proteins Facilitate is recruited to the Listeria surface and colocalizes with Actin Polymerization Ena/VASP. However, in human platelet extracts, a muThe Ena/VASP family of proteins has been studied in tant profilin which has been reported not to bind to many systems and appears to have a universal role in proline-rich sequences can still enhance the translocacontrol of cell motility and actin dynamics. Ena (Enabled) tion of Listeria in a similar fashion to wild type. Furtherwas originally discovered as a genetic suppressor of more, VASP still enhances translocation of Listeria in mutations in Drosophila Abl (Abelson) tyrosine kinase. profilin-depleted extracts (Laurent et al., 1999). Thus, at Mammals contain a family of Ena-related proteins, inleast in cell extracts, VASP and profilin can act indepencluding VASP (vasodilator-stimulated phosphoprotein), dently of one another. It would be interesting to test EVL (Ena-VASP like), Mena (Mammalian Enabled), and whether Ena/VASP mutants that are unable to bind to Mena1, Mena11, and Mena111 (three Mena splice profilin can still enhance Listeria translocation. Laurent variants; Gertler et al., 1996). Together, Drosophila Ena et al. speculated that the ability of VASP to bind to actin and its mammalian homologs are commonly referred to filaments and thus connect the actin tail to the bacterium as the Ena/VASP family. Members of this family share suggests that in uninfected cells, Ena/VASP proteins common sequence motifs (Figure 1), including an EVH1 might connect newly assembling actin networks to the N-terminal domain that binds to proteins containing a membrane/cortical areas. Clearly this is an area that D/E FPPPPXD motif and targets family members to focal needs more future study. adhesions (Niebuhr et al., 1997). The middle portions of In addition to an important role in Listeria transloca-