Actin reorganization is a tightly regulated process that co-ordinates complex cellular events, such as cell migration, chemotaxis, phagocytosis and adhesion, but the molecular mechanisms that underlie these processes are not well understood. SCAR (suppressor of cAMP receptor)/WAVE [WASP (Wiskott–Aldrich syndrome protein)-family verprolin homology protein] proteins are members of the conserved WASP family of cytoskeletal regulators, which play a critical role in actin dynamics by triggering Arp2/3 (actin-related protein 2/3)-dependent actin nucleation. SCAR/WAVEs are thought to be regulated by a pentameric complex which also contains Abi (Abl-interactor), Nap (Nck-associated protein), PIR121 (p53-inducible mRNA 121) and HSPC300 (haematopoietic stem progenitor cell 300), but the structural organization of the complex and the contribution of its individual components to the regulation of SCAR/WAVE function remain unclear. Additional features of SCAR/WAVE regulation are highlighted by the discovery of other interactors and distinct complexes. It is likely that the combinatorial assembly of different components of SCAR/WAVE complexes will prove to be vital for their roles at the centre of dynamic actin reorganization.
- actin-related protein 2/3 complex (Arp2/3 complex)
- suppressor of cAMP receptor/Wiskott–Aldrich syndrome protein family verprolin homologous protein (SCAR/WAVE)
Roles of SCAR/WAVE proteins in actin reorganization
There has been great deal of progress in our understanding of the SCAR (suppressor of cAMP receptor)/WAVE [WASP (Wiskott–Aldrich syndrome protein)-family verprolin homology protein] proteins since their discovery in 1999. SCAR was first identified in Dictyostelium as a WASP-related protein . Mammals express three isoforms, SCAR1–3, also known as WAVE1–3, bringing the total number of WASP family members to five: WASP, N-WASP (neural WASP), SCAR/WAVE1, 2 and 3 [2–4]. Yeast expresses a single WASP family protein, whose domain structure more closely resembles WASP than SCAR, whereas Dictyostelium, Caenorhabditis elegans and Drosophila each express one WASP and one SCAR [1,5–8].
The first connection between the SCAR proteins and actin polymerization was established when it was discovered that human SCAR1 binds a major nucleator of actin polymerization, the Arp2/3 (actin-related protein 2/3) complex, and that this interaction stimulates actin polymerization in cells . More recently, it has been shown that SCAR/WAVE and WASP proteins stimulate the ability of the Arp2/3 complex to nucleate actin filaments .
The downstream effects of SCARs in several organisms have been associated with a wide range of essential events, such as cell–cell and cell–substrate adhesion, cell migration, invasion and spreading, cytoskeletal remodelling, embryogenesis, cardiovascular development, cognitive processes, neurodevelopment, bacterial internalization, macropinocytosis, exocytosis and endosome traffic, as well as the dynamics of membrane protrusion [11–22]. However, the precise role of SCAR proteins is not completely known in any of these events.
Actin structures induced by the SCAR/WAVE proteins
The SCAR/WAVE–Arp2/3 complex signalling pathway has a clear, if poorly defined, role in the generation of actin structures. SCAR/WAVEs are directly involved in the protrusion of both lamellipodia and filopodia in a wide range of species and cell types [23–28]. In mammalian cells, there may be a level of control which uses different SCAR/WAVE isoforms, as co-ordination of SCAR/WAVE1 and 2 activities appears to be necessary for formation of normal actin structures in stable lamellipodia . The role of the Arp2/3 complex in filopodia is very unclear; some authors report that SCAR is absent from filopodia , while others find SCAR/WAVE and Arp2/3 puncta in the tips and sometimes in the body of filopodia , so SCAR/WAVEs may regulate actin polymerization through transient recruitment of Arp2/3 and actin molecules to filopodia tips.
Regulation of SCAR/WAVE proteins
It is clear that the SCAR/WAVE proteins can bind and activate the Arp2/3 complex, but the mechanisms by which this activity is regulated are now beginning to emerge. Because most of the GTP-stimulated actin polymerization in cell extracts depends on the Arp2/3 complex , a number of recent studies have sought a link between SCAR proteins and Rac GTPase [27,28,32]. SCAR/WAVE1 is present in a complex with four other proteins, PIR121 (p53-inducible mRNA), Nap (Nck-associated protein), Abi (Abl-interactor) and HSPC300 (haematopoietic stem progenitor cell 300)  (Table 1). SCAR/WAVEs 2 and 3 have been found recently to be included in similar signalling complexes to SCAR/WAVE1 . Evidence is emerging that the different mammalian SCAR isoforms have specific functions and may be regulated differentially, possibly acting in different cellular locations or in response to different extracellular environments. Furthermore, it has been suggested that the different SCAR isoforms are not regulated by exclusive participation in specific complexes, but rather that the regulation is likely to be more subtle.
In an elegant biochemical study, Gautreau et al.  have described the interactions between the five SCAR complex subunits. The complex seems to be organized around a core of Nap and Abi which recruit PIR121 and SCAR/WAVE–HSPC300. Interestingly, all the subunits are only present in vivo in the complexed form, with the exception of HSPC300, which is also found in a free pool.
There has been some conflict about whether the other members of the complex are inhibitors or stimulators of SCAR/WAVE function. The Nap subunit seems to antagonize SCAR function , whereas PIR121 is the direct link between the complex and Rac. Initial reports suggested that PIR121 binding directly inhibits SCAR/WAVE function , and that Rac binding to PIR121 may trigger a conformational change that induces the dissociation of the SCAR–HSPC300 unit. PIR121 is also likely to mediate the indirect interaction between the adaptor Nck and Nap  which itself directly interacts with Abi . Finally, Abi is thought to connect PIR121/Nap to SCAR/WAVEs [28,34,37], although this account seems rather simple and linear. HSPC300, for example, can interact with Abi as well as SCAR/WAVE [26,28,34]. A detailed role for Abi proteins in regulating SCAR/WAVE function has been reported recently . The data reveal that Abi-mediated coupling to the Abelson kinase, Abl, promotes tyrosine phosphorylation on SCAR/WAVE that is required to link it with activated Rac and with actin remodelling at the cell periphery.
Studies using purified proteins originally suggested that actin polymerization only occurred in the presence of Rac or receptor tyrosine kinases, mediated via the adaptor protein Nck, whose binding resulted in the dissociation of PIR121, Nap and Abi from a SCAR–HSPC300 subcomplex . This model is insufficient to account for the mechanisms through which the activity of the complex is restricted to specific sites within cells destined for the formation of membrane extensions. Genetic and biochemical studies performed in mammalian systems and Drosophila challenge this transinhibition model. In particular, recent work suggests that PIR121, Nap and Abi play a positive, rather than inhibitory, role in SCAR/WAVE regulation [27,28]. It was found that the SCAR/WAVE-containing complexes are not disrupted following Rac activation [27,28]. In particular, it was suggested that PIR121–Nap–Abi–SCAR assembles together with the Arp2/3 complex to form the core of a huge, multiprotein actin nucleating complex with multiple catalytic and regulatory subunits that is targeted and activated by Rac-GTP [27,28] (Figure 1). There are several factors that could account for the conflicting accounts of SCAR/WAVE regulation; for instance, the different experimental organisms and species used, the SCAR/WAVE isoforms present in various cellular contexts and the different viewpoints provided by genetic, biochemical and cell biological approaches.
Abi, PIR121, Nap and HSPC300 seem to promote SCAR/WAVE function by acting at different levels. They modulate Arp2/3-mediated actin polymerization, serve as a physical link to upstream pathways in signalling cascades centred on Rac, localize the complex to its proper sites and control SCAR/WAVE degradation, presumably by proteolysis via the proteasome pathway.
One intriguing question that remains to be answered is why SCAR/WAVE requires four large proteins to transduce upstream signals to the Arp2/3 complex. One plausible explanation is the existence of multiple levels of regulation (many of which are currently unknown), as well as a potential central role for SCAR/WAVE in a range of differently assembled actin-regulating complexes. This idea is reinforced by the increasing number of cell signalling molecules known to influence cytoskeletal dynamics through SCAR/WAVE pathways. We believe that SCAR/WAVE-interacting proteins serve a more complex role than merely acting as inhibitors or activators, and a detailed understanding of actin dynamics will require a more complete description of the pathways that lead to SCAR/WAVE.
Cell Architecture: from Structure to Function: A Focus Topic at BioScience2005, held at SECC Glasgow, U.K., 17–21 July 2005. Edited by S. Cockroft (University College London, U.K.), Y. Goda (University College London, U.K.), R. Insall (Birmingham, U.K.) and M. Wakelam (Birmingham, U.K.).
Abbreviations: Abi, Abl-interactor; Arp2/3, actin-related protein 2/3; HSPC300, haematopoietic stem progenitor cell 300; Nap, Nck-associated protein; PIR121, p53-inducible mRNA 121; SCAR, suppressor of cAMP receptor; WASP, Wiskott–Aldrich syndrome protein; WAVE, WASP-family verprolin homology protein
- © 2005 The Biochemical Society