The WASP and WAVE family proteins

Protein family review The WASP and WAVE family proteins Shusaku Kurisu and Tadaomi Takenawa 0 Address: Division of Lipid Biochemistry, Department of Biochemistry and Molecular Biology, Kobe University Graduate School of Medicine , 7-5-1 Kusunoki-cho, Chuo-ku, Kobe, Hyogo 650-0017 , Japan All eukaryotic cells need to reorganize their actin cytoskeleton to change shape, divide, move, and take up nutrients for survival. The Wiskott-Aldrich syndrome protein (WASP) and WASPfamily verprolin-homologous protein (WAVE) family proteins are fundamental actin-cytoskeleton reorganizers found throughout the eukaryotes. The conserved function across species is to receive upstream signals from Rho-family small GTPases and send them to activate the Arp2/3 complex, leading to rapid actin polymerization, which is critical for cellular processes such as endocytosis and cell motility. Molecular and cell biological studies have identified a wide array of regulatory molecules that bind to the WASP and WAVE proteins and give them diversified roles in distinct cellular locations. Genetic studies using model organisms have also improved our understanding of how the WASP- and WAVE-family proteins act to shape complex tissue architectures. Current efforts are focusing on integrating these pieces of molecular information to draw a unified picture of how the actin cytoskeleton in a single cell works dynamically to build multicellular organization. - Gene organization and evolutionary history The human Wiskott-Aldrich syndrome protein (WASP) gene was the first of the WASP and WAVE family genes to be isolated, in 1994, as a mutated gene associated with WiskottAldrich syndrome (WAS), an X-linked recessive disease characterized by immunodeficiency, thrombocytopenia and eczema, clinical features caused by complex defects in lymphocyte and platelet function [1]. Another WASP family member, neural (N-) WASP, was then identified from a proteomic search for mammalian proteins that interact with the Src homology 3 (SH3) domain of growth factor receptor binding protein 2 (Grb2, also known as Ash) [2]. Although expressed ubiquitously, N-WASP is most abundant in the brain - hence its name. The first WAVE protein was identified in humans by our group and another group independently as a WASP-like molecule and was named WAVE and SCAR1, respectively [3,4]. Currently, it is agreed that mammals possess five genes for the WASP and WAVE family, WASP, N-WASP, WAVE1/SCAR1, WAVE2, and WAVE3 [5-9]. Human WASP and WAVE family genes are located on different chromosomes, with each gene showing a unique expression pattern (Figure 1). The human WASP gene is carried on the X chromosome and is expressed exclusively in hematopoietic cells, which explains the inheritance pattern and the immunodeficiency and platelet deficiency characteristic of WAS. WAVE1 and WAVE3 are strongly enriched in the brain and are moderately expressed in some hematopoietic lineages, whereas WAVE2 appears to be ubiquitous. Human WASP and WAVE proteins are between 498 and 559 amino acids long and are encoded by 9 to 12 exons. The length of the genes is relatively similar, ranging from 67.1 kb for N-WASP to 131.2 kb for WAVE3, with the exception of WASP, which is a compact 7.6 kb. The restricted expression of WASP in hematopoietic cells is dependent on a 137-bp region upstream of the transcription start site [10]. It is unclear how brain-specific expression of WAVE1 and WAVE3 is regulated, but the proximal promoter region of mouse WAVE1 retains potential recognition motifs for the transcription factor hepatocyte nuclear factor 3 (HNF3) and putative E2-box sequences that can be recognized by some basic helix-loop-helix transcription factors, such as MyoD and Twist, upstream of the transcription start site [11]. The WASP and WAVE family proteins possess a carboxyterminal homologous sequence, the VCA region, consisting of the verprolin homology (also known as WASP homology 2 (WH2)) domain, the cofilin homology (also known as central) domain, and the acidic region, through which they bind to and activate the Arp2/3 complex, a major actin nucleator in cells (Figure 1). Besides the VCA region, the WASP subfamily proteins are characterized by the amino-terminal WH1 (WASP homology 1; also known as an Ena-VASP homology 1, EVH1) domain, which functions as a protein-protein interaction domain. In contrast, WAVE subfamily proteins are characterized by the presence of the WHD/SHD domain (WAVE homology domain/SCAR homology domain), which is located at the amino terminus. This domain is highly conserved between species, for even the distantly related Arabidopsis WHD/SHD domain has 74% amino acid similarity to the WHD/SHD domain of human WAVE1. This domain seems to be involved in the formation of the WAVE complex (see later). Using these sequence signatures together with genomic information (a) V/WH2+C phylogeny (b) WH1/EVH1 phylogeny WASP (c) WHD/SHD phylogeny Dr N-WASPb Vertebrate from various organisms, WASP and WAVE homologs have been discovered in a wide variety of eukaryotic species; WASP and WAVE homologs (one of each) are found in Dictyostelium discoideum (WASP and SCAR) [12,13], Caenorhabditis elegans (WSP-1 and WVE-1) [14-16], and Drosophila melanogaster (WASP and SCAR) [17,18]. Budding yeast has only one WASP homolog, Las17/Bee1 [19,20], and seems to lack WAVEs. In contrast, the plant Arabidopsis thaliana appears to have four WAVE genes, SCAR1-4 [21], but no WASPs. Given that even plants have WAVE homologs, the evolutionary history of the WASP and WAVE family is likely to extend back to before the divergence of the eukaryotes. Along with the evolution of the actin cytoskeleton, eukaryotic cells must have needed means to control actin polymerization and reorganize the actin cytoskeleton, which presumably led to the development of the WASP/WAVE-Arp2/3 axis of actinpolymerizing mechanisms. Although it is difficult to determine whether the WASP and WAVE subfamilies evolved from a common ancestral gene, Arabidopsis SCARs seem to have evolved independently of the evolution of WASPs and other fungal and metazoan WAVE/SCARs, which is suggested by the alignment of conserved verprolin domain (V) and cofilin homology domain (C) sequences (Figure 2a). More detailed phylogenetic trees can be drawn from the alignment P P P P P P Closed WAVE complex (?) P PPPP P P PPP V V C A V C A Open VCA (?) of highly conserved WH1/EVH1 domains of WASPs and the alignment of WHD/SHD domains of WAVEs. Zebrafish homologs of human WASP and N-WASP have been reported recently [22], and a TBLAST search over the Ensembl zebrafish genome (Zv8) revealed at least one homolog of WAVE1, one of WAVE2 and two of WAVE3 (see the legend to Figure 2 for the zebrafish gene accession numbers). Phylogenetic analyses that include the zebrafish amino acid sequences give us some interesting insights into the evolution of these proteins in vertebrates. First, both ancestral WASP and N-WASP seem to (...truncated)