WASP family proteins: their evolution and its physiological implications

Actin-based motility: WAVE regulatory complex structure reopens old SCARs

The SCAR/WAVE complex controls actin polymerization at the leading edges of moving cells, but its mechanism of regulation remains unclear. The recent determination of its crystal structure, and identification of the binding sites for upstream regulators, mean its workings can finally start to be revealed.

High-resolution X-ray structure of the trimeric Scar/WAVE-complex precursor Brk1

The Scar/WAVE-complex links upstream Rho-GTPase signaling to the activation of the conserved Arp2/3-complex. Scar/WAVE-induced and Arp2/3-complex-mediated actin nucleation is crucial for actin assembly in protruding lamellipodia to drive cell migration. The heteropentameric Scar/WAVE-complex is composed of Scar/WAVE, Abi, Nap, Pir and a small polypeptide Brk1/HSPC300, and recent work suggested that free Brk1 serves as a homooligomeric precursor in the assembly of this complex. Here we characterized the Brk1 trimer from Dictyostelium by analytical ultracentrifugation and gelfiltration. We show for the first time its dissociation at concentrations in the nanomolar range as well as an exchange of subunits within different DdBrk1 containing complexes. Moreover, we determined the three-dimensional structure of DdBrk1 at 1.5 Å resolution by X-ray crystallography. Three chains of DdBrk1 are associated with each other forming a parallel triple coiled-coil bundle. Notably, this structure is highly similar to the heterotrimeric α-helical bundle of HSPC300/WAVE1/Abi2 within the human Scar/WAVE-complex. This finding, together with the fact that Brk1 is collectively sandwiched by the remaining subunits and also constitutes the main subunit connecting the triple-coil domain of the HSPC300/WAVE1/Abi2/ heterotrimer to Sra1(Pir1), implies a critical function of this subunit in the assembly process of the entire Scar/WAVE-complex.

Dysbindin-containing complexes and their proposed functions in brain: from zero to (too) many in a decade

Dysbindin (also known as dysbindin-1 or dystrobrevin-binding protein 1) was identified 10 years ago as a ubiquitously expressed protein of unknown function. In the following years, the protein and its encoding gene, DTNBP1, have become the focus of intensive research owing to genetic and histopathological evidence suggesting a potential role in the pathogenesis of schizophrenia. In this review, we discuss published results demonstrating that dysbindin function is required for normal physiology of the mammalian central nervous system. In tissues other than brain and in non-neuronal cell types, the protein has been characterized as a stable component of a multi-subunit complex, named BLOC-1 (biogenesis of lysosome-related organelles complex-1), which has been implicated in intracellular protein trafficking and the biogenesis of specialized organelles of the endosomal-lysosomal system. In the brain, however, dysbindin has been proposed to associate into multiple complexes with alternative binding partners, and to play a surprisingly wide variety of functions including transcriptional regulation, neurite and dendritic spine formation, synaptic vesicle biogenesis and exocytosis, and trafficking of glutamate and dopamine receptors. This puzzling array of molecular and functional properties ascribed to the dysbindin protein from brain underscores the need of further research aimed at ascertaining its biological significance in health and disease.

Actin polymerization driven by WASH causes V-ATPase retrieval and vesicle neutralization before exocytosis

WASH coats mature lysosomes and is required for exocytosis of indigestible material.

WASP and SCAR homologue (WASH) is a recently identified and evolutionarily conserved regulator of actin polymerization. In this paper, we show that WASH coats mature Dictyostelium discoideum lysosomes and is essential for exocytosis of indigestible material. A related process, the expulsion of the lethal endosomal pathogen Cryptococcus neoformans from mammalian macrophages, also uses WASH-coated vesicles, and cells expressing dominant negative WASH mutants inefficiently expel C. neoformans. D. discoideum WASH causes filamentous actin (F-actin) patches to form on lysosomes, leading to the removal of vacuolar adenosine triphosphatase (V-ATPase) and the neutralization of lysosomes to form postlysosomes. Without WASH, no patches or coats are formed, neutral postlysosomes are not seen, and indigestible material such as dextran is not exocytosed. Similar results occur when actin polymerization is blocked with latrunculin. V-ATPases are known to bind avidly to F-actin. Our data imply a new mechanism, actin-mediated sorting, in which WASH and the Arp2/3 complex polymerize actin on vesicles to drive the separation and recycling of proteins such as the V-ATPase.

Wsp1, a GBD/CRIB domain-containing WASP homolog, is required for growth, morphogenesis, and virulence of Cryptococcus neoformans

Human endocytic protein ITSN1 regulates actin reorganization by activating Rho family GTPases, such as Cdc42. The process is enhanced by ITSN binding of WASP, an effector of Cdc42 and a potent activator of actin polymerization. In the human pathogen Cryptococcus neoformans, endocytic protein Cin1 also interacts with Cdc42 and Wsp1, an uncharacterized WASP homolog, but the significance of these interactions remains unknown. Wsp1 contains several conserved domains, including a WASP homology 1 domain (WH1), a GTPase binding/Cdc42 and Rac interactive binding domain (GBD/CRIB), and a C-terminal domain composed of verprolin-like, central, and acidic motifs (VCA). Thus, Wsp1 exhibits domain compositions more similar to human WASP proteins than Saccharomyces cerevisiae Las17/Bee1, a WASP homolog lacking the GDB/CRIB domain. Wsp1 is not an essential protein; however, the wsp1 mutant exhibited defects in growth, cytokinesis, chitin distribution, and endocytosis and exocytosis. The wsp1 mutant was also unable to undergo genetic cross, produce the polysaccharide capsule, or secrete the enzyme urease. An in vitro phagocytosis assay showed a higher phagocytic index for the wsp1 mutant, whose ability to cause lethal infection in a murine model of cryptococcosis was also attenuated. Our studies reveal divergent evolution of WASP proteins in the fungal phylum and suggest that the conserved function of WASP proteins in the actin cytoskeleton may also impact fungal virulence.

Actin on disease--studying the pathobiology of cell motility using Dictyostelium discoideum

The actin cytoskeleton in eukaryotic cells provides cell structure and organisation, and allows cells to generate forces against membranes. As such it is a central component of a variety of cellular structures involved in cell motility, cytokinesis and vesicle trafficking. In multicellular organisms these processes contribute towards embryonic development and effective functioning of cells of all types, most obviously rapidly moving cells like lymphocytes. Actin also defines and maintains the architecture of complex structures such as neuronal synapses and stereocillia, and is required for basic housekeeping tasks within the cell. It is therefore not surprising that misregulation of the actin cytoskeleton can cause a variety of disease pathologies, including compromised immunity, neurodegeneration, and cancer spread. Dictyostelium discoideum has long been used as a tool for dissecting the mechanisms by which eukaryotic cells migrate and chemotax, and recently it has gained precedence as a model organism for studying the roles of conserved pathways in disease processes. Dictyostelium's unusual lifestyle, positioned between unicellular and multicellular organisms, combined with ease of handling and strong conservation of actin regulatory machinery with higher animals, make it ideally suited for studying actin-related diseases. Here we address how research in Dictyostelium has contributed to our understanding of immune deficiencies and neurological defects in humans, and briefly discuss its future prospects for furthering our understanding of neurodegenerative disorders.

WASH, WHAMM and JMY: regulation of Arp2/3 complex and beyond

Arp2/3 complex mediates the nucleation of actin filaments in multiple subcellular processes, and is activated by nucleation-promoting factors (NPFs) from the Wiskott-Aldrich Syndrome family. In exciting new developments, this family has grown by three members: WASH, WHAMM and JMY, which extend the repertoire of dynamic membrane structures that are remodeled following Arp2/3 activation in vivo. These novel NPFs share an actin- and Arp2/3-interacting WCA module, and combine Arp2/3 activation with additional biochemical functions, including capping protein inhibition, microtubule engagement or Arp2/3-independent actin nucleation, none of which had been previously associated with canonical WCA-harboring proteins. Uncovering the physiological relevance of these unique activities will require concerted efforts from multiple disciplines, and is sure to impact our understanding of how the cytoskeleton controls so many dynamic subcellular events.