Abi1 is essential for the formation and activation of a WAVE2 signalling complex

WAVE/SCAR, a multifunctional complex coordinating different aspects of neuronal connectivity

Although it is well established that the WAVE/SCAR complex transduces Rac1 signaling to trigger Arp2/3-dependent actin nucleation, regulatory mechanisms of this complex and its versatile function in the nervous system are poorly understood. Here we show that the Drosophila proteins SCAR, CYFIP and Kette, orthologs of WAVE/SCAR complex components, all show strong accumulation in axons of the central nervous system and indeed form a complex in vivo. Neuronal defects of SCAR, CYFIP and Kette mutants are, despite the initially proposed function of CYFIP and Kette as SCAR silencers, indistinguishable and are as diverse as ectopic midline crossing and nerve branching as well as synapse undergrowth at the larval neuromuscular junction. The common phenotypes of the single mutants are readily explained by the finding that loss of any one of the three proteins leads to degradation of its partners. As a consequence, each mutant is unambiguously to be judged as defective in multiple components of the complex even though each component affects different signaling pathways. Indeed, SCAR-Arp2/3 signaling is known to control axonogenesis whereas CYFIP signaling to the Fragile X Mental Retardation Protein fly ortholog contributes to synapse morphology. Thus, our results identify the Drosophila WAVE/SCAR complex as a multifunctional unit orchestrating different pathways and aspects of neuronal connectivity.

The Arabidopsis GNARLED gene encodes the NAP125 homolog and controls several actin-based cell shape changes

In plants many aspects of cell shape regulation are controlled by actin-dependent processes. The ARP2/3 complex has been recognized as a regulator of actin organization. Mutations in genes encoding components of the ARP2/3 complex lead to cell shape defects in several cell types, including trichomes, epidermal pavement cells and hypocotyl cells. We show here that mutations in the GNARLED (GRL) gene cause a similar range of phenotypes. The GRL gene encodes the Arabidopsis homolog of NAP125, which in animals is known to act as one regulator of the ARP2/3-regulating complex WAVE-HSPC300. As an HSPC300 homolog is present in the Arabidopsis genome but no WAVE homolog has yet been found, the existence of a related regulation pathway was doubtful. Our finding that GRL encodes a putative regulator of the WAVE-HSPC300 complex, NAP125, combined with the phenotypic similarity between arp2/3 and grl mutants, provides evidence that the ARP2/3 complex is indeed regulated by the above mentioned pathway in Arabidopsis.

Interchangeable functions of Arabidopsis PIROGI and the human WAVE complex subunit SRA1 during leaf epidermal development

The WAVE complex is an essential regulator of actin-related protein (ARP) 2/3-dependent actin filament nucleation and cell shape change in migrating cells. Although the composition of the WAVE complex is well characterized, the cellular mechanisms that control its activity and localization are not well known. The 'distorted group' defines a set of Arabidopsis genes that are required to remodel the actin cytoskeleton and maintain the polarized elongation of branched, hair-like cells termed trichomes. Several loci within this group encode homologs of ARP2/3 subunits. In addition to trichome distortion, ARP2/3 subunit mutants have reduced shoot fresh weight and widespread defects in epidermal cell-cell adhesion. The precise cellular function of plant ARP2/3, and the means by which it is regulated, is not known. In this paper, we report that the 'distorted group' gene PIROGI encodes a homolog of the WAVE complex subunit SRA1. The similar cell shape and actin phenotypes of pir and ARP2/3 complex subunit mutants suggest that PIROGI positively regulates ARP2/3. PIROGI directly interacts with the small GTPase ATROP2 with isoform specificity and with selectivity for active forms of the protein. PIROGI shares only 30% amino acid identity with its human homolog. However, both WAVE subunit homologs are functionally interchangeable and display identical physical interactions with RHO family GTPases and the Arabidopsis homolog of the WAVE complex subunit NAP125. These results demonstrate the utility of the 'distorted group' mutants to study ARP2/3 complex functions from signaling input to cell shape output.

Arabidopsis NAP1 Is Essential for Arp2/3-Dependent Trichome Morphogenesis

The dynamic nature of the eukaryotic actin cytoskeleton is essential for the locomotion of animal cells and the morphogenesis of plant and fungal cells. The F-actin nucleating/branching activity of the Arp2/3 complex is a key function for all of these processes. The SCAR/WAVE family represents a group of Arp2/3 activators that are associated with lamellipodia formation. A protein complex of PIR121, NAP1, ABI, and HSPC300 is required for SCAR regulation by cell signaling pathways, but the exact nature of this interaction is controversial and represents a continually evolving model. The mechanism originally proposed was of a SCAR trans repressing complex supported by evidence from in vitro experiments. This model was reinforced by genetic studies in the Drosophila central nervous system and Dictyostelium, where the knockout of certain SCAR-complex components leads to excessive SCAR-mediated actin polymerization. Conflicting data have steadily accumulated from animal tissue culture experiments suggesting that the complex activates rather than represses in vivo SCAR activity. Recent biochemical evidence supports the SCAR-complex activator model. Here, we show that genetic observations in Arabidopsis are compatible with an activation model and provide one potential mechanism for the regulation of the newly identified Arabidopsis Arp2/3 complex.

Regulation of cytoskeletal dynamics by actin-monomer-binding proteins

The actin cytoskeleton is a vital component of several key cellular and developmental processes in eukaryotes. Many proteins that interact with filamentous and/or monomeric actin regulate the structure and dynamics of the actin cytoskeleton. Actin-filament-binding proteins control the nucleation, assembly, disassembly and crosslinking of actin filaments, whereas actin-monomer-binding proteins regulate the size, localization and dynamics of the large pool of unpolymerized actin in cells. In this article, we focus on recent advances in understanding how the six evolutionarily conserved actin-monomer-binding proteins - profilin, ADF/cofilin, twinfilin, Srv2/CAP, WASP/WAVE and verprolin/WIP - interact with actin monomers and regulate their incorporation into filament ends. We also present a model of how, together, these ubiquitous actin-monomer-binding proteins contribute to cytoskeletal dynamics and actin-dependent cellular processes.

Beginning and ending an actin filament: control at the barbed end

Dynamic actin filaments contribute to cell migration, organelle movements, memory, and gene regulation. These dynamic processes are often regulated by extracellular and?or cell cycle signals. Regulation targets, not actin itself, but the factors that determine it's dynamic properties. Thus, filament nucleation, rate and duration of elongation, and depolymerization are each controlled with regard to time and?or space. Two mechanisms exist for nucleating filaments de novo, the Arp23 complex and the formins; multiple pathways regulate each. A new filament elongates rapidly but transiently before its barbed end is capped. Rapid capping allows the cell to maintain fine temporal and spatial control over F-actin distribution. Modulation of capping protein activity and its access to barbed ends is emerging as a site of local regulation. Finally, to maintain a steady state filaments must depolymerize. Depolymerization can limit the rate of new filament nucleation and elongation. The activity of ADF?cofilin, which facilitates depolymerization, is also regulated by multiple inputs. This chapter describes (1) mechanism and regulation of new filament formation, (2) mechanism of enhancing elongation at barbed ends, (3) capping proteins and their regulators, and (4) recycling of actin monomers from filamentous actin (F-actin) back to globular actin (G-actin).

Anti-actin antibodies generated against profilin:actin distinguish between non-filamentous and filamentous actin, and label cultured cells in a dotted pattern

Actin polymerization is a prominent feature of migrating cells, where it powers the protrusion of the leading edge. Many studies have characterized the well-ordered and dynamic arrangement of filamentous actin in this submembraneous space. However, less is known about the organization of unpolymerized actin. Previously, we reported on the use of covalently coupled profilin:actin to study actin dynamics and presented evidence that profilin-bound actin is a major source of actin for filament growth. To locate profilin:actin in the cell we have now used this non-dissociable complex for antibody generation, and obtained monospecific anti-actin and anti-profilin antibodies from two separate immunizations. Fluorescence microscopy revealed drastic differences in the staining pattern generated by the anti-actin antibody preparations. With one, distinct puncta appeared at the actin-rich leading edge and sometimes aligned with microtubules in the interior of the lamella, while the other displayed typical actin filament staining. Labelling experiments in vitro demonstrated failure of the first antibody to recognize filamentous actin and none of the two bound microtubules. The two anti-profilin antibodies purified in parallel generated a punctated pattern similar to that seen with the first anti-actin antibody. All antibody preparations labelled the nuclei.

CRMP-2 induces axons in cultured hippocampal neurons

In cultured hippocampal neurons, one axon and several dendrites differentiate from a common immature process. Here we found that CRMP-2/TOAD-64/Ulip2/DRP-2 (refs. 2-4) level was higher in growing axons of cultured hippocampal neurons, that overexpression of CRMP-2 in the cells led to the formation of supernumerary axons and that expression of truncated CRMP-2 mutants suppressed the formation of primary axon in a dominant-negative manner. Thus, CRMP-2 seems to be critical in axon induction in hippocampal neurons, thereby establishing and maintaining neuronal polarity.