Background Arp2/3 complex is a key actin cytoskeletal regulator that creates branched actin filament networks in response to cellular signals. WASP-activated Arp2/3 complex assembles branched actin networks by nucleating new filaments from the sides of pre-existing ones. WASP-mediated activation requires seed filaments, to which the WASP-bound Arp2/3 complex can bind to form branches, but the source of the first substrate filaments for branching is unknown. Results Here we show that Dip1, a member of the WISH/DIP/SPIN90 family of actin regulators, potently activates Arp2/3 complex without preformed filaments. Unlike other Arp2/3 complex activators, Dip1 does not bind actin monomers or filaments, and interacts with the complex using a non-WASP-like binding mode. In addition, Dip1-activated Arp2/3 complex creates linear instead of branched actin filament networks. Conclusions Our data show the mechanism by which Dip1 and other WISH/DIP/SPIN90 proteins can provide seed filaments to Arp2/3 complex to serve as master switches in initiating branched actin assembly. This mechanism is distinct from other known activators of Arp2/3 complex.

Background: How cortactin and WASP proteins coordinately regulate branched actin assembly is poorly understood. Results: The Arp2/3 complex-and actin-interacting regions of cortactin and WASP proteins are functionally distinct and tailored to their regulatory roles. Conclusion: Mechanistic distinctions between activators are important in allowing coordinate regulation of branched actin networks. Significance: Understanding mechanistic distinctions between activators is required to understand cellular structures like lamellipodia.

Vestibular disorders pose a substantial burden on the healthcare system due to a high prevalence and the severity of symptoms. Currently, a large portion of patients experiencing vestibular symptoms receive an ambiguous diagnosis or one that is based solely on history, unconfirmed by any objective measures. As patients primarily experience perceptual symptoms (e.g., dizziness), recent studies have investigated the use of vestibular perceptual thresholds, a quantitative measure of vestibular perception, in clinical populations. This review provides an overview of vestibular perceptual thresholds and the current literature assessing use in clinical populations as a potential diagnostic tool. Patients with peripheral and central vestibular pathologies, including bilateral vestibulopathy and vestibular migraine, show characteristic changes in vestibular thresholds. Vestibular perceptual thresholds have also been found to detect subtle, sub-clinical declines in vestibular function in asymptomatic older adults, suggesting a potential use of vestibular thresholds to augment or complement existing diagnostic methods in multiple populations. Vestibular thresholds are a reliable, sensitive, and specific assay of vestibular precision, however, continued research is needed to better understand the possible applications and limitations, especially with regard to the diagnosis of vestibular disorders.

Summary When activated by WASP family proteins, Arp2/3 complex nucleates branched actin filaments important for processes like cellular motility and endocytosis [1]. WASP-mediated activation of Arp2/3 complex requires a preformed actin filament, ensuring that activation by WASP creates branched instead of linear filaments. However, this biochemical requirement also means that assembly of branched actin networks must be primed with an initial seed filament [2–4]. We recently described a class of activators called WISH/DIP/SPIN90 (WDS) proteins, that unlike WASP, activate Arp2/3 complex without a preformed filament [4]. While this property may allow WDS proteins to serve as seed filament generators, it is unknown if actin filaments nucleated by WDS-activated Arp2/3 complex can activate WASP-bound Arp2/3 complex. Further, despite their potential importance as branched actin network initiators, little is known about how WDS proteins turn on Arp2/3 complex. Here we use two color single molecule TIRF microscopy to show that Dip1, the S. pombe WDS protein [5], co-opts features of branching nucleation to activate Arp2/3 complex. Specifically, it activates Arp2/3 complex to nucleate linear filaments analogous to the branch created by WASP-mediated activation. The barbed ends of Dip1-Arp2/3 nucleated filaments are free to elongate and their pointed ends remain anchored to Dip1-bound Arp2/3 complex. The linear filaments nucleated by Dip1-bound Arp2/3 complex activate WASP-bound Arp2/3 complex as potently as spontaneously nucleated or branched actin filaments. These observations provide important insights into the regulation of Arp2/3 complex by its activators and the molecular basis for initiation of branched actin networks.