The Arp2/3 (Actin-related proteins 2/3) complex is activated by WASP (Wiskott-Aldrich syndrome protein) family proteins to nucleate branched actin filaments that are important for cellular motility. WASP recruits actin monomers to the complex and stimulates movement of Arp2 and Arp3 into a "short-pitch" conformation that mimics the arrangement of actin subunits within filaments. The relative contribution of these functions in Arp2/3 complex activation and the mechanism by which WASP stimulates the conformational change have been unknown. We purified budding yeast Arp2/3 complex held in or near the short-pitch conformation by an engineered covalent cross-link to determine if the WASP-induced conformational change is sufficient for activity. Remarkably, cross-linked Arp2/3 complex bypasses the need for WASP in activation and is more active than WASP-activated Arp2/3 complex. These data indicate that stimulation of the short-pitch conformation is the critical activating function of WASP and that monomer delivery is not a fundamental requirement for nucleation but is a specific requirement for WASP-mediated activation. During activation, WASP limits nucleation rates by releasing slowly from nascent branches. The cross-linked complex is inhibited by WASP's CA region, even though CA potently stimulates cross-linking, suggesting that slow WASP detachment masks the activating potential of the short-pitch conformational switch. We use structure-based mutations and WASP-Arp fusion chimeras to determine how WASP stimulates movement toward the short-pitch conformation. Our data indicate that WASP displaces the autoinhibitory Arp3 C-terminal tail from a hydrophobic groove at Arp3′s barbed end to destabilize the inactive state, providing a mechanism by which WASP stimulates the short-pitch conformation and activates Arp2/3 complex.actin | Arp2/3 | WASP

Nucleation of branched actin filaments by Arp2/3 complex is tightly regulated to control actin assembly in cells. Arp2/3 complex activation involves conformational changes brought about by ATP, Nucleation Promoting Factor (NPF) proteins, actin filaments and NPF-recruited actin monomers. To understand how these factors promote activation, we must first understand how the complex is held inactive in their absence. Here we demonstrate that the Arp3 C-terminal tail is a structural switch that prevents Arp2/3 complex from adopting an active conformation. The interaction between the tail and a hydrophobic groove in Arp3 blocks movement of Arp2 and Arp3 into an activated filament-like (short pitch) conformation. Our data indicate ATP binding destabilizes this interaction via an allosteric link between the Arp3 nucleotide cleft and the hydrophobic groove, thereby promoting the short-pitch conformation. Our results help explain how Arp2/3 complex is locked in an inactive state without activators and how autoinhibition is relieved.

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.

Background:Smoking and tobacco use continue to be the largest preventable causes of death globally. A novel therapeutic approach has recently been proposed: administration of an enzyme that degrades nicotine, the main addictive component of tobacco, minimizing brain exposure and reducing its reinforcing effects. Pre-clinical proof of concept has been previously established through dosing the amine oxidase NicA2 from Pseudomonas putida in rat nicotine self-administration models of addiction. Results: This paper describes efforts towards optimizing NicA2 for potential therapeutic use: enhancing potency, improving its pharmacokinetic profile, and attenuating immunogenicity. Libraries randomizing residues located in all 22 active site positions of NicA2 were screened. 58 single mutations with 2-to 19-fold enhanced catalytic activity compared to wt at 10 μM nicotine were identified. A novel nicotine biosensor assay allowed efficient screening of the many primary hits for activity at nicotine concentrations typically found in smokers. 10 mutants with improved activity in rat serum at or below 250 nM were identified. These catalytic improvements translated to increased potency in vivo in the form of further lowering of nicotine blood levels and nicotine accumulation in the brains of Sprague-Dawley rats. Examination of the X-ray crystal structure suggests that these mutants may accelerate the rate limiting re-oxidation of the flavin adenine dinucleotide cofactor by enhancing molecular oxygen's access. PEGylation of NicA2 led to prolonged serum half-life and lowered immunogenicity observed in a human HLA DR4 transgenic mouse model, without impacting nicotine degrading activity.Conclusions: Systematic mutational analysis of the active site of the nicotine-degrading enzyme NicA2 has yielded 10 variants that increase the catalytic activity and its effects on nicotine distribution in vivo at nicotine plasma concentrations found in smokers. In addition, PEGylation substantially increases circulating half-life and reduces the enzyme's immunogenic potential. Taken together, these results provide a viable path towards generation of a drug candidate suitable for human therapeutic use in treating nicotine addiction.