Structure of the nucleation‐promoting factor SPIN90 bound to the actin filament nucleator Arp2/3 complex

• Published in: The EMBO journal Vol. 37; no. 22
• Main Authors: Luan, Qing, Liu, Su‐Ling, Helgeson, Luke A, Nolen, Brad J
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 15.11.2018; Springer Nature B.V; John Wiley and Sons Inc
• Subjects: Actin; Arp2/3 complex; Assembly; Binding sites; Crystal structure; EMBO05; EMBO40; Filaments; Nucleation; Protein structure; Proteins; SPIN90; WASP; Arp2/3 complex; nucleation; actin; SPIN90; WASP
• ISSN: 0261-4189; 1460-2075; 1460-2075
• DOI: 10.15252/embj.2018100005

Unlike the WASP family of Arp2/3 complex activators, WISH/DIP/SPIN90 (WDS) family proteins activate actin filament nucleation by the Arp2/3 complex without the need for a preformed actin filament. This allows WDS proteins to initiate branched actin network assembly by providing seed filaments that activate WASP‐bound Arp2/3 complex. Despite their important role in actin network initiation, it is unclear how WDS proteins drive the activating steps that require both WASP and pre‐existing actin filaments during WASP‐mediated nucleation. Here, we show that SPIN90 folds into an armadillo repeat domain that binds a surface of Arp2/3 complex distinct from the two WASP sites, straddling a hinge point that may stimulate movement of the Arp2 subunit into the activated short‐pitch conformation. SPIN90 binds a surface on Arp2/3 complex that overlaps with actin filament binding, explaining how it could stimulate the same structural rearrangements in the complex as pre‐existing actin filaments. By revealing how WDS proteins activate the Arp2/3 complex, these data provide a molecular foundation to understand initiation of dendritic actin networks and regulation of Arp2/3 complex by its activators. Synopsis Unlike WASP, WDS proteins do not need a pre‐existing actin filament to activate new filament nucleation by the Arp2/3 complex and can therefore trigger the assembly of branched actin networks. A new crystal structure of WDS protein SPIN190 in complex with Arp2/3 sheds light on the underlying mechanism. We solved the crystal structure of the Arp2/3 complex activator SPIN90 alone and bound to the complex. Unlike the canonical activator WASP, SPIN90 activates the Arp2/3 complex without requiring a preformed filament. SPIN90 binds a putative hinge region that may rotate Arp2 into a short pitch helical conformation, explaining how it could stimulate the same activating structural change as WASP. The SPIN90 binding site overlaps with the binding site for actin filaments, providing a mechanism for it to induce the same activating conformational changes as filaments. Graphical Abstract New crystal structures reveal how WDS proteins facilitate Arp2/3‐dependent actin filament nucleation in the absence of pre‐existing filaments.