Cryo-EM reveals the transition of Arp2/3 complex from inactive to nucleation-competent state

• Published in: Nature structural & molecular biology Vol. 27; no. 11; pp. 1009 - 1016
• Main Authors: Shaaban, Mohammed, Chowdhury, Saikat, Nolen, Brad J.
• Format: Journal Article
• Language: English
• Published: New York Nature Publishing Group US 01.11.2020; Nature Publishing Group
• Subjects: 101/28; 631/535/1258/1259; 631/80/128/1276; 82/80; 82/83; Actin; Actin Cytoskeleton - chemistry; Actin Cytoskeleton - metabolism; Actin Cytoskeleton - ultrastructure; Actin-Related Protein 2-3 Complex - chemistry; Actin-Related Protein 2-3 Complex - metabolism; Actin-Related Protein 2-3 Complex - ultrastructure; Biochemistry; Biological Microscopy; Biomedical and Life Sciences; Cellular structure; Conformation; Cryoelectron Microscopy; Dimers; Life Sciences; Membrane Biology; Models, Molecular; Nucleation; Protein Binding; Protein Conformation; Protein Multimerization; Protein Structure; Proteins; Schizosaccharomyces - chemistry; Schizosaccharomyces - metabolism; Schizosaccharomyces pombe Proteins - chemistry; Schizosaccharomyces pombe Proteins - metabolism; Schizosaccharomyces pombe Proteins - ultrastructure; Twisting; Yeast
• ISSN: 1545-9993; 1545-9985; 1545-9985
• DOI: 10.1038/s41594-020-0481-x

Arp2/3 complex, a crucial actin filament nucleator, undergoes structural rearrangements during activation by nucleation-promoting factors (NPFs). However, the conformational pathway leading to the nucleation-competent state is unclear due to lack of high-resolution structures of the activated state. Here we report a ~3.9 Å resolution cryo-EM structure of activated Schizosaccharomyces pombe Arp2/3 complex bound to the S. pombe NPF Dip1 and attached to the end of the nucleated actin filament. The structure reveals global and local conformational changes that allow the two actin-related proteins in Arp2/3 complex to mimic a filamentous actin dimer and template nucleation. Activation occurs through a clamp-twisting mechanism, in which Dip1 forces two core subunits in Arp2/3 complex to pivot around one another, shifting half of the complex into a new activated position. By showing how Dip1 stimulates activation, the structure reveals how NPFs can activate Arp2/3 complex in diverse cellular processes. A cryo-EM structure reveals how nucleation-promoting factor Dip1 activates Arp2/3 complex and shows the actin-related proteins in Arp2/3 in a conformation that mimics a filamentous actin dimer, thus templating nucleation.