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Structural-Functional Mechanisms of Arp2/3 Complex Activation and Inhibition - 2 November 2021

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Speaker: Professor Roberto Dominguez, William Maul Measey Presidential Professor of Physiology, Perelman School of Medicine, University of Pennsylvania 

Host: Matthias Krause

Abstract: Activated by nucleation-promoting factors (NPFs), Arp2/3 complex binds to the side of a pre-existing (mother) filament and nucleates a new (branch) filament. NPFs are unrelated, but their conserved C-terminal WH2/Central/Acidic (WCA) region is necessary and sufficient to activate Arp2/3 complex.

We determined the 3.8-Å cryo-EM structure of Arp2/3 complex with N-WASP WCA bound. The structure revealed two NPFs bound to Arp2/3 complex, one on Arp2-ArpC1 and one on Arp3. Biochemical studies further revealed: a) actin binding to the W domains of NPFs favors the transition toward the active, filament-like conformation, b) actin-NPF binding to Arp2-ArpC1 precedes binding to Arp3 and is sufficient to shift the equilibrium toward the filament-like conformation, c) Arp2/3 complex activation requires NPF-mediated delivery of actin at the barbed end of both Arps.

The NPF-bound structure, however, showed the inactive conformation of Arp2/3 complex. In another structure (3.5-Å), we added actin and capping protein (CP) to form a mini-filament branch. This structure revealed a series of states along the transition from inactive to activated Arp2/3 complex.

Arpin is an inhibitor that controls cell migration by interrupting a Rac-WAVE-Arp2/3 complex feedback loop. The 3.24-Å cryo-EM structure of Arpin bound to Arp2/3 complex unexpectedly showed that Arpin binds Arp2/3 complex similarly to NPFs but interacts only with Arp3. Like NPFs, Arpin has a helix that binds at the barbed end of Arp3. Sequence differences within this helix define the molecular basis for inhibition by Arpin vs. activation by NPFs.

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