The Arp2/3 complex is thus a molecular machinery that provides the directionality, orientation, and large-scale architecture to the actin cytoskeleton that is necessary for its function.
Structure
What are SD1-SD4 domains of Arp2/3 complex?
| Subdomain | Location / Function | Structural role |
|---|---|---|
| SD1 | N-terminal lobe | Contacts other actin subunits; part of the “barbed end” interface |
| SD2 | Small domain, adjacent to SD1 | Contributes to the nucleotide-binding cleft |
| SD3 | Larger domain | Anchors the nucleotide pocket (along with SD2) |
| SD4 | Large C-terminal domain | Contains the “back” of the molecule; key for domain rotation during flattening |
Difference of structure in activated and non-activated state
Previous experiments suggest that during activation, Arp2 and Arp3 move into, or close to, the side-by-side arrangement of consecutive actin subunits along the short-pitch helical axis of an actin filament, hereafter called the short-pitch conformation. In this position, Arp2 and Arp3 are thought to mimic a filamentous actin dimer, thereby creating a template for new filament assembly. In the inactive complex, however, Arp2 and Arp3 do not mimic a filamentous actin dimer. Instead, two core subunits of the Arp2/3 complex, ARPC2 and ARPC4, form a clamp that holds the Arps in an end-to-end (splayed) arrangement distinct from the short-pitch conformation. Furthermore, in the inactive complex, Arp2 and Arp3 adopt conformations that resemble unpolymerized actin monomers rather than filamentous actin subunits.
Reference: @shaabanCryoEMRevealsTransition2020