Scar, a WASp-related protein, activates nucleation of actin filaments by the Arp2/3 complex

The Arp2/3 complex, a stable assembly of two actin-related proteins (Arp2 a nd Arp3) with five other subunits, caps the pointed end of actin filaments and nucleates actin polymerization with low efficiency. WASp and Scar are t wo similar proteins that bind the p21 subunit of the Arp2/3 complex, but th eir effect on the nucleation activity of the complex was not known. We repo rt that full-length, recombinant human Scar protein, as well as N-terminall y truncated Scar proteins, enhance nucleation by the Arp2/3 complex. By the mselves, these proteins either have no effect or inhibit actin polymerizati on. The actin monomer-binding W domain and the p21-binding A domain from th e C terminus of Scar are both required to activate Arp2/3 complex. A prolin e-rich domain in the middle of Scar enhances the activity of the W and A do mains. Preincubating Scar and Arp2/3 complex with actin filaments overcomes the initial lag in polymerization, suggesting that efficient nucleation by the Arp2/3 complex requires assembly on the side of a preexisting filament -a dendritic nucleation mechanism. The Arp2/3 complex with full-length Scar , Scar containing P, Wand A domains, or Scar containing W and A domains ove rcomes inhibition of nucleation by the actin monomer-binding protein profil in, giving active nucleation over a low background of spontaneous nucleatio n. These results show that Scar and, likely, related proteins, such as the Cdc42 targets WASp and N-WASp, are endogenous activators of actin polymeriz ation by the Arp2/3 complex.