Regulates actin polymerization by inhibiting the actin-nucleating activity of the Arp2/3 complex; the function is competitive with nucleation promoting factors. Participates in an incoherent feedforward loop at the lamellipodium tip where it inhibits the ARP2/2 complex in response to Rac signaling and where Rac also stimulates actin polymerization through the WAVE complex. Involved in steering cell migration by controlling its directional persistence (By similarity). Associates with the Arp2/3 complex. Interacts with ARPC2; enhanced by activated RAC1. Interacts with ARPC5; the interaction is dependent on RAC1. Colocalized with the WAVE complex at lamellipodium tip. The acidic C-terminus is necessary and sufficient to inhibit ARP2/3 complex activity. Belongs to the Arpin family.