Depending on their function, actins can form straight bundles or branched filaments. Straight actin filaments are generated by the dimeric protein formin, whose FH1 domain acts as a binding site for the profilin-ATP-actin complex. The donut-shaped FH2 domain initiates actin nucleation with two actin monomers. The FH2 domain receives one actin monomer at a time from the FH1 domain, generating a straight filament. Branched actin filaments are generated by the Arp2/3 complex. The Arp2/3 complex binds with CDC42-activated nucleation-promoting factor with attached actin monomer. The binding of Arp2/3 with NPFs alters its conformation allowing it to bind to the sides of a pre-existing actin filament for elongation. Upon binding to the actin filament, NPFs add actin monomers to the activated Arp2/3 complex for nucleation before dissociating. The elongation of the branched filament proceeds at a seventy-degree angle, forming a Y-shaped network.