Figure 1. The role of Abi within the SCAR complex. The SCAR complex is composed of PIR121 (blue), Nap1 (green), HSPC300 (hidden in this image), SCAR (magenta), and Abi (orange). The SCAR complex promotes activation of the Arp2/3 complex (magenta arrow). Abi has long been considered a key regulator of the SCAR complex. In combination with the Rac/PIR121 interaction (blue arrow), Abi was thought to activate actin polymerization by coupling SCAR to various signaling and adaptor proteins (orange arrows) via its C-terminal SH3 domain (metazoans only) and polyproline tail (dashed ribbons). However, having deleted the majority of Dictyostelium Abi, it is now evident that Abi is not required for SCAR complex activation. Instead, Abi likely acts to tune the activity of the SCAR by integrating both positive and negative signals (dashed lines). One possible negative input could be acting through the N-terminal first α-helix to suppress SCAR activity during mitosis. However, the regulators that bind this domain have yet to be identified.