The Scar/WAVE complex drives normal actin protrusions without the VCA domain, but proline-rich domains are required

The Scar/WAVE regulatory complex (WRC) is the main catalyst of pseudopod and lamellipodium formation during cell migration. Its actin nucleation activity has been attributed to its ability to activate the Arp2/3 complex through the VCA domain of Scar/WAVE. Here we show in both B16-F1 mouse melanoma cells and Dictyostelium discoideum cells that the WRC with its VCA domain deleted can still induce the formation of morphologically normal actin protrusions. Its function is lost only after further deletion of Scar/WAVE’s (in Dictyostelium cells) or both Abi and WAVE’s (in B16-F1 cells) proline-rich domains. Moreover, mass spectrometry analysis identified BAIAP2 and Vasp (both promotors of actin polymerization) as specific interactors of WRC polyproline domains. Thus we conclude that proline-rich domains play a central role in actin nucleation by concentrating other effectors needed to form actin protrusions. Together, these findings suggest a new mechanism for WRC action independent of the VCA-Arp2/3 interaction.