Tight regulation of F-actin assembly and disassembly at specific sites in the cell is crucial for its function in different cellular processes — for example, cell motility, intracellular organization and membrane trafficking. Yeast Las17 and its mammalian orthologue Wiskott–Aldrich syndrome protein (WASP) are known activators of actin-related protein 2/3 (ARP2/3)-dependent actin nucleation, which promotes the formation of branched actin networks. Here, Ayscough and colleagues report that a central Pro-rich domain in Las17 and WASP has a previously uncharacterized actin-binding activity. Importantly, they find that binding of this domain to actin promotes actin nucleation independently of ARP2/3. Furthermore, they show that Las17-mediated actin binding has a role in endocytosis.