 The study found that ERK3, an atypical MAPK, regulates the formation of filipodia and actin polymerization through its interaction with the ARP2-3 complex. This interaction allows ERK3 to activate the kinase activity of the ARP2-3 complex, which leads to the formation of actin-rich protrusions in mammalian cells. This article was authored by Katasina Bagucca-Janchi, Gregory Harms, Marie Mayquassia, and others.