ERK3/MAPK6 dictates Cdc42/Rac1 activity and ARP2/3-dependent actin polymerization

Previously we have identified a role for ERK3/MAPK6, an atypical MAPK, in the regulation epithelial secretome, immune cell chemotaxis and tumour metastasis52 (Bogucka et al. 2020). Here we uncover a critical role for ERK3 in the regulation of actin cytoskeleton. The actin cytoskeleton is tightly controlled by RhoGTPases, actin binding proteins and nucleation-promoting factors to perform fundamental cellular functions. We show that ERK3 directly acts as a guanine nucleotide exchange factorfor Cdc42 and phosphorylates the ARP3 subunit of the ARP2/3 complex at S418 to promote filopodia formation and actin polymerization, respectively. Consistently, depletion of ERK3 prevented both basal and EGF-dependent Rac1 and Cdc42 activation, maintenance of F-actin content, filopodia formation and epithelial cell migration. Further, ERK3 protein binds directly to the purified ARP2/3 complex and augments polymerization of actin in vitro. ERK3 kinase activity is required for the formation of actin-rich protrusions in mammalian cells. These findings unveil a fundamentally unique pathway employed by cells to control actin-dependent cellular functions.