The phosphorylation of Pak1 by Erk1/2 for cell migration requires Arl4D acting as a scaffolding protein
Activation of extracellular signal-regulated kinases 1 and 2 (Erk1/2) at the plasma membrane typically results in their translocation to other intracellular sites for substrate targeting. This targeting requires scaffolding proteins to bring Erk1/2 and their substrates together. In the case of platelet-derived growth factor (PDGF)-induced signaling that activates Erk1/2 to phosphorylate Pak1 for cell migration, Erk1/2 remain at the plasma membrane. Thus, it has been unclear in this case whether a scaffolding protein is needed for substrate targeting by Erk1/2. The small GTPase Arf-like protein (Arl) 4D also promotes cell migration by targeting Pak1 to the plasma membrane. However, as this recruitment also results in the phosphorylation of Pak1, it has been unclear how this phosphorylation is achieved. Furthermore, because both Erk1/2 and Arl4D promote the role of Pak1 in cell migration, the relationship between these two processes has also been unclear. Addressing these outstanding questions, we show that Arl4D acts as a scaffolding protein by recruiting Erk1/2 and Pak1 to the plasma membrane for their assembly into a protein complex. Our findings identify Arl4D as a novel regulator of Erk1/2, reveal a conserved role for scaffolding proteins in substrate targeting by Erk1/2, as well as uncovering a previously unknown interplay among Arl4D, Erk1/2, and Pak1. Moreover, as all these factors are known to be involved in cell migration, we have shed new mechanistic insights into how this fundamental cellular process occurs.