The atypical MAP kinases ERK3 and ERK4 are the main characters in this study. ERK3 and ERK4 were for long considered as orphan proteins without known upstream activators. However, in 2011 two studies had detected that group I p-21 protein activated kinase (PAKs) can constitutively phosphorylate ERK3 and ERK4 in their activation loop. The only well-characterized substrate for ERK3 and ERK4 at that ...