Mechanistic insight into the dual phosphorylation of ERK1 and ERK2 by MEK1

Abstract

Activation of ERK1 and ERK2 through MEK1 and MEK2-catalyzed phosphorylation of conserved tyrosine and threonine residues in the ERK1/2 activation loops is a key step in the RAS-RAF-MEK-ERK signaling cascade. However, the molecular mechanisms underlying MEK1/2-catalyzed ERK1/2 phosphorylation remain unclear. Here, we report the cryo-EM structures of phosphorylated MEK1 (pMEK1) bound to unphosphorylated ERK1/2 and monophosphorylated ERK1, together with a crosslinked pMEK1:ERK1 complex. These structures elucidate pMEK1-catalyzed dual phosphorylation of ERK1/2: the ERK1/2 activation loops interact with the MEK1 activation loop to enable tyrosine phosphorylation, and adopt a distinct conformation by interacting with the ERK1/2 HRD motifs to promote threonine phosphorylation in the ERK1/2 activation loops. Notably, we find that pMEK1 catalyzes ERK1/2 dephosphorylation and reveal that it is attributed to the reverse transfer of the phosphate from ERK1/2 to ADP. These findings delineate the catalytic cycle of MEK1/2-catalyzed ERK1/2 phosphorylation and provide insights for targeting this oncogenic pathway.