Molecular basis of mitogen-activated protein kinase ERK2 activation by its upstream kinase MEK1

The RAS-RAF-MEK-ERK mitogen-activated protein kinase (MAPK) pathway relays extracellular signals into a cellular response and its dysregulation leads to many pathologies, particularly cancer. Here, we determined cryo-EM structures of the MAP2K MEK1 activating its substrate MAPK ERK2, the final event in the cascade. We define the molecular details of specificity and phosphoryl transfer to the tyrosine of the ERK2 activation loop and examine the mechanism of substrate recognition using solution techniques and molecular dynamics. Binding of the substrate MAPK leads to release of the MAP2K catalytic machinery, explaining the mechanism of many disease-causing mutations, and ERK2 release is not required for nucleotide exchange, suggesting a processive mechanism. Our data advance the understanding of MAPK signalling and provide a starting point for drug development.