Architecture of the MKK6-p38α complex defines the basis of MAPK specificity and activation

The MAP kinase p38α is a central component of signalling in inflammation and the immune response, and is therefore an important drug target. Little is known about the molecular mechanism of its activation by double-phosphorylation from MAP2Ks, due to the challenge of trapping a transient and dynamic hetero-kinase complex. Here, we applied a multidisciplinary approach to generate the first structure of p38α in complex with its MAP2K MKK6 and understand the activation mechanism. Integrating cryo-EM with MD simulations and in cellulo experiments, we demonstrate a dynamic, multi-step, phosphorylation mechanism, reveal new catalytically relevant interactions, and show that MAP2K disordered N-termini determine pathway specificity. Our work captures, for the first time, a fundamental step of cell signalling: a kinase phosphorylating its downstream target kinase