Protein kinases are crucial components of the cell-signalling machinery that orchestrate and convey messages to their downstream targets. Most often, kinases are activated upon a phosphorylation to their activation loop, which will shift the kinase into the active conformation. The Dual specificity mitogen-activated protein kinase kinase 4 (MKK4) exists in a unique conformation in its inactive unphosphorylated state, where its activation segment appears in a stable α-helical conformation. However, the precise role of this unique conformational state of MKK4 is unknown. Here, by all-atom molecular dynamics simulations (MD simulations), we show that this inactive state is unstable as monomer even when unphosphorylated and that the phosphorylation of the activation segment further destabilizes the autoinhibited α-helix. The specific phosphorylation pattern of the activation segment has also a unique influence on MKK4 dynamics. Furthermore, we observed that this specific inactive state is stable as a dimer, which becomes destabilized upon phosphorylation. Finally, we noticed that the most frequent MKK4 mutation observed in cancer, R134W, which role has not been disclosed to date, contributes to the dimer stability. Based on these data we postulate that MKK4 occurs as a dimer in its inactive autoinhibited state, providing an additional layer for its activity regulation.
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