Crystal and solution structures disclose a putative transient state of mitogen-activated protein kinase kinase 4

► X-ray crystal and SAXS experiments represented a transient state of apo npMAP2K4. ► Formation of β-barrel conferred the ATP site to be exposed to the bulk solvent. ► This conformation displayed the transient state after the product ADP release. ► The ATP binding allows the transient structure to assume the canonical kinase fold. ► The depletion of ATP perhaps enhances the aggregation via hydrophobic interactions.