Structural insights into the activation of MST3 by MO25

► Elucidation of the structure of the catalytic core of MST3 complexed with its MO25β regulatory subunit. ► Define key conserved interface residues on MO25β and MST3 that interact with one another. ► First structure of MO25β isoform to be reported. ► Findings provide greater insight into how MO25 isoforms can function as master regulators of STE20 kinase.