The crystal structure of the cysteine protease Xylellain from Xylella fastidiosa reveals an intriguing activation mechanism

► The plant pathogen Xylella fastidiosa has a single cysteine protease, Xylellain. ► Xylellain crystallographic structure was solved at 1.65 Å resolution. ► A ribonucleotide, UDP, is present in a hinge region of Xylellain. ► The UDP may play an important role in stabilizing the N-terminal pro-region. ► This nucleotide suggests a unique mechanism of activation for this cysteine protease.