Jumonji C-terminal (JmjC) domain-containing proteins are protein hydroxylases and histone demethylases that control gene expression. Jumonji domain-containing protein 6 (Jmjd6) is indispensable for embryonic development and has both histone arginine demethylase and lysyl-hydroxylase activities. The protein undergoes post-translational homo-oligomerization, but the underlying mechanism remains unknown. In this study, we examined the enzymatic activity of Jmjd6 and uncovered the mechanism underlying its homo-oligomerization. An in vitro enzymatic assay monitored by matrix-assisted laser desorption-ionization time-of-flight (MALDI-TOF) mass spectrometry indicates that Jmjd6 is unable to remove the methyl group from histone arginine residues but can hydroxylate the histone H4 tail at lysine residues in a 2-oxoglutarate (2-OG)- and Fe (II)-dependent manner. A mutational analysis reveals that the homo-oligomerization of Jmjd6 requires its enzymatic activity and the N- and C-termini. Using an in vitro enzymatic assay, we further demonstrate that Jmjd6 can hydroxylate its N-terminus but not its C-terminus. In summary, we did not detect arginine demethylase activity for Jmjd6, but we did confirm that it could catalyze the lysyl-hydroxylation of histone peptides. In addition, we demonstrated that the homo-oligomerization of Jmjd6 requires its own enzymatic activity and the N- and C-termini. We propose that Jmjd6 forms intermolecular covalent bonds between its N- and C-termini via autohydroxylation.
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