O-GlcNAc Transferase Catalyzes Site-Specific Proteolysis of HCF-1

The human epigenetic cell-cycle regulator HCF-1 undergoes an unusual proteolytic maturation process resulting in stably associated HCF-1N and HCF-1C subunits that regulate different aspects of the cell cycle. Proteolysis occurs at six centrally located HCF-1PRO-repeat sequences and is important for activation of HCF-1C-subunit functions in M phase progression. We show here that the HCF-1PRO repeat is recognized by O-linked β-N-acetylglucosamine transferase (OGT), which both O-GlcNAcylates the HCF-1N subunit and directly cleaves the HCF-1PRO repeat. Replacement of the HCF-1PRO repeats by a heterologous proteolytic cleavage signal promotes HCF-1 proteolysis but fails to activate HCF-1C-subunit M phase functions. These results reveal an unexpected role of OGT in HCF-1 proteolytic maturation and an unforeseen nexus between OGT-directed O-GlcNAcylation and proteolytic maturation in HCF-1 cell-cycle regulation.