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O-Linked GlcNAc transferase (OGT) possesses dual glycosyltransferase–protease activities. OGT thereby stably glycosylates serines and threonines of numerous proteins and, via a transient glutamate glycosylation, cleaves a single known substrate—the so-called HCF-1 PRO repeat of the transcriptional co-regulator host-cell factor 1 (HCF-1). Here, we probed the relationship between these distinct glycosylation and proteolytic activities. For proteolysis, the HCF-1 PRO repeat possesses an important extended threonine-rich region that is tightly bound by the OGT tetratricopeptide-repeat (TPR) region. We report that linkage of this HCF-1 PRO-repeat, threonine-rich region to heterologous substrate sequences also potentiates robust serine glycosylation with the otherwise poor R p-S-UDP-GlcNAc diastereomer phosphorothioate and UDP-5S-GlcNAc OGT co-substrates. Furthermore, it potentiated proteolysis of a non-HCF-1 PRO-repeat cleavage sequence, provided it contained an appropriately positioned glutamate residue. Using serine- or glutamate-containing HCF-1 PRO-repeat sequences, we show that proposed OGT-based or UDP-GlcNAc– based serine-acceptor residue activation mechanisms can be circumvented independently, but not when disrupted together. In contrast, disruption of both proposed activation mechanisms even in combination did not inhibit OGT-mediated proteolysis. These results reveal a multiplicity of OGT glycosylation strategies, some leading to proteolysis, which could be targets of alternative molecular regulatory strategies.
- Host-cell factor-1
- enzyme mechanism
- post-translational modification (PTM)
- O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT)