Regulating the Regulators: Mechanisms of Substrate Selection of the O-GlcNAc Cycling Enzymes OGT and OGA

Abstract

Thousands of nuclear and cytosolic proteins are modified with a single β-N-acetylglucosamine on serine and threonine residues in mammals, a modification termed O-GlcNAc. This modification is essential for normal development and plays important roles in virtually all intracellular processes. Additionally, O-GlcNAc is involved in many disease states, including cancer, diabetes, and X-linked intellectual disability. Given the myriad of functions of the O-GlcNAc modification, it is therefore somewhat surprising that O-GlcNAc cycling is mediated by only two enzymes: the O-GlcNAc transferase (OGT), which adds O-GlcNAc, and the O-GlcNAcase (OGA), which removes it. A significant outstanding question in the O-GlcNAc field is how do only two enzymes mediate such an abundant and dynamic modification. In this review, we explore the current understanding of mechanisms for substrate selection for the O-GlcNAc cycling enzymes. These mechanisms include direct substrate interaction with specific domains of OGT or OGA, selection of interactors via partner proteins, posttranslational modification of OGT or OGA, nutrient sensing, and localization alteration. Altogether, current research paints a picture of an exquisitely regulated and complex system by which OGT and OGA select substrates. We also make recommendations for future work, toward the goal of identifying interaction mechanisms for specific substrates that may be able to be exploited for various research and medical treatment goals.

Keywords: O-GlcNAc; OGA; OGT; substrate selection.

Fig. 1

The O-GlcNAc transferase. (A) Schematic of the addition of O-GlcNAc to an acceptor substrate by OGT. (B) Linear representation of the structure and domains of OGT. Interacting proteins and their location of interaction are shown above the structure. Known posttranslational modifications are shown below. (C) Possible mechanisms of OGT substrate selection, shown on a surface and cartoon representation of OGT in complex with a substrate peptide. A structure for the N-terminal TPR domain (PDB: 1W3B, Jinek et al. 2004) was aligned with an overlapping structure containing the remaining C-terminal TPR and catalytic domains (PDB: 4GYY, Lazarus et al. 2012) using PyMOL 2.4.1.

Fig. 2

The O-GlcNAcase. (A) Schematic of the removal of O-GlcNAc from an acceptor substrate by OGA. (B) Linear representation of the structure and domains of OGA. Interacting proteins and their location of interaction are shown above the structure. Known posttranslational modifications are shown below. (C) Possible mechanisms of OGA substrate selection, shown on a surface and cartoon representation of OGA in complex with a substrate O-GlcNAc-modified glycopeptide. A structure of OGA in complex with thiamet-G (PDB: 5UN93) was aligned with a structure of an OGA D175N mutant in complex with a glycopeptide substrate (PDB: 5VVU, Li et al. 2017a) using PyMOL 2.4.1.