Chemically Induced Proximity for Targeted Protein O-GlcNAcylation.

O-GlcNAcylation is a ubiquitous post-translational modification across the mammalian proteome, yet determining how individual O-GlcNAcylation events control specific protein functions remains a central challenge. The O-GlcNAc cycle is governed by two enzymes: O-GlcNAc transferase (OGT), which installs O-GlcNAc, and O-GlcNAcase (OGA), which removes it. Because these enzymes act on thousands of proteins, conventional methods, such as pharmacological inhibition of OGT or OGA, produce broad cellular effects that are difficult to assign to individual substrates. This gap has motivated the development of targeted O-GlcNAcylation strategies, including genetically encoded and chemical biology tools, which have shown that O-GlcNAc can be selectively installed on defined proteins to interrogate protein-specific O-GlcNAc functions. This Account describes the development of O-GlcNAc TArgeting Chimeras, or OGTACs, as chemical tools for targeted O-GlcNAc installation. OGTACs are bifunctional molecules that can recruit OGT to a specific protein of interest. Specifically, an OGTAC consists of an OGT-engaging element, a linker, and a target-recognition motif, which together bring OGT into proximity with a selected substrate. Our studies show that chemically induced proximity can redirect OGT toward selected proteins, thereby creating targeted O-GlcNAc writing that can partially bypass TPR-domain-mediated substrate recognition under engineered recruitment conditions. A central lesson is that productive O-GlcNAc writing requires not only recruitment, but also proper ternary-complex formation and noninhibitory OGT engagement. Moreover, the target-recognition motif determines the targetable protein space and achievable selectivity of the platform. By linking local O-GlcNAc installation to changes in protein turnover, signaling, and transcriptional output, OGTACs provide a conceptual framework for using induced proximity to modulate protein functions.