Ubiquitination of Glycogen and Metabolites in Cells and Tissues

Why It Matters

The findings reveal a previously hidden layer of metabolic regulation by ubiquitin, offering new targets for treating glycogen‑related disorders and metabolic diseases. By exposing non‑protein ubiquitination, the work broadens drug discovery opportunities within the ubiquitin system.

Key Takeaways

• NoPro-clipping reveals ubiquitinated glycogen across mouse tissues.
• Liver glycogen ubiquitination spikes during fasting, aiding catabolism.
• Ubiquitination directs glycogen to lysosomes, reducing cellular stores.
• Met1‑polyubiquitin machinery modulates glycogen ubiquitination in storage diseases.
• Ubiquitination also observed on glycerol and spermine metabolites.

Pulse Analysis

The study introduces Non‑Protein Ub‑clipping (NoPro‑clipping), a mass‑spectrometry workflow that couples ubiquitin‑specific clippases with sortase‑mediated labeling. By bypassing the protein‑centric bias of traditional ubiquitinomics, the method captures ubiquitin conjugates on sugars, lipids and nucleotides that were previously invisible. Validation in cultured cells and mouse and human tissues uncovered a surprisingly rich landscape of non‑protein ubiquitination, expanding the biochemical definition of ubiquitin from a protein modifier to a universal tag for diverse biomolecules. This technical advance opens a new frontier for systems‑level mapping of ubiquitin signaling.

Among the newly identified substrates, glycogen emerged as a prominent target. The authors demonstrate that ubiquitinated glycogen is present in every glycogen‑rich mouse tissue, with the liver and skeletal muscle showing the highest levels. Ubiquitination tags glycogen for lysosomal delivery, accelerating its degradation and lowering cellular glycogen stores. Notably, fasting triggers a surge in hepatic glycogen ubiquitination, linking the modification to physiological catabolism. In models of glycogen storage disease, dysregulated ubiquitination correlates with abnormal glycogen accumulation, suggesting that the Met1‑linked polyubiquitin chain machinery could be a therapeutic lever.

The discovery that ubiquitin also modifies small metabolites such as glycerol and spermine reshapes our understanding of cellular regulation. It implies that ubiquitin may coordinate metabolic fluxes alongside protein turnover, integrating energy balance with signaling networks. Pharmaceutical interest in ubiquitin pathways has focused on proteasomal degradation; this broader view invites drug developers to explore modulators of non‑protein ubiquitination for metabolic disorders. Future research will need to map the enzymes that attach and remove ubiquitin from these unconventional substrates, and to determine how these tags influence disease phenotypes across organ systems.