Ubiquitin beyond the proteome: lipids, glycans, metabolites, nucleic acids, and an expanding molecular landscape.

Ubiquitination is a versatile post-translational modification process in which the small globular protein ubiquitin is covalently attached to substrate proteins to generate diverse cellular signals. Although originally characterized by its role in proteasome-mediated protein degradation, ubiquitination is now recognized as a central regulator of numerous processes, including signaling, trafficking, and immunity. Canonical ubiquitination is mediated by a cascade of E1 (activating), E2 (conjugating), and E3 (ligase) enzymes that repeatedly conjugate ubiquitin molecules to lysine residues on substrate proteins, leading to the formation of polyubiquitin chains with distinct topologies. The modification is reversed by deubiquitinating enzymes. Notably, components of the ubiquitin system comprise approximately 7% of the human proteome, underscoring its importance in biological regulation. Recent advances have revealed the broad scope of ubiquitination. Ubiquitin was found to conjugate not only to lysine but also to serine, threonine, and cysteine, indicating its unexpected chemical flexibility. Furthermore, ubiquitination can be directed toward other post-translational modifications, particularly glycosylation and ADP-ribosylation, highlighting the extensive crosstalk between modification systems. Strikingly, lipids, sugars, metabolites, nucleic acids, and even synthetic small-molecule compounds have been identified as ubiquitinated substrates. The hypothesis that virtually all classes of molecules are targeted by ubiquitination has become increasingly plausible. Taken together, these findings redefine ubiquitination as a far more general modification process than previously appreciated. In this mini-review, we focus on recent progress in non-proteinaceous ubiquitination research, summarize emerging substrate classes, and discuss key challenges in elucidating the underlying mechanisms and physiological roles of this expanding modification landscape.