Monoubiquitylation

Monoubiquitylation is a post-translational modification where a single ubiquitin molecule is attached to a substrate protein. Ubiquitin is a small, highly conserved protein found in eukaryotes. The attachment of ubiquitin is catalyzed by a cascade of enzymes, including E1 (ubiquitin-activating enzyme), E2 (ubiquitin-conjugating enzyme), and E3 (ubiquitin ligase). E3 ligases are crucial as they provide specificity, determining which proteins are ubiquitylated.

Unlike polyubiquitylation, which involves the formation of ubiquitin chains, monoubiquitylation typically occurs on a single lysine residue of the target protein or at the N-terminus. The monoubiquitylation mark is reversible, mediated by deubiquitinating enzymes (DUBs). This reversibility allows for dynamic regulation of protein function.

Monoubiquitylation plays diverse roles in cellular processes. It can regulate protein localization, stability, and activity. For instance, monoubiquitylation is involved in endocytosis, DNA repair, gene transcription, and signal transduction pathways. It often acts as a signal for non-degradative processes, distinguishing it from many polyubiquitylation events that target proteins for proteasomal degradation. The precise function of monoubiquitylation is highly dependent on the specific substrate protein and the cellular context.