MONylation

Monylation is a post-translational modification where a single ubiquitin molecule is attached to a substrate protein. Unlike polyubiquitination, which involves the formation of ubiquitin chains, monoubiquitination adds only one ubiquitin. This modification is reversible, meaning ubiquitin can be removed from the substrate protein by deubiquitinating enzymes. The process of monoubiquitination is carried out by a cascade of enzymes known as the ubiquitin conjugation system, which involves an E1 activating enzyme, an E2 conjugating enzyme, and an E3 ubiquitin ligase. The E3 ligase plays a crucial role in conferring substrate specificity.

Monoubiquitination can occur on lysine residues, N-terminal amino groups, or even cysteine residues within a protein. The specific site of monoubiquitination can significantly influence the protein's function, localization, and interactions. This modification is involved in a wide array of cellular processes. It is particularly important in regulating protein trafficking and endocytosis, where it can target proteins to specific cellular compartments or mark them for degradation. Furthermore, monoubiquitination plays roles in DNA repair, transcriptional regulation, and signal transduction pathways. The functional outcome of monoubiquitination is diverse and context-dependent, highlighting its versatility as a regulatory mechanism in cells.