Monoubiquitinated

Monoubiquitinated refers to a substrate protein that has a single ubiquitin molecule covalently attached to one lysine residue. This attachment is carried out through an isopeptide bond between the ubiquitin C-terminus and the ε-amino group of the lysine on the substrate. Monoubiquitination can also occur on protein N-termini in some cases.

In contrast to polyubiquitination, which forms chains of ubiquitin molecules, monoubiquitination involves only one ubiquitin and often alters the substrate’s activity, interactions, or localization rather than marking it for degradation. Polyubiquitin chains, especially those linked through Lys48, commonly target proteins for proteasomal destruction, while other linkages can signal signaling, trafficking, or DNA repair pathways. Monoubiquitination is typically a regulatory rather than a degradative signal.

Biological roles of monoubiquitination are diverse. It participates in DNA damage tolerance, exemplified by PCNA monoubiquitination by Rad6/Rad18, which promotes translesion synthesis. It also regulates endocytosis and membrane trafficking, influencing receptor turnover and signaling. In chromatin biology, monoubiquitination of histones affects transcriptional regulation and chromatin dynamics; for instance, H2B monoubiquitination by RNF20/40 and H2A monoubiquitination by a Polycomb complex modulate gene expression and DNA damage responses.

The modification is dynamic and reversible. E1 activating enzymes, E2 conjugating enzymes, and E3 ligases catalyze the attachment, while deubiquitinases remove ubiquitin. Specific E3 ligases and DUBs determine the substrate scope and cellular context of monoubiquitination.

Detection and study rely on mass spectrometry, ubiquitin remnant antibodies, and functional assays to understand its regulatory roles and implications in health and disease. Dysregulation of monoubiquitination has been linked to cancer and neurological disorders, highlighting its importance as a non-degradative signaling mechanism.