Ubiquitin

Ubiquitin is a small, highly conserved protein that cells attach to other proteins to regulate their degradation, location, and activity. Its effects depend on how ubiquitin molecules are attached and linked.

Ubiquitination: A post-translational modification in which ubiquitin is covalently attached to a protein or another molecule. This is the broader process that uses ubiquitin as a regulatory tag.
Ubiquitin–proteasome system: A cellular pathway that marks many proteins with ubiquitin and degrades them in the proteasome. It is the best-known route by which ubiquitin directs protein destruction.
Ubiquitin fold: A compact protein architecture consisting of a beta-grasp arrangement of beta sheets and an alpha helix. This fold gives ubiquitin a stable surface for recognition by many binding partners.
DNA damage response: A network of cellular pathways that detects DNA lesions, coordinates repair, and controls cell-cycle progression. Ubiquitin modifications organize proteins at DNA lesions and regulate repair factors.
SUMO: A ubiquitin-like protein that is covalently attached to other proteins, often altering their interactions or localization. SUMO is another reversible protein tag, but it usually changes protein behavior rather than directing proteasomal degradation.
E1 ubiquitin-activating enzyme: An enzyme that activates ubiquitin using ATP and transfers it to an E2 conjugating enzyme. E1 enzymes initiate the cascade that attaches ubiquitin to target proteins.
26S proteasome: A large ATP-dependent protease complex that recognizes ubiquitinated proteins and degrades them into peptides. It receives many ubiquitin-tagged substrates for degradation.
Lysine: An amino acid with a positively charged side chain that can form isopeptide bonds with ubiquitin. Ubiquitin chains often link through lysine residues.
NF-κB: A family of transcription factors that controls immune, inflammatory, and cell-survival gene expression. Ubiquitin signaling helps activate and restrain this major transcriptional pathway.
ISG15: An interferon-induced ubiquitin-like protein that is conjugated to cellular proteins during immune responses. ISG15 resembles ubiquitin chemically but is especially associated with antiviral signaling.

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