Ubiquitin–proteasome system

The ubiquitin–proteasome system is a cellular pathway that marks selected proteins with ubiquitin and degrades many of them in the proteasome. It regulates protein quality, abundance, and timing.

Ubiquitination: The covalent attachment of ubiquitin to a protein or other molecule. This tagging reaction marks many substrates for recognition by the proteasome.
Ubiquitin: A small, highly conserved protein that cells attach to other proteins to alter their fate or activity. Its attachment creates the signal that many proteasome substrates carry.
Cell cycle: The ordered sequence of growth and division through which a cell produces two daughter cells. Proteasomal destruction of regulatory proteins helps make cell-cycle transitions directional.
Autophagy: A cellular process that delivers cytoplasmic material to lysosomes for degradation or recycling. Unlike the proteasome, autophagy can engulf large structures and aggregates.
E1 ubiquitin-activating enzyme: An enzyme that uses ATP to activate ubiquitin and begin its transfer to a substrate. E1 enzymes supply activated ubiquitin to the tagging cascade.
Polyubiquitin chain: A chain of ubiquitin molecules linked together through one of ubiquitin’s amino groups. Chain length and linkage type help distinguish degradation signals from other ubiquitin messages.
NF-κB: A family of transcription factors that regulates immune, inflammatory, and survival-related genes. Ubiquitin-dependent degradation of its inhibitor helps activate NF-κB signaling.
Lysosome: A membrane-bound organelle containing enzymes that digest cellular material. Lysosomes provide a distinct degradation compartment for cargo outside the proteasome’s usual scope.
E2 ubiquitin-conjugating enzyme: An enzyme that carries activated ubiquitin and transfers it with an E3 ligase. E2 enzymes help determine how ubiquitin is delivered during substrate tagging.
Lysine: An amino acid whose side chain contains a reactive amino group. Ubiquitin commonly attaches to lysine residues on substrate proteins.

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