Proteasome

The proteasome is a large protein complex that breaks down proteins, especially those marked with ubiquitin. This controlled degradation helps regulate protein quality, abundance, and cellular processes.

Ubiquitin: A small protein attached to other proteins to alter their fate, location, or activity. Ubiquitin chains commonly mark proteins for recognition by the proteasome.
Polyubiquitination: The attachment of a chain of ubiquitin molecules to a protein. Certain ubiquitin chains provide recognition signals for proteasomal degradation.
Antigen processing: The generation and presentation of peptide fragments that enable immune recognition of intracellular proteins. Proteasomal peptides supply many fragments presented by major histocompatibility complex class I.
Cell cycle: The ordered series of events through which a cell grows, duplicates its DNA, and divides. Timed destruction of regulatory proteins helps drive transitions between cell-cycle stages.
Lysosome: An acidic organelle containing enzymes that digest cellular material. Lysosomes handle bulk and membrane-associated cargo that is not directly fed into the proteasome.
26S proteasome: The ATP-dependent proteasome complex formed by a 20S core particle and 19S regulatory particles. This is the principal complex that recognizes ubiquitin tags and degrades targeted proteins.
Ubiquitin ligase: An enzyme that helps transfer ubiquitin onto selected substrate proteins. Ligases help determine which proteins receive degradation-linked ubiquitin marks.
Immunoproteasome: A proteasome variant with inducible catalytic subunits that changes peptide production during immune responses. Its altered cleavage preferences help shape peptides available for antigen presentation.
Cyclin: A regulatory protein whose changing abundance activates cyclin-dependent kinases during the cell cycle. Proteasomal destruction of cyclins helps make cell-cycle transitions directional.
Calpain: A family of calcium-dependent cysteine proteases that cleave proteins in regulated cellular processes. Calpains cut selected proteins rather than generally digesting ubiquitin-tagged substrates.

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