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ubiquitin A small protein (consisting of 76 amino acid residues), found universally in prokaryotes and eukaryotes, that tags proteins destined for degradation by proteasomes. It forms a covalent bond with lysine residues in an ATP-dependent reaction termed ubiquitination. Ubiquitin is important in both the normal life of the cell and in a cell's response to stress; it is considered to be a heat-shock protein.