Ubiquitin is a small (76-residue), heat-stable protein found in all eukaryotic cells. It derives its name from its widespread (ubiquitous) distribution. Ubiquitin undergoes an ATP-dependent reaction with proteins, which condenses C-terminal glycine residues of ubiquitin with lysine amino groups on the target protein, as shown in Figure 20.11. Such modified proteins are degraded soon afterward, by a large protease complex whose assembly requires more ATP and that recognizes the ubiquitin marker.