A small protein (consisting of 76 amino acid residues), found universally in eukaryotes, that tags proteins destined for degradation by proteasomes or alters the protein’s function, location, or trafficking. It forms a covalent bond with lysine residues in an ATP-dependent reaction termed ubiquitination or ubiquitinylation. Any of the seven lysine residues of a ubiquitin molecule can also form bonds with other ubiquitin molecules, giving rise to chains. The particular lysine residues involved, and the polyubiquitin chain length, are crucial in determining whether ubiquitination tags proteins for delivery to proteasomes or signals other modifications to the protein that affect its properties or fate in the cell. 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. See also sumo.

https://www.rcsb.org/structure/1UBQ Explore the structure and function of ubiquitin on the Protein Data Bank website