Ubiquitin

Ubiquitin is a highly conserved small regulatory protein that is ubiquitous in eukaryotes. Ubiquitination (or Ubiquitylation) refers to the post-translational modification of a protein by the covalent attachment (via an isopeptide bond) of one or more ubiquitin monomers. The most prominent function of Ubiquitin is labeling proteins for proteasomal degradation (see: Proteasome). Beside this function ubiquitination controls the stability, function, and intracellular localization of a wide variety of proteins.
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U·biq·ui·tin n. A polypeptide found in all eukaryotic cells, including plant cells, that participates in a variety of cellular functions including [more]Ubiquitin - Community and Resources

A small protein that becomes covalently linked to a protein targeted for degradation.

UBIQUITINA