Ubiquitin is a small regulatory protein found in most tissues of eukaryotic organisms, i.e. it occurs ubiquitously. Four genes in the human genome code for ubiquitin: UBB, UBC, UBA52 and RPS27A.
The addition of ubiquitin to a substrate protein is called ubiquitination . Ubiquitination affects proteins in many ways: it can mark them for degradation via the proteasome, alter their cellular location, affect their activity, and promote or prevent protein interactions. Ubiquitination involves three main steps: activation, conjugation, and ligation, performed by ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), and ubiquitin ligases (E3s), respectively. The result of this sequential cascade is to bind ubiquitin to lysine residues on the protein substrate via an isopeptide bond, cysteine residues through a thioester bond, serine and threonine residues through an ester bond, or the amino group of the protein's N-terminus via a peptide bond.