Tools to investigate the ubiquitin proteasome system.

Ubiquitin is a 76-amino acid regulatory protein involved in many important cellular processes. Ubiquitin can be attached to other proteins at either a lysine residue or to the N-terminus by the consecutive actions of E1, E2, and E3 enzymes. Ubiquitin can also be attached to itself, resulting in poly-ubiquitin chains. Ubiquitination affects substrate proteins in different ways, for example by resulting in degradation of the substrate protein by the 26S proteasome. Ubiquitination can be reversed by deubiquitinating enzymes, which either trim or remove ubiquitin chains from proteins. Many proteins involved in either the ubiquitination, deubiquitination or degradation of proteins are implicated in human diseases and are currently under investigation as potential drug targets.