The ubiquitin proteasome system. (a) Ubiquitin is activated by a ubiquitin-activating enzyme (E1) and transferred onto substrate proteins by ubiquitin-conjugating enzymes (E2) and ubiquitin ligases (E3), resulting in (b) either attachment of a single ubiquitin molecule (mono-ubiquitination), attachment of multiple ubiquitin units to several substrate lysine residues on the same protein (multi-ubiquitination) or synthesis of ubiquitin chains (poly-ubiquitination). (c) Many poly-ubiquitinated proteins are subsequently degraded by the 26S proteasome, which consists of the catalytic 20S complex and the regulatory 19S particles. Degradation substrates are either delivered to the proteasome by soluble ubiquitin receptors or recognized by the intrinsic ubiquitin-binding activity of the 19S particle. At the 19S proteasome the ubiquitin chain is disassembled, and the substrate is unfolded before it can enter the cavity of the 20S subunit where proteolysis takes place. Finally, proteolytic fragments exit the proteasome in a poorly understood way. (d) Ubiquitination can also directly regulate protein function in a proteolysis-independent manner, via mono-, multi- or poly-ubiquitinated proteins.