Figure 1. Principles of ubiquitin signalling.

Ubiquitylation is mediated by the sequential activity of a set of enzymes including activating (E1), conjugating (E2) and ligating (E3) enzymes. This leads to the conjugation of monoubiquitin or polyubiquitin chains of different lengths and link ages to target proteins. Depending on the differents types of ubiquitin chains, proteins are subsequently degraded via the proteasome or participate in various cell ularfunctions including signalling, DNA repair or endocytosis. See text for more details.