Figure 1: Ubiquitination and ubiquitin-mediated trafficking. Ubiquitin (Ub) is activated in an ATP-dependent manner by an E1, passed to an E2 ubiquitin conjugase, and finally transferred to a target protein by an E2/E3 ubiquitin ligase complex. Following monoubiquitination, the addition of further ubiquitin moieties occurs at specific lysine residues and results in one of a variety of polyubiquitin chains, each possessing a unique set of known consequences for protein regulation and trafficking. The ubiquitination state of a protein is regulated both via the addition of ubiquitin and also via the removal of single moieties or chains by deubiquitinases (DUBs).