The process of the ubiquitin proteasome system (UPS). 1) Ubiquitin (Ub) is activated by E1 activating enzymes in an ATP-dependent manner, then transferred to E2 conjugating enzymes. E2 conjugating enzymes can recruit E3 ligases enzymes with target substrates. E3 ligase enzymes directly catalyze transfer of activated ubiquitin from E2 conjugating enzymes to substrates, leading to the formation of polyubiquitin chains on target substrates. This process is called ubiquitination. Lys48- and Lys11-linked polyubiquitin chains usually induce degradation of target substrate through proteasome activation. Conversely, Lys63-linked polyubiquitin chains regulate cellular signaling and trafficking. 2) Ubiquitination of target substrate is reversed by deubiquitinases (DUBs). DUBs are critical roles for regulating the function of ubiquitinated proteins by removal of polyubiquitin chains. This process is called deubiquitination. In addition, ubiquitin released from substrates by DUBs can be recycled for activation of ubiquitination.