Ubiquitin/Ubl E2 Conjugating Enzymes
The ubiquitin (Ub)/Ub-like (Ubl) conjugating enzymes are a conserved family of ligases which mediate the conjugation of Ub and Ubl proteins to target substrates. There have been 35 E2 enzymes identified in the human genome to date.
The ubiquitin (Ub)/Ub-like (Ubl) conjugating enzymes are a conserved family of ligases which mediate the conjugation of Ub and Ubl proteins to target substrates. There have been 35 E2 enzymes identified in the human genome to date, all of which are characterized by a highly conserved catalytic core Ub-conjugating (UBC) domain. The UBC domain contains an active-site cysteine (Cys) residue essential for E2 enzyme catalytic activity. E2 enzymes are classified based on N- and C-terminal extensions to this catalytic core, which determine the functional differences between the ligases. Class I E2 enzymes consist of the UBC catalytic core, whereas class II and III enzymes have additional C-or N-terminal extensions respectively and class IV enzymes have both.
E2 enzymes mediate Ub/Ubl conjugation through interactions with Ub-activating enzymes (E1s) and Ub ligases (E3s). Ub/Ubl is activated by E1 enzymes before forming a thioester linkage with the Cys residue in the E2 enzyme's UBC domain. E2 enzymes can then act independently or in conjunction with E3 ligases to catalyze the attachment of Ub/Ubl to acceptor lysine (Lys) residues of target substrates through the formation of isopeptide bonds. Whilst E3 enzymes usually determine substrate selection, E2 enzymes are vital for determining the topology and length of Ub/Ubl chains, which are linked by Ub/Ubl Lys residues. The Lys residues used for chain formation determine the fate and function of the ubiquitinated protein. For example, proteins modified with Lys48 and Lys11-linked Ub chains are targeted to the 26S proteasome for degradation, whilst Lys63-linked chains mediate the recruitment of binding partners and NF-κB.
Whilst there are many E2 enzymes capable of mediating ubiquitination (e.g. UBE2D1, UBE2L3 and UBE2N), only a few are specific for the conjugation of Ubls such as SUMO proteins and NEDD8. The conjugation of SUMO and NEDD8 to target proteins are termed SUMOylation and neddylation respectively. The E2 enzyme UBE2I is exclusive for the mechanism of SUMOylation, whilst UBE2M and UBE2F mediate neddylation. Other UBLs include ISG15 and Ufm1, which are ligated to target substrates by ISGylation and ufmylation respectively. ISGylation is mediated by UbcH8, whilst Ufc1 is involved in the mechanism of ufmylation. Furthermore, Apg3 and Apg10, which are heterologous in sequence to the classical E2 enzymes, are involved in autophagy.
The dysfunction of the Ub-proteasome system has been implicated in the pathology of many forms of cancer. Studies inhibiting the E2 enzyme UBE2N demonstrated the involvement of the enzyme in cancer progression and survival. Inhibition of UBE2N resulted in the inhibition of the NFκB signaling pathway, suppressing the proliferation and survival of diffuse large B-cell lymphoma cells. UBE2N inhibition also induced p53 transcriptional activity and tumor suppressor function in chemo-resistant neuroblastoma cell lines, thus impeding cell growth and inducing apoptosis. Therefore, E2 enzymes may represent novel therapeutic targets for the treatment of malignant tumors.