Identification and Characterization of a New E1 that Activates Ubiquitin and FAT10

Ubiquitination is one of many post-translational modifications in eukaryotes. Three enzymes (E1, E2, and E3) are involved in conjugating ubiquitin to protein substrates. I identified a novel E1-like protein, E1-L2, which is homologous to the ubiquitin E1 and another E1 involved in the activation of the ubiquitin-like protein ISG-15 (E1-L1). E1-L2 activates both ubiquitin and FAT10, a ubiquitin-like protein. Interestingly, E1-L2 can transfer ubiquitin to only a subset of E2 enzymes, Ubc5 and Ubc13, but not Ubc3 and E2-25K, suggesting that E1-L2 may function in certain ubiquitination reactions. E1-L2, but not E1 or E1-L1, forms a thioester with FAT10 in vitro. The formation of the thioester bond requires the active site cysteine residue of E1-L2 and the C-terminal diglycine motif of FAT10. In addition, endogenous FAT10 forms a thioester with E1-L2 in cells stimulated with tumor necrosis factor-alpha and interferon-gamma, which induce FAT10 expression. Silencing of E1-L2 expression by RNAi impaired the formation of FAT10 conjugates in cells. Furthermore, E1-L2 deficient embryos died before embryonic day 13.5, suggesting that E1-L2 is essential for early embryonic development. Since the FAT10-deficient mice develop normally, it is likely that specific ubiquitination reactions catalyzed by E1-L2 play an important role in animal development.