The ubiquitin-like modifier FAT10 stimulates the activity of deubiquitylating enzyme OTUB1.

The deubiquitylation of target proteins is mediated by deubiquitylating enzymes (DUB) such as OTUB1, which plays an important role in immune response, cell cycle progression, and DNA repair. Within these processes, OTUB1 reduces the ubiquitylation of target proteins in two distinct ways, either by using its catalytic DUB activity or in a noncatalytic manner by inhibiting the E2-conjugating enzyme. Here, we show that the ubiquitin-like modifier FAT10 regulates OTUB1 stability and functionality in different ways. Covalent FAT10ylation of OTUB1 resulted in its proteasomal degradation, whereas a noncovalent interaction stabilized OTUB1. We provide evidence that OTUB1 interacts directly with FAT10 and the E2-conjugating enzyme USE1. This interaction strongly stimulated OTUB1 DUB activity toward Lys-48-linked diubiquitin. Furthermore, the noncovalent interaction between FAT10 and OTUB1 not only enhanced its isopeptidase activity toward Lys-48-linked ubiquitin moieties but also strengthened its noncatalytic activity in reducing Lys-63 polyubiquitylation of its target protein TRAF3 (TNF receptor-associated factor 3). Additionally, the cellular clearance of overall polyubiquitylation by OTUB1 was strongly stimulated through the presence of FAT10. The addition of FAT10 also led to an increased interaction between OTUB1 and its cognate E2 UbcH5B, implying a function of FAT10 in the inhibition of polyubiquitylation. Overall, these data indicate that FAT10 not only plays a role in covalent modification, leading its substrates to proteasomal degradation, but also regulates the stability and functionality of target proteins by interacting in a noncovalent manner. FAT10 is thereby able to exert a major influence on ubiquitylation processes.