Monoubiquitination is a reversible post-translational protein modification that has an important regulatory function in many biological processes, including DNA repair. Deubiquitinating enzymes (DUBs) are proteases that are negative regulators of monoubiquitination, but little is known about their regulation and contribution to the control of conjugated-substrate levels. Here, we show that the DUB ubiquitin specific protease 1 (USP1) deubiquitinates the DNA replication processivity factor, PCNA, as a safeguard against error-prone translesion synthesis (TLS) of DNA. Ultraviolet (UV) irradiation inactivates USP1 through an autocleavage event, thus enabling monoubiquitinated PCNA to accumulate and to activate TLS. Significantly, the site of USP1 cleavage is immediately after a conserved internal ubiquitin-like diglycine (Gly-Gly) motif. This mechanism is reminiscent of the processing of precursors of ubiquitin and ubiquitin-like modifiers by DUBs. Our results define a regulatory mechanism for protein ubiquitination that involves the signal-induced degradation of an inhibitory DUB.Monoubiquitination is a highly regulated process that is conserved in all eukaryotes 1,2 and controls a broad range of cellular functions, including DNA repair. Protein monoubiquitination is a reversible post-translational event that can be influenced by the opposing activities of a ubiquitin E3 ligase and a deubiquitinating enzyme (DUB), similar to the regulation of protein phosphorylation by kinases and phosphatases 3,4 . Protein monoubiquitination regulates the rescue of stalled DNA replication forks -an important cellular process required for cell survival 5 . The E2 ubiquitin conjugating enzyme RAD6 and the E3 ligase RAD18 are conserved in both yeast and human and they coordinate and activate the monoubiquitination of PCNA in response to UV damage or stalled replication forks [6][7][8] . Recent studies have shown that polη, a specialized TLS polymerase, is recruited to the replication fork through a specific interaction with monoubiquitinated PCNA 7 . The deployment of TLS polymerases during replication ensures timely bypass of the diverse DNA lesions encountered by the replication fork. Although many TLS polymerases are intrinsically mutagenic, polη allows replication past UV-damaged bases with high fidelity 9-11 . Thus, a model has emerged in which polη binds to monoubiquitinated PCNA and ensures accurate (error-free) replicative bypass of UV lesions. However, other (error-prone) TLS polymerases (such as polι and Rev1) have recently been shown to rely on monoubiquitinated PCNA for their function 12,13 . How cells limit PCNA monoubiquitination and the unwanted deployment of polη and/or other error-prone TLS polymerases in the absence or presence of extrinsic DNA damage during the synthesis of DNA in S phase is not known.In humans, protein deubiquitination is controlled by a family of approximately 95 distinct DUB enzymes 14,15 but the function of most of these proteins is unknown. DUBs are cysteine proteases that cleave ubiquitin fro...

The transcription factor NF-kappaB is critical for setting the cellular sensitivities to apoptotic stimuli, including DNA damaging anticancer agents. Central to NF-kappaB signaling pathways is NEMO/IKKgamma, the regulatory subunit of the cytoplasmic IkappaB kinase (IKK) complex. While NF-kappaB activation by genotoxic stress provides an attractive paradigm for nuclear-to-cytoplasmic signaling pathways, the mechanism by which nuclear DNA damage modulates NEMO to activate cytoplasmic IKK remains unknown. Here, we show that genotoxic stress causes nuclear localization of IKK-unbound NEMO via site-specific SUMO-1 attachment. Surprisingly, this sumoylation step is ATM-independent, but nuclear localization allows subsequent ATM-dependent ubiquitylation of NEMO to ultimately activate IKK in the cytoplasm. Thus, genotoxic stress induces two independent signaling pathways, SUMO-1 modification and ATM activation, which work in concert to sequentially cause nuclear targeting and ubiquitylation of free NEMO to permit the NF-kappaB survival pathway. These SUMO and ubiquitin modification pathways may serve as anticancer drug targets.

Protein ubiquitination and deubiquitination are dynamic processes implicated in the regulation of numerous cellular pathways. Monoubiquitination of the Fanconi anemia (FA) protein FANCD2 appears to be critical in the repair of DNA damage because many of the proteins that are mutated in FA are required for FANCD2 ubiquitination. By screening a gene family RNAi library, we identify the deubiquitinating enzyme USP1 as a novel component of the Fanconi anemia pathway. Inhibition of USP1 leads to hyperaccumulation of monoubiquitinated FANCD2. Furthermore, USP1 physically associates with FANCD2, and the proteins colocalize in chromatin after DNA damage. Finally, analysis of crosslinker-induced chromosomal aberrations in USP1 knockdown cells suggests a role in DNA repair. We propose that USP1 deubiquitinates FANCD2 when cells exit S phase or recommence cycling after a DNA damage insult and may play a critical role in the FA pathway by recycling FANCD2.