USP7 is a deubiquitinating enzyme that plays a pivotal role in multiple oncogenic pathways and therefore is a desirable target for new anti-cancer therapies. However, the lack of structural information about the USP7-inhibitor interactions has been a critical gap in the development of potent inhibitors. USP7 is unique among USPs in that its active site is catalytically incompetent, and is postulated to rearrange into a productive conformation only upon binding to ubiquitin. Surprisingly, we found that ubiquitin alone does not induce an active conformation in solution. Using a combination of nuclear magnetic resonance, mass spectrometry, computational modeling, and cell-based assays, we found that DUB inhibitors P22077 and P50429 covalently modify the catalytic cysteine of USP7 and induce a conformational switch in the enzyme associated with active site rearrangement. This work represents the first experimental insights into USP7 activation and inhibition and provides a structural basis for rational development of potent anti-cancer therapeutics.

Genetic alterations, including DNA mutations and chromosomal abnormalities, are primary drivers of tumor formation and cancer progression. These alterations can endow cells with a selective growth advantage, enabling cancers to evade cell death, proliferation limits, and immune checkpoints, to metastasize throughout the body. Genetic alterations occur due to failures of the genome stability pathways. In many cancers, the rate of alteration is further accelerated by the deregulation of these processes. The deubiquitinating enzyme ubiquitin specific protease 7 (USP7) has recently emerged as a key regulator of ubiquitination in the genome stability pathways. USP7 is also deregulated in many cancer types, where deviances in USP7 protein levels are correlated with cancer progression. In this work, we review the increasingly evident role of USP7 in maintaining genome stability, the links between USP7 deregulation and cancer progression, as well as the rationale of targeting USP7 in cancer therapy.

Reversible protein ubiquitination is an essential signaling mechanism within eukaryotes. Deubiquitinating enzymes are critical to this process, as they mediate removal of ubiquitin from substrate proteins. Ubiquitin-specific protease 7 (USP7) is a prominent deubiquitinating enzyme, with an extensive network of interacting partners and established roles in cell cycle activation, immune responses and DNA replication. Characterized USP7 substrates primarily interact with one of two major binding sites outside the catalytic domain. These are located on the USP7 N-terminal TRAF-like (TRAF) domain and the first and second UBL domains (UBL1-2) within the C-terminal tail. Here, we report that DNA polymerase iota (Pol ι) is a novel USP7 substrate that interacts with both TRAF and UBL1-2. Through the use of biophysical approaches and mutational analysis, we characterize both interfaces and demonstrate that bipartite binding to both USP7 domains is required for efficient Pol ι deubiquitination. Together, these data establish a new bipartite mode of USP7 substrate binding.

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