SummaryDeubiquitinating enzymes (DUBs) are important regulators of ubiquitin signaling. Here, we report the discovery of deubiquitinating activity in ZUFSP/C6orf113. High-resolution crystal structures of ZUFSP in complex with ubiquitin reveal several distinctive features of ubiquitin recognition and catalysis. Our analyses reveal that ZUFSP is a novel DUB with no homology to any known DUBs, leading us to classify ZUFSP as the seventh DUB family. Intriguingly, the minimal catalytic domain does not cleave polyubiquitin. We identify two ubiquitin binding domains in ZUFSP: a ZHA (ZUFSP helical arm) that binds to the distal ubiquitin and an atypical UBZ domain in ZUFSP that binds to polyubiquitin. Importantly, both domains are essential for ZUFSP to selectively cleave K63-linked polyubiquitin. We show that ZUFSP localizes to DNA lesions, where it plays an important role in genome stability pathways, functioning to prevent spontaneous DNA damage and also promote cellular survival in response to exogenous DNA damage.

Branched ubiquitin (Ub) chains make up a significant proportion of Ub polymers in human cells and are formed when two or more sites on a single Ub molecule are modified with Ub creating bifurcated architectures. Despite their abundance, we have a poor understanding of the cellular functions of branched Ub signals that stems from a lack of facile tools and methods to study them. Here we develop a comprehensive pipeline to define branched Ub function, using K48-K63-branched chains as a case study. We discover branch-specific binders and, by developing a method that monitors cleavage of linkages within complex polyUb, we discover the VCP/p97-associated ATXN3, and MINDY family deubiquitinases to act as debranching enzymes. By engineering and utilizing a branched K48-K63-Ub chain-specific nanobody, we reveal roles for these chains in VCP/p97-related processes. In summary, we provide a blueprint to investigate branched Ub function that can be readily applied to study other branched chain types.