Yogesh Kulathu

Sixteen ovarian tumor (OTU) family deubiquitinases (DUBs) exist in humans, and most members regulate cell-signaling cascades. Several OTU DUBs were reported to be ubiquitin (Ub) chain linkage specific, but comprehensive analyses are missing, and the underlying mechanisms of linkage specificity are unclear. Using Ub chains of all eight linkage types, we reveal that most human OTU enzymes are linkage specific, preferring one, two, or a defined subset of linkage types, including unstudied atypical Ub chains. Biochemical analysis and five crystal structures of OTU DUBs with or without Ub substrates reveal four mechanisms of linkage specificity. Additional Ub-binding domains, the ubiquitinated sequence in the substrate, and defined S1' and S2 Ub-binding sites on the OTU domain enable OTU DUBs to distinguish linkage types. We introduce Ub chain restriction analysis, in which OTU DUBs are used as restriction enzymes to reveal linkage type and the relative abundance of Ub chains on substrates.

The linear ubiquitin (Ub) chain assembly complex (LUBAC) is an E3 ligase that specifically assembles Met1-linked (also known as linear) Ub chains that regulate nuclear factor κB (NF-κB) signaling. Deubiquitinases (DUBs) are key regulators of Ub signaling, but a dedicated DUB for Met1 linkages has not been identified. Here, we reveal a previously unannotated human DUB, OTULIN (also known as FAM105B), which is exquisitely specific for Met1 linkages. Crystal structures of the OTULIN catalytic domain in complex with diubiquitin reveal Met1-specific Ub-binding sites and a mechanism of substrate-assisted catalysis in which the proximal Ub activates the catalytic triad of the protease. Mutation of Ub Glu16 inhibits OTULIN activity by reducing kcat 240-fold. OTULIN overexpression or knockdown affects NF-κB responses to LUBAC, TNFα, and poly(I:C) and sensitizes cells to TNFα-induced cell death. We show that OTULIN binds LUBAC and that overexpression of OTULIN prevents TNFα-induced NEMO association with ubiquitinated RIPK1. Our data suggest that OTULIN regulates Met1-polyUb signaling.

Deubiquitinating enzymes (DUBs) remove ubiquitin (Ub) from Ub-conjugated substrates to regulate the functional outcome of ubiquitylation. Here we report the discovery of a new family of DUBs, which we have named MINDY (motif interacting with Ub-containing novel DUB family). Found in all eukaryotes, MINDY-family DUBs are highly selective at cleaving K48-linked polyUb, a signal that targets proteins for degradation. We identify the catalytic activity to be encoded within a previously unannotated domain, the crystal structure of which reveals a distinct protein fold with no homology to any of the known DUBs. The crystal structure of MINDY-1 (also known as FAM63A) in complex with propargylated Ub reveals conformational changes that realign the active site for catalysis. MINDY-1 prefers cleaving long polyUb chains and works by trimming chains from the distal end. Collectively, our results reveal a new family of DUBs that may have specialized roles in regulating proteostasis.

Active-site directed probes are powerful in studies of enzymatic function. We report an active-site directed probe based on a warhead so far considered unreactive. By replacing the C-terminal carboxylate of ubiquitin (Ub) with an alkyne functionality, a selective reaction with the active-site cysteine residue of de-ubiquitinating enzymes was observed. The resulting product was shown to be a quaternary vinyl thioether, as determined by X-ray crystallography. Proteomic analysis of proteins bound to an immobilized Ub alkyne probe confirmed the selectivity toward de-ubiquitinating enzymes. The observed reactivity is not just restricted to propargylated Ub, as highlighted by the selective reaction between caspase-1 (interleukin converting enzyme) and a propargylated peptide derived from IL-1β, a caspase-1 substrate.