OTU deubiquitinases reveal mechanisms of linkage specificity and enable ubiquitin chain restriction analysis.

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.

Structural and functional characterization of a cell cycle associated HDAC1/2 complex reveals the structural basis for complex assembly and nucleosome targeting.

Recent proteomic studies have identified a novel histone deacetylase complex that is upregulated during mitosis and is associated with cyclin A. This complex is conserved from nematodes to man and contains histone deacetylases 1 and 2, the MIDEAS corepressor protein and a protein called DNTTIP1 whose function was hitherto poorly understood. Here, we report the structures of two domains from DNTTIP1. The amino-terminal region forms a tight dimerization domain with a novel structural fold that interacts with and mediates assembly of the HDAC1:MIDEAS complex. The carboxy-terminal domain of DNTTIP1 has a structure related to the SKI/SNO/DAC domain, despite lacking obvious sequence homology. We show that this domain in DNTTIP1 mediates interaction with both DNA and nucleosomes. Thus, DNTTIP1 acts as a dimeric chromatin binding module in the HDAC1:MIDEAS corepressor complex.

RNF168, a new RING finger, MIU-containing protein that modifies chromatin by ubiquitination of histones H2A and H2AX.

BACKGROUND: Modulation of chromatin structure has emerged as a critical molecular device to control gene expression. Histones undergo different post-translational modifications that increase chromatin accessibility to a number of regulatory factors. Among them, histone ubiquitination appears relevant in nuclear processes that govern gene silencing, either by inhibiting or activating transcription, and maintain genome stability, acting as scaffold to properly organize the DNA damage response. Thus, it is of paramount importance the identification and the characterization of new ubiquitin ligases that address histones. RESULTS: We identified and characterized RNF168, a new chromatin-associated RING finger protein. We demonstrated that RNF168 is endowed with ubiquitin ligase activity both in vitro and in vivo, which targets histones H2A and H2AX, but not H2B, forming K63 polyubiquitin chains. We previously described the presence within RNF168 sequence of two MIU domains, responsible for the binding to ubiquitinated proteins. Here we showed that inactivation of the MIUs impairs ubiquitin binding ability in vitro and reduces chromatin association of RNF168 in vivo. Moreover, upon formation of DNA double strand breaks induced by chemical and physical agents, RNF168 is recruited to the DNA damage foci, where it co-localizes with gammaH2AX and 53BP1. The localization of RNF168 at the site of damage highly increases the local concentration of ubiquitinated proteins and determines the prolonged ubiquitination signal. CONCLUSION: The RING finger protein RNF168 is a new ubiquitin ligase that functions as chromatin modifier, through histone ubiquitination. We hypothesize a dual function for RNF168. In normal condition RNF168 modifies chromatin structure by modulating ubiquitination of histone H2A. Upon DNA lesions, RNF168 is recruited to DNA damage response foci where it contributes to increase the amount of ubiquitinated proteins, thereby facilitating the downstream signalling cascade.

Regulation of A20 and other OTU deubiquitinases by reversible oxidation.

Protein ubiquitination is a highly versatile post-translational modification that regulates as diverse processes as protein degradation and kinase activation. Deubiquitinases hydrolyse ubiquitin modifications from proteins and are hence key regulators of the ubiquitin system. Ovarian tumour deubiquitinases comprise a family of fourteen human enzymes, many of which regulate cellular signalling pathways. Ovarian tumour deubiquitinases are cysteine proteases that cleave polyubiquitin chains in vitro and in cells, but little is currently known about their regulation. Here we show that ovarian tumour deubiquitinases are susceptible to reversible oxidation of the catalytic cysteine residue. High-resolution crystal structures of the catalytic domain of A20 in four different oxidation states reveal that the reversible form of A20 oxidation is a cysteine sulphenic acid intermediate, which is stabilised by the architecture of the catalytic centre. Using chemical tools to detect sulphenic acid intermediates, we show that many ovarian tumour deubiquitinases undergo reversible oxidation upon treatment with H2O2, revealing a new mechanism to regulate deubiquitinase activity.

The N-terminal acetylation of Sir3 stabilizes its binding to the nucleosome core particle.

The N-terminal acetylation of Sir3 is essential for heterochromatin establishment and maintenance in yeast, but its mechanism of action is unknown. The crystal structure of the N-terminally acetylated BAH domain of Saccharomyces cerevisiae Sir3 bound to the nucleosome core particle reveals that the N-terminal acetylation stabilizes the interaction of Sir3 with the nucleosome. Additionally, we present a new method for the production of protein-nucleosome complexes for structural analysis.