The E3 ubiquitin (Ub) ligase RNF168 plays a critical role in the initiation of the DNA damage response to double-strand breaks (DSBs). The recruitment of RNF168 by ubiquitylated targets involves two distinct regions, Ub-dependent DSB recruitment module (UDM) 1 and UDM2. Here we report the crystal structures of the complex between UDM1 and Lys63-linked diUb (K63-Ub2) and that between the C-terminally truncated UDM2 (UDM2ΔC) and K63-Ub2. In both structures, UDM1 and UDM2ΔC fold as a single α-helix. Their simultaneous bindings to the distal and proximal Ub moieties provide specificity for Lys63-linked Ub chains. Structural and biochemical analyses of UDM1 elucidate an Ub-binding mechanism between UDM1 and polyubiquitylated targets. Mutations of Ub-interacting residues in UDM2 prevent the accumulation of RNF168 to DSB sites in U2OS cells, whereas those in UDM1 have little effect, suggesting that the interaction of UDM2 with ubiquitylated and polyubiquitylated targets mainly contributes to the RNF168 recruitment.

RAP80 has a key role in the recruitment of the Abraxas-BRCC36-BRCA1-BARD1 complex to DNA-damage foci for DNA repair through specific recognition of Lys 63-linked polyubiquitinated proteins by its tandem ubiquitin-interacting motifs (UIMs). Here, we report the crystal structure of the RAP80 tandem UIMs (RAP80-UIM1-UIM2) in complex with Lys 63-linked di-ubiquitin at 2.2 A resolution. The two UIMs, UIM1 and UIM2, and the alpha-helical inter-UIM region together form a continuous 60 A-long alpha-helix. UIM1 and UIM2 bind to the proximal and distal ubiquitin moieties, respectively. Both UIM1 and UIM2 of RAP80 recognize an Ile 44-centered hydrophobic patch on ubiquitin but neither UIM interacts with the Lys 63-linked isopeptide bond. Our structure suggests that the inter-UIM region forms a 12 A-long alpha-helix that ensures that the UIMs are arranged to enable specific binding of Lys 63-linked di-ubiquitin. This was confirmed by pull-down analyses using RAP80-UIM1-UIM2 mutants of various length inter-UIM regions. Further, we show that the Epsin1 tandem UIM, which has an inter-UIM region similar to that of RAP80-UIM1-UIM2, also selectively binds Lys 63-linked di-ubiquitin.

TAB2 and TAB3 activate the Jun N-terminal kinase and nuclear factor-kappaB pathways through the specific recognition of Lys 63-linked polyubiquitin chains by its Npl4 zinc-finger (NZF) domain. Here we report crystal structures of the TAB2 and TAB3 NZF domains in complex with Lys 63-linked diubiquitin at 1.18 and 1.40 A resolutions, respectively. Both NZF domains bind to the distal ubiquitin through a conserved Thr-Phe dipeptide that has been shown to be important for the interaction of the NZF domain of Npl4 with monoubiquitin. In contrast, a surface specific to TAB2 and TAB3 binds the proximal ubiquitin. Both the distal and proximal binding sites of the TAB2 and TAB3 NZF domains recognize the Ile 44-centred hydrophobic patch on ubiquitin but do not interact with the Lys 63-linked isopeptide bond. Mutagenesis experiments show that both binding sites are required to enable binding of Lys 63-linked diubiquitin. We therefore propose a mechanism for the recognition of Lys 63-linked polyubiquitin chains by TAB2 and TAB3 NZF domains in which diubiquitin units are specifically recognized by a single NZF domain.

Npl4 is likely to be the most upstream factor recognizing Lys48-linked polyubiquitylated substrates in the proteasomal degradation pathway in yeast. Along with Ufd1, Npl4 forms a heterodimer (UN), and functions as a cofactor for the Cdc48 ATPase. Here, we report the crystal structures of yeast Npl4 in complex with Lys48-linked diubiquitin and with the Npl4-binding motif of Ufd1. The distal and proximal ubiquitin moieties of Lys48-linked diubiquitin primarily interact with the C-terminal helix and N-terminal loop of the Npl4 C-terminal domain (CTD), respectively. Mutational analysis suggests that the CTD contributes to linkage selectivity and initial binding of ubiquitin chains. Ufd1 occupies a hydrophobic groove of the Mpr1/Pad1 N-terminal (MPN) domain of Npl4, which corresponds to the catalytic groove of the MPN domain of JAB1/MPN/Mov34 metalloenzyme (JAMM)-family deubiquitylating enzyme. This study provides important structural insights into the polyubiquitin chain recognition by the Cdc48-UN complex and its assembly.