SUMOylation regulates Lem2 function in centromere clustering and silencing.

Regulation by the small modifier SUMO is heavily dependent on spatial control of enzymes that mediate the attachment and removal of SUMO on substrate proteins. Here, we show that in the fission yeast Schizosaccharomyces pombe, delocalisation of the SUMO protease Ulp1 from the nuclear envelope results in centromeric defects that can be attributed to hyper-SUMOylation at the nuclear periphery. Unexpectedly, we find that although this localised hyper-SUMOylation impairs centromeric silencing, it can also enhance centromere clustering. Moreover, both effects are at least partially dependent on SUMOylation of the inner nuclear membrane protein Lem2. Lem2 has previously been implicated in diverse biological processes, including the promotion of both centromere clustering and silencing, but how these distinct activities are coordinated was unclear; our observations suggest a model whereby SUMOylation serves as a regulatory switch, modulating Lem2 interactions with competing partner proteins to balance its roles in alternative pathways. Our findings also reveal a previously unappreciated role for SUMOylation in promoting centromere clustering.

Insights into the molecular composition of endogenous unanchored polyubiquitin chains.

The diverse influences of ubiquitin, mediated by its post-translational covalent modification of other proteins, have been extensively investigated. However, more recently roles for unanchored (nonsubstrate linked) polyubiquitin chains have also been proposed. Here we describe the use of ubiquitin-binding domains to affinity purify endogenous unanchored polyubiquitin chains and their subsequent characterization by mass spectrometry (MS). Using the A20 Znf domain of the ubiquitin receptor ZNF216 we isolated a protein from skeletal muscle shown by a combination of nanoLC-MS and LC-MS/MS to represent an unmodified and unanchored K48-linked ubiquitin dimer. Selective purification of unanchored polyubiquitin chains using the Znf UBP (BUZ) domain of USP5/isopeptidase-T allowed the isolation of K48 and K11-linked ubiquitin dimers, as well as revealing longer chains containing as many as 15 ubiquitin moieties, which include the K48 linkage. Top-down nanoLC-MS/MS of the A20 Znf-purified ubiquitin dimer generated diagnostic ions consistent with the presence of the K48 linkage, illustrating for the first time the potential of this approach to probe connectivity within endogenous polyubiquitin modifications. As well as providing initial proteomic insights into the molecular composition of endogenous unanchored polyubiquitin chains, this work also represents the first definition of polyubiquitin chain length in vivo.

Probing affinity and ubiquitin linkage selectivity of ubiquitin-binding domains using mass spectrometry.

Non-covalent interactions between ubiquitin (Ub)-modified substrates and Ub-binding domains (UBDs) are fundamental to signal transduction by Ub receptor proteins. Poly-Ub chains, linked through isopeptide bonds between internal Lys residues and the C-terminus of Ub, can be assembled with varied topologies to mediate different cellular processes. We have developed and applied a rapid and sensitive electrospray ionization-mass spectrometry (ESI-MS) method to determine isopeptide linkage-selectivity and affinity of poly-Ub·UBD interactions. We demonstrate the technique using mono-Ub and poly-Ub complexes with a number of α-helical and zinc-finger (ZnF) UBDs from proteins with roles in neurodegenerative diseases and cancer. Affinities in the 2-200 µM range were determined to be in excellent agreement with data derived from other biophysical techniques, where available. Application of the methodology provided further insights into the poly-Ub linkage specificity of the hHR23A-UBA2 domain, confirming its role in Lys48-linked poly-Ub signaling. The ZnF UBP domain of isopeptidase-T showed no linkage specificity for poly-Ub chains, and the Rabex-5 MIU also exhibited little or no specificity. The discovery that a number of domains are able to bind cyclic Lys48 di-Ub with affinities similar to those for the acyclic form indicates that cyclic poly-Ub may be capable of playing a role in Ub-signaling. Detection of a ternary complex involving Ub interacting simultaneously with two different UBDs demonstrated the co-existence of multi-site interactions, opening the way for the study of crosstalk between individual Ub-signaling pathways.

Structural insights into specificity and diversity in mechanisms of ubiquitin recognition by ubiquitin-binding domains.

UBDs [Ub (ubiquitin)-binding domains], which are typically small protein motifs of <50 residues, are used by receptor proteins to transduce post-translational Ub modifications in a wide range of biological processes, including NF-κB (nuclear factor κB) signalling and proteasomal degradation pathways. More than 20 families of UBDs have now been characterized in structural detail and, although many recognize the canonical Ile44/Val70-binding patch on Ub, a smaller number have alternative Ub-recognition sites. The A20 Znf (A20-like zinc finger) of the ZNF216 protein is one of the latter and binds with high affinity to a polar site on Ub centred around Asp58/Gln62. ZNF216 shares some biological function with p62, with both linked to NF-κB signal activation and as shuttle proteins in proteasomal degradation pathways. The UBA domain (Ub-associated domain) of p62, although binding to Ub through the Ile44/Val70 patch, is unique in forming a stable dimer that negatively regulates Ub recognition. We show that the A20 Znf and UBA domain are able to form a ternary complex through independent interactions with a single Ub molecule, supporting functional models for Ub as a 'hub' for mediating multi-protein complex assembly and for enhancing signalling specificity.

Independent interactions of ubiquitin-binding domains in a ubiquitin-mediated ternary complex.

Ubiquitin (Ub) modifications are transduced by receptor proteins that use Ub-binding domains (UBDs) to recognize distinct interaction faces on the Ub surface. We report the nuclear magnetic resonance (NMR) solution structures of the A20-like zinc finger (A20 Znf) UBD of the Ub receptor ZNF216, and its complex with Ub, and show that the binding surface on Ub centered on Asp58 leaves the canonical hydrophobic Ile44 patch free to participate in additional interactions. We have modeled ternary complexes of the different families of UBDs and show that while many are expected to bind competitively to the same Ile44 surface or show steric incompatibility, other combinations (in particular, those involving the A20 Znf domain) are consistent with a single Ub moiety simultaneously participating in multiple interactions with different UBDs. We subsequently demonstrate by NMR that the A20 Znf domain of ZNF216 and the UBA domain of the p62 protein (an Ile44-binding UBD), which function in the same biological pathways, are able to form such a Ub-mediated ternary complex through independent interactions with a single Ub. This work supports an emerging concept of Ub acting as a scaffold to mediate multiprotein complex assembly.

Human BRCA1-BARD1 ubiquitin ligase activity counteracts chromatin barriers to DNA resection.

The opposing activities of 53BP1 and BRCA1 influence pathway choice in DNA double-strand-break repair. How BRCA1 counteracts the inhibitory effect of 53BP1 on DNA resection and homologous recombination is unknown. Here we identify the site of BRCA1-BARD1 required for priming ubiquitin transfer from E2∼ubiquitin and demonstrate that BRCA1-BARD1's ubiquitin ligase activity is required for repositioning 53BP1 on damaged chromatin. We confirm H2A ubiquitination by BRCA1-BARD1 and show that an H2A-ubiquitin fusion protein promotes DNA resection and repair in BARD1-deficient cells. BRCA1-BARD1's function in homologous recombination requires the chromatin remodeler SMARCAD1. SMARCAD1 binding to H2A-ubiquitin and optimal localization to sites of damage and activity in DNA repair requires its ubiquitin-binding CUE domains. SMARCAD1 is required for 53BP1 repositioning, and the need for SMARCAD1 in olaparib or camptothecin resistance is alleviated by 53BP1 loss. Thus, BRCA1-BARD1 ligase activity and subsequent SMARCAD1-dependent chromatin remodeling are critical regulators of DNA repair.

Cyclisation of Lys48-linked diubiquitin in vitro and in vivo.

Ubiquitin (Ub) is able to form polymeric isopeptide-linked chains through condensation of any of its seven lysine (Lys) residues with the C-terminus of an adjacent Ub monomer. Electrospray ionisation mass spectrometry (ESI-MS) of commercial in vitro-generated Lys48-linked di-Ub (Lys48-Ub(2)) revealed a major population of cyclised dimer. The absence of a free C-terminus in this population was confirmed by an inability to bind the zinc finger ubiquitin-binding domain (ZnF-UBP) of USP5/isopeptidase-T. Endogenous Ub(2) purified from skeletal muscle and cultured mammalian cells was found to contain cyclic Lys48-Ub(2), demonstrating that cyclisation of poly-Ub can also occur in vivo.