UbE2E1/UBCH6 Is a Critical in Vivo E2 for the PRC1-catalyzed Ubiquitination of H2A at Lys-119.

UbE2E1/UbcH6 is an E2 ubiquitin-conjugating enzyme that is regulated by USP7. We identified UbE2E1 as a novel component of Polycomb repressive complex 1 (PRC1), the E3 ligase complex responsible for histone H2A ubiquitination and gene silencing. We demonstrate that UbE2E1 is critical for the monoubiquitination of H2A at residue Lys-119 (uH2AK119) through its association with the PRC1 complex. UbE2E1 interacts with PRC1 subunits including Ring1A and Ring1B. Overexpression of UbE2E1 results in increased levels of uH2AK119, whereas overexpression of catalytically inactive UbE2E1_C131A or UbE2E1 knockdown results in decreased levels of uH2AK119. The down-regulation of H2A ubiquitination by loss of function of UbE2E1 is correlated with alleviated p16INK4a promoter repression and induced growth inhibition in HCT116 cells. These results are specific to UbE2E1 as knockdown of UbE2D E2s does not show any effect on uH2AK119. We extended the UbE2E1 regulation of uH2AK119 to USP7 and showed that USP7 is also a key regulator for monoubiquitination at H2A Lys-119 as both knockdown and deletion of USP7 results in decreased levels of uH2AK119. This study reveals that UbE2E1 is an in vivo E2 for the PRC1 ligase complex and thus plays an important role in the regulation of H2A Lys-119 monoubiquitination.

The Role of Support Services in Promoting Social Inclusion for the Disadvantaged Urban-dwelling Elderly.

BACKGROUND: Disadvantaged older adults living in non-family situations in Toronto are more likely than older adults living in family situations to have less economic security, less social support, and less choice in housing. Older adults who live in poverty and are precariously housed are more likely to be chronically ill, to live with multiple illnesses, to have poor nutrition, high stress and loneliness, all of which are strongly associated with the determinant of health social exclusion. The aim of this study is to: 1) evaluate the level of social disadvantage and exclusion experienced by low-income older adults 65 years of age and older living alone or in non-family situations; 2) assess the level of dependency on government and community services (support services) to maintain a reasonable standard of living (minimize effects of social exclusion); and 3) identify consequences of social exclusion not addressed by current available services. METHODS: Fifteen male older adult members of the Good Neighbours' Club in downtown Toronto were interviewed. Semi-structured questionnaires assessed barriers to, utility of, and perceived impact of support services available to disadvantaged older adults living in the central core of southeast Toronto. RESULTS: Support services for income, housing, food security, social support, and health care do mitigate the effects of social exclusion in the study participants. Data gathered from interviews identified factors that counter the efforts by support services to increase social inclusion in this population. CONCLUSIONS: Support services reduce social isolation experienced by these older adults. Evidence of the detrimental impact of low financial literacy suggests a need to design and implement training programs to build the older adults' capacity to manage their own finances effectively, and resist falling victim to financial fraud.

Ubiquitin-specific protease 7 is a regulator of ubiquitin-conjugating enzyme UbE2E1.

Ubiquitin-specific protease 7 (USP7) is a deubiquitinating enzyme found in all eukaryotes that catalyzes the removal of ubiquitin from specific target proteins. Here, we report that UbE2E1, an E2 ubiquitin conjugation enzyme with a unique N-terminal extension, is a novel USP7-interacting protein. USP7 forms a complex with UbE2E1 in vitro and in vivo through the ASTS USP7 binding motif within its N-terminal extension in an identical manner with other known USP7 binding proteins. We show that USP7 attenuates UbE2E1-mediated ubiquitination, an effect that requires the N-terminal ASTS sequence of UbE2E1 as well as the catalytic activity of USP7. Additionally, USP7 is critical in maintaining the steady state levels of UbE2E1 in cells. This study reveals a new cellular mechanism that couples the opposing activities of the ubiquitination machinery and a deubiquitinating enzyme to maintain and modulate the dynamic balance of the ubiquitin-proteasome system.

The herpesvirus associated ubiquitin specific protease, USP7, is a negative regulator of PML proteins and PML nuclear bodies.

The PML tumor suppressor is the founding component of the multiprotein nuclear structures known as PML nuclear bodies (PML-NBs), which control several cellular functions including apoptosis and antiviral effects. The ubiquitin specific protease USP7 (also called HAUSP) is known to associate with PML-NBs and to be a tight binding partner of two herpesvirus proteins that disrupt PML NBs. Here we investigated whether USP7 itself regulates PML-NBs. Silencing of USP7 was found to increase the number of PML-NBs, to increase the levels of PML protein and to inhibit PML polyubiquitylation in nasopharyngeal carcinoma cells. This effect of USP7 was independent of p53 as PML loss was observed in p53-null cells. PML-NBs disruption was induced by USP7 overexpression independently of its catalytic activity and was induced by either of the protein interaction domains of USP7, each of which localized to PML-NBs. USP7 also disrupted NBs formed from some single PML isoforms, most notably isoforms I and IV. CK2α and RNF4, which are known regulators of PML, were dispensable for USP7-associated PML-NB disruption. The results are consistent with a novel model of PML regulation where a deubiquitylase disrupts PML-NBs through recruitment of another cellular protein(s) to PML NBs, independently of its catalytic activity.

USP7/HAUSP promotes the sequence-specific DNA binding activity of p53.

The p53 tumor suppressor invokes cellular responses to stressful stimuli by coordinating distinct gene expression programs. This function relies heavily on the ability of p53 to function as a transcription factor by binding promoters of target genes in a sequence specific manner. The DNA binding activity of the core domain of p53 is subject to regulation via post-translational modifications of the C-terminal region. Here we show that the ubiquitin specific protease, USP7 or HAUSP, known to stabilize p53, also regulates the sequence-specific DNA binding mediated by the core domain of p53 in vitro. This regulation is contingent upon interaction between USP7 and the C-terminal regulatory region of p53. However, our data suggest that this effect is not mediated through the N-terminal domain of USP7 previously shown to bind p53, but rather involves the USP7 C-terminal domain and is independent of the deubiquitylation activity of USP7. Consistent with our in vitro observations, we found that overexpression of catalytically inactive USP7 in cells promotes p53 binding to its target sequences and p21 expression, without increasing the levels of p53 protein. We also found that the USP7 C-terminal domain was sufficient for p21 induction. Our results suggest a novel mode of regulation of p53 function by USP7, which is independent of USP7 deubiquitylating activity.

Further insight into substrate recognition by USP7: structural and biochemical analysis of the HdmX and Hdm2 interactions with USP7.

Ubiquitin-specific protease 7 (USP7) catalyzes the deubiquitination of several substrate proteins including p53 and Hdm2. We have previously shown that USP7, and more specifically its amino-terminal domain (USP7-NTD), interacts with distinct regions on p53 and Hdm2 containing P/AxxS motifs. The ability of USP7 to also deubiquitinate and control the turnover of HdmX was recently demonstrated. We utilized a combination of biochemistry and structural biology to identify which domain of USP7 interacts with HdmX as well as to identify regions of HdmX that interact with USP7. We showed that USP7-NTD recognized two of six P/AxxS motifs of HdmX ((8)AQCS(11) and (398)AHSS(401)). The crystal structure of the USP7-NTD:HdmX(AHSS) complex was determined providing the molecular basis of complex formation between USP7-NTD and the HdmX(AHSS) peptide. The HdmX peptide interacted within the same residues of USP7-NTD as previously demonstrated with p53, Hdm2, and EBNA1 peptides. We also identified an additional site on Hdm2 ((397)PSTS(400)) that interacts with USP7-NTD and determined the crystal structure of this complex. Finally, analysis of USP7-interacting peptides on filter arrays confirmed the importance of the serine residue at the fourth position for the USP7-NTD interaction and showed that phosphorylation of serines within the binding sequence prevents this interaction. These results lead to a better understanding of the mechanism of substrate recognition by USP7-NTD.

EBNA1-mediated recruitment of a histone H2B deubiquitylating complex to the Epstein-Barr virus latent origin of DNA replication.

The EBNA1 protein of Epstein-Barr virus (EBV) plays essential roles in enabling the replication and persistence of EBV genomes in latently infected cells and activating EBV latent gene expression, in all cases by binding to specific recognition sites in the latent origin of replication, oriP. Here we show that EBNA1 binding to its recognition sites in vitro is greatly stimulated by binding to the cellular deubiquitylating enzyme, USP7, and that USP7 can form a ternary complex with DNA-bound EBNA1. Consistent with the in vitro effects, the assembly of EBNA1 on oriP elements in human cells was decreased by USP7 silencing, whereas assembly of an EBNA1 mutant defective in USP7 binding was unaffected. USP7 affinity column profiling identified a complex between USP7 and human GMP synthetase (GMPS), which was shown to stimulate the ability of USP7 to cleave monoubiquitin from histone H2B in vitro. Accordingly, silencing of USP7 in human cells resulted in a consistent increase in the level of monoubquitylated H2B. The USP7-GMPS complex formed a quaternary complex with DNA-bound EBNA1 in vitro and, in EBV infected cells, was preferentially detected at the oriP functional element, FR, along with EBNA1. Down-regulation of USP7 reduced the level of GMPS at the FR, increased the level of monoubiquitylated H2B in this region of the origin and decreased the ability of EBNA1, but not an EBNA1 USP7-binding mutant, to activate transcription from the FR. The results indicate that USP7 can stimulate EBNA1-DNA interactions and that EBNA1 can alter histone modification at oriP through recruitment of USP7.

Epstein-Barr nuclear antigen 1 contributes to nasopharyngeal carcinoma through disruption of PML nuclear bodies.

Latent Epstein-Barr virus (EBV) infection is strongly associated with several cancers, including nasopharyngeal carcinoma (NPC), a tumor that is endemic in several parts of the world. We have investigated the molecular basis for how EBV latent infection promotes the development of NPC. We show that the viral EBNA1 protein, previously known to be required to maintain the EBV episomes, also causes the disruption of the cellular PML (promyelocytic leukemia) nuclear bodies (or ND10s). This disruption occurs both in the context of a native latent infection and when exogenously expressed in EBV-negative NPC cells and involves loss of the PML proteins. We also show that EBNA1 is partially localized to PML nuclear bodies in NPC cells and interacts with a specific PML isoform. PML disruption by EBNA1 requires binding to the cellular ubiquitin specific protease, USP7 or HAUSP, but is independent of p53. We further observed that p53 activation, DNA repair and apoptosis, all of which depend on PML nuclear bodies, were impaired by EBNA1 expression and that cells expressing EBNA1 were more likely to survive after induction of DNA damage. The results point to an important role for EBNA1 in the development of NPC, in which EBNA1-mediated disruption of PML nuclear bodies promotes the survival of cells with DNA damage.

Molecular recognition of p53 and MDM2 by USP7/HAUSP.

The ubiquitin-specific protease, USP7, has key roles in the p53 pathway whereby it stabilizes both p53 and MDM2. We show that the N-terminal domain of USP7 binds two closely spaced 4-residue sites in both p53 and MDM2, falling between p53 residues 359-367 and MDM2 residues 147-159. Cocrystal structures with USP7 were determined for both p53 peptides and for one MDM2 peptide. These peptides bind the same surface of USP7 as Epstein-Barr nuclear antigen-1, explaining the competitive nature of the interactions. The structures and mutagenesis data indicate a preference for a P/AXXS motif in peptides that bind USP7. Contacts made by serine are identical and crucial for all peptides, and Trp165 in the peptide-binding pocket of USP7 is also crucial. These results help to elucidate the mechanism of substrate recognition by USP7 and the regulation of the p53 pathway.

Structure of the p53 binding domain of HAUSP/USP7 bound to Epstein-Barr nuclear antigen 1 implications for EBV-mediated immortalization.

USP7/HAUSP is a key regulator of p53 and Mdm2 and is targeted by the Epstein-Barr nuclear antigen 1 (EBNA1) protein of Epstein-Barr virus (EBV). We have determined the crystal structure of the p53 binding domain of USP7 alone and bound to an EBNA1 peptide. This domain is an eight-stranded beta sandwich similar to the TRAF-C domains of TNF-receptor associated factors, although the mode of peptide binding differs significantly from previously observed TRAF-peptide interactions in the sequence (DPGEGPS) and the conformation of the bound peptide. NMR chemical shift analyses of USP7 bound by EBNA1 and p53 indicated that p53 binds the same pocket as EBNA1 but makes less extensive contacts with USP7. Functional studies indicated that EBNA1 binding to USP7 can protect cells from apoptotic challenge by lowering p53 levels. The data provide a structural and conceptual framework for understanding how EBNA1 might contribute to the survival of Epstein-Barr virus-infected cells.