Adenovirus early region 1A protein binds to mammalian SUG1-a regulatory component of the proteasome

Adenovirus E1A binding to DCAF10 targets proteasomal degradation of RUVBL1/2 AAA+ ATPases required for quaternary assembly of multiprotein machines, innate immunity, and responses to metabolic stress

IMPORTANCE

Inactivation of EP300/CREBB paralogous cellular lysine acetyltransferases (KATs) during the early phase of infection is a consistent feature of DNA viruses. The cell responds by stabilizing transcription factor IRF3 which activates transcription of scores of interferon-stimulated genes (ISGs), inhibiting viral replication. Human respiratory adenoviruses counter this by assembling a CUL4-based ubiquitin ligase complex that polyubiquitinylates RUVBL1 and 2 inducing their proteasomal degradation. This inhibits accumulation of active IRF3 and the expression of anti-viral ISGs, allowing replication of the respiratory HAdVs in the face of inhibition of EP300/CBEBBP KAT activity by the N-terminal region of E1A.

Prolyl hydroxylase 3 stabilizes the p53 tumor suppressor by inhibiting the p53-MDM2 interaction in a hydroxylase-independent manner

Prolyl hydroxylase 3 (PHD3) has initially been reported to hydroxylase hypoxia-inducible factor α (HIFα) and mediate HIFα degradation. More recent studies have shown that, in addition to HIFα, PHD3 has also other substrates. Moreover, pHD3 is believed to act as a tumor suppressor, but the underlying mechanism remains to be elucidated. Here, we demonstrate that PHD3 stabilizes p53 in a hydroxylase-independent manner. We found that PHD3 overexpression increases and PHD3 knockdown decreases p53 levels. Mechanistically, PHD3 bound MDM2 proto-oncogene (MDM2) and prevented MDM2 from interacting with p53, thereby inhibiting MDM2-mediated p53 degradation. Interestingly, we found that PHD3 overexpression could enhance p53 in the presence of the prolyl hydroxylase inhibitor dimethyloxalylglycine, and the prolyl hydroxylase activity-deficient variant PHD3-H196A also inhibited the p53-MDM2 interaction and stabilized p53. Genetic ablation of PHD3 decreased p53 protein levels in mice intestinal epithelial cells, but a genetic knockin of PHD3-H196A did not affect p53 protein levels in vivo These results suggest that the prolyl hydroxylase activity of PHD3 is dispensable for its ability to stabilize p53. We found that both PHD3 and PHD3-H196A suppress the expression of the stem cell-associated gene NANOG and inhibited the properties of colon cancer stem cells through p53. Our results reveal an additional critical mechanism underlying the regulation of p53 expression and highlight that PHD3 plays a role in the suppression of colon cancer cell stemness in a hydroxylase-independent manner.

PHD3 Regulates p53 Protein Stability by Hydroxylating Proline 359

Cellular p53 protein levels are regulated by a ubiquitination/de-ubiquitination cycle that can target the protein for proteasomal destruction. The ubiquitination reaction is catalyzed by a multitude of ligases, whereas the removal of ubiquitin chains is mediated by two deubiquitinating enzymes (DUBs), USP7 (HAUSP) and USP10. Here, we show that PHD3 hydroxylates p53 at proline 359, a residue that is in the p53-DUB binding domain. Hydroxylation of p53 upon proline 359 regulates its interaction with USP7 and USP10, and its inhibition decreases the association of p53 with USP7/USP10, increases p53 ubiquitination, and rapidly reduces p53 protein levels independently of mRNA expression. Our results show that p53 is a PHD3 substrate and that hydroxylation by PHD3 regulates p53 protein stability through modulation of ubiquitination.

Hacking the Cell: Network Intrusion and Exploitation by Adenovirus E1A

As obligate intracellular parasites, viruses are dependent on their infected hosts for survival. Consequently, viruses are under enormous selective pressure to utilize available cellular components and processes to their own advantage. As most, if not all, cellular activities are regulated at some level via protein interactions, host protein interaction networks are particularly vulnerable to viral exploitation. Indeed, viral proteins frequently target highly connected “hub” proteins to “hack” the cellular network, defining the molecular basis for viral control over the host. This widespread and successful strategy of network intrusion and exploitation has evolved convergently among numerous genetically distinct viruses as a result of the endless evolutionary arms race between pathogens and hosts. Here we examine the means by which a particularly well-connected viral hub protein, human adenovirus E1A, compromises and exploits the vulnerabilities of eukaryotic protein interaction networks. Importantly, these interactions identify critical regulatory hubs in the human proteome and help define the molecular basis of their function.

Rice stripe tenuivirus nonstructural protein 3 hijacks the 26S proteasome of the small brown planthopper via direct interaction with regulatory particle non-ATPase subunit 3

The ubiquitin/26S proteasome system plays a vital role in regulating host defenses against pathogens. Previous studies have highlighted different roles for the ubiquitin/26S proteasome in defense during virus infection in both mammals and plants, but their role in the vectors that transmit those viruses is still unclear. In this study, we determined that the 26S proteasome is present in the small brown planthopper (SBPH) (Laodelphax striatellus) and has components similar to those in plants and mammals. There was an increase in the accumulation of Rice stripe virus (RSV) in the transmitting vector SBPH after disrupting the 26S proteasome, indicating that the SBPH 26S proteasome plays a role in defense against RSV infection by regulating RSV accumulation. Yeast two-hybrid analysis determined that a subunit of the 26S proteasome, named RPN3, could interact with RSV NS3. Transient overexpression of RPN3 had no effect on the RNA silencing suppressor activity of RSV NS3. However, NS3 could inhibit the ability of SBPH rpn3 to complement an rpn3 mutation in yeast. Our findings also indicate that the direct interaction between RPN3 and NS3 was responsible for inhibiting the complementation ability of RPN3. In vivo, we found an accumulation of ubiquitinated protein in SBPH tissues where the RSV titer was high, and silencing of rpn3 resulted in malfunction of the SBPH proteasome-mediated proteolysis. Consequently, viruliferous SBPH in which RPN3 was repressed transmitted the virus more effectively as a result of higher accumulation of RSV. Our results suggest that the RSV NS3 protein is able to hijack the 26S proteasome in SBPH via a direct interaction with the RPN3 subunit to attenuate the host defense response.

IMPORTANCE

We show, for the first time, that the 26S proteasome components are present in the small brown planthopper and play a role in defense against its vectored plant virus (RSV). In turn, RSV encodes a protein that subverts the SBPH 26S proteasome via direct interaction with the 26S proteasome subunit RPN3. Our results imply that the molecular arms race observed in plant hosts can be extended to the insect vector that transmits those viruses.

Viral retasking of hBre1/RNF20 to recruit hPaf1 for transcriptional activation

Upon infection, human adenovirus (HAdV) must activate the expression of its early genes to reprogram the cellular environment to support virus replication. This activation is orchestrated in large part by the first HAdV gene expressed during infection, early region 1A (E1A). E1A binds and appropriates components of the cellular transcriptional machinery to modulate cellular gene transcription and activate viral early genes transcription. Previously, we identified hBre1/RNF20 as a target for E1A. The interaction between E1A and hBre1 antagonizes the innate antiviral response by blocking H2B monoubiquitination, a chromatin modification necessary for the interferon (IFN) response. Here, we describe a second distinct role for the interaction of E1A with hBre1 in transcriptional activation of HAdV early genes. Furthermore, we show that E1A changes the function of hBre1 from a ubiquitin ligase involved in substrate selection to a scaffold which recruits hPaf1 as a means to stimulate transcription and transcription-coupled histone modifications. By using hBre1 to recruit hPaf1, E1A is able to optimally activate viral early transcription and begin the cycle of viral replication. The ability of E1A to target hBre1 to simultaneously repress cellular IFN dependent transcription while activating viral transcription, represents an elegant example of the incredible economy of action accomplished by a viral regulatory protein through a single protein interaction.

Characterization of the 55-residue protein encoded by the 9S E1A mRNA of species C adenovirus

Early region 1A (E1A) of human adenovirus (HAdV) has been the focus of over 30 years of investigation and is required for the oncogenic capacity of HAdV in rodents. Alternative splicing of the E1A transcript generates mRNAs encoding multiple E1A proteins. The 55-residue (55R) E1A protein, which is encoded by the 9S mRNA, is particularly interesting due to the unique properties it displays relative to all other E1A isoforms. 55R E1A does not contain any of the conserved regions (CRs) present in the other E1A isoforms. The C-terminal region of the 55R E1A protein contains a unique sequence compared to all other E1A isoforms, which results from a frameshift generated by alternative splicing. The 55R E1A protein is thought to be produced preferentially at the late stages of infection. Here we report the first study to directly investigate the function of the species C HAdV 55R E1A protein during infection. Polyclonal rabbit antibodies (Abs) have been generated that are capable of immunoprecipitating HAdV-2 55R E1A. These Abs can also detect HAdV-2 55R E1A by immunoblotting and indirect immunofluorescence assay. These studies indicate that 55R E1A is expressed late and is localized to the cytoplasm and to the nucleus. 55R E1A was able to activate the expression of viral genes during infection and could also promote productive replication of species C HAdV. 55R E1A was also found to interact with the S8 component of the proteasome, and knockdown of S8 was detrimental to viral replication dependent on 55R E1A.

Adenovirus E1A interacts directly with, and regulates the level of expression of, the immunoproteasome component MECL1

Proteasomes represent the major non-lysosomal mechanism responsible for the degradation of proteins. Following interferon γ treatment 3 proteasome subunits are replaced producing immunoproteasomes. Adenovirus E1A interacts with components of the 20S and 26S proteasome and can affect presentation of peptides. In light of these observations we investigated the relationship of AdE1A to the immunoproteasome. AdE1A interacts with the immunoproteasome subunit, MECL1. In contrast, AdE1A binds poorly to the proteasome β2 subunit which is replaced by MECL1 in the conversion of proteasomes to immunoproteasomes. Binding sites on E1A for MECL1 correspond to the N-terminal region and conserved region 3. Furthermore, AdE1A causes down-regulation of MECL1 expression, as well as LMP2 and LMP7, induced by interferon γ treatment during Ad infections or following transient transfection. Consistent with previous reports AdE1A reduced IFNγ-stimulated STAT1 phosphorylation which appeared to be responsible for its ability to reduce expression of immunoproteasome subunits.

Comparison of E1A CR3-dependent transcriptional activation across six different human adenovirus subgroups

The largest E1A isoform of human adenovirus (Ad) includes a C-4 zinc finger domain within conserved region 3 (CR3) that is largely responsible for activating transcription of the early viral genes. CR3 interacts with multiple cellular factors, but its mechanism of action is modeled primarily on the basis of the mechanism for the prototype E1A protein of human Ad type 5. We expanded this model to include a representative member from each of the six human Ad subgroups. All CR3 domains tested were capable of transactivation. However, there were dramatic differences in their levels of transcriptional activation. Despite these functional variations, the interactions of these representative CR3s with known cellular transcriptional regulators revealed only modest differences. Four common cellular targets of all representative CR3s were identified: the proteasome component human Sug1 (hSug1)/S8, the acetyltransferases p300/CREB binding protein (CBP), the mediator component mediator complex subunit 23 (MED23) protein, and TATA binding protein (TBP). The first three factors appear to be critical for CR3 function. RNA interference against human TBP showed no significant reduction in transactivation by any CR3 tested. These results indicate that the cellular factors previously shown to be important for transactivation by Ad5 CR3 are similarly bound by the E1A proteins of other types. This was confirmed experimentally using a transcriptional squelching assay, which demonstrated that the CR3 regions of each Ad type could compete with Ad5 CR3 for limiting factors. Interestingly, a mutant of Ad5 CR3 (V147L) was capable of squelching wild-type Ad5 CR3, despite its failure to bind TBP, MED23, p300/CBP-associated factor (pCAF), or p300/CBP, suggestive of the possibility that an additional as yet unidentified cellular factor is required for transactivation by E1A CR3.

Isolation of human proteasomes and putative proteasome-interacting proteins using a novel affinity chromatography method

The proteasome is the primary subcellular organelle responsible for protein degradation. It is a dynamic assemblage of 34 core subunits and many differentially expressed, transiently interacting, modulatory proteins. This paper describes a novel affinity chromatography method for the purification of functional human holoproteasome complexes using mild conditions. Human proteasomes purified by this simple procedure maintained the ability to proteolytically process synthetic peptide substrates and degrade ubiquitinated parkin. Furthermore, the entire purification fraction was analyzed by mass spectrometry in order to identify proteasomal proteins and putative proteasome-interacting proteins. The mild purification conditions maintained transient physical interactions between holoproteasomes and a number of known modulatory proteins. In addition, several classes of putative interacting proteins co-purified with the proteasomes, including proteins with a role in the ubiquitin proteasome system for protein degradation or DNA repair. These results demonstrate the efficacy of using this affinity purification strategy for isolating functional human proteasomes and identifying proteins that may physically interact with human proteasomes.

Roles for APIS and the 20S proteasome in adenovirus E1A-dependent transcription

We have determined distinct roles for different proteasome complexes in adenovirus (Ad) E1A-dependent transcription. We show that the 19S ATPase, S8, as a component of 19S ATPase proteins independent of 20S (APIS), binds specifically to the E1A transactivation domain, conserved region 3 (CR3). Recruitment of APIS to CR3 enhances the ability of E1A to stimulate transcription from viral early gene promoters during Ad infection of human cells. The ability of CR3 to stimulate transcription in yeast is similarly dependent on the functional integrity of yeast APIS components, Sug1 and Sug2. The 20S proteasome is also recruited to CR3 independently of APIS and the 26S proteasome. Chromatin immunoprecipitation reveals that E1A, S8 and the 20S proteasome are recruited to both Ad early region gene promoters and early region gene sequences during Ad infection, suggesting their requirement in both transcriptional initiation and elongation. We also demonstrate that E1A CR3 transactivation and degradation sequences functionally overlap and that proteasome inhibitors repress E1A transcription. Taken together, these data demonstrate distinct roles for APIS and the 20S proteasome in E1A-dependent transactivation.

Accumulation of p53 in response to adenovirus early region 1A sensitizes human cells to tumor necrosis factor alpha-induced apoptosis

Many tumor cells are resistant to tumor necrosis factor alpha (TNFalpha)-induced apoptosis. Adenovirus early region 1A (AdE1A) sensitizes the otherwise resistant cells to TNFalpha. AdE1A also stabilizes the p53 protein. The present study demonstrates a correlation between AdE1A-induced sensitization and stabilization of p53 in TNFalpha-induced apoptosis since the N-terminal and CR2 regions, the binding sites for CBP/p300, Rb and 26S proteasome regulatory components, are required for both these actions of AdE1A. TNFalpha does not induce apoptosis and AdE1A fails to sensitize TNFalpha cytotoxicity in p53-negative cells. However, introduction of exogenous p53 overcomes the cellular resistance to TNFalpha toxicity and enhances AdE1A sensitization, demonstrating that AdE1A sensitizes TNFalpha-induced apoptosis by its stabilization of p53. A proteasome inhibitor, lactacystin, enhances TNFalpha cytotoxicity in p53-positive and -negative cells, suggesting that accumulation of cellular proteins other than p53 might also regulate the cellular response to TNFalpha signaling.

Comprehensive sequence analysis of the E1A proteins of human and simian adenoviruses

Despite extensive study of human adenovirus type 5 E1A, surprisingly little is known about the E1A proteins of other adenoviruses. We report here a comprehensive analysis of the sequences of 34 E1A proteins. These represent all six primate adenovirus subgroups and include all human representatives of subgroups A, C, E, and F, eight from subgroup B, nine from subgroup D, and seven simian adenovirus E1A sequences. We observed that many, but not all, functional domains identified in human adenovirus type 5 E1A are recognizably present in the other E1A proteins. Importantly, we identified highly conserved sequences without known activities or binding partners, suggesting that previously unrecognized determinants of E1A function remain to be uncovered. Overall, our analysis forms a solid foundation for future study of the activities and features of the E1A proteins of different serotypes and identifies new avenues for investigating E1A function.

The targeting of the proteasomal regulatory subunit S2 by adenovirus E1A causes inhibition of proteasomal activity and increased p53 expression

Although adenovirus early region 1A (AdE1A) can modulate protein expression through its interaction with transcriptional regulators it can also influence the ability of the cell to degrade proteins by binding to components of the 26 S proteasome. We demonstrate here that AdE1A interacts with the S2 subunit of the 19 S regulatory complex in addition to the ATPase subunits S4 and S8 previously identified. S2 forms complexes with both the 13 and 12 S AdE1A proteins both in vivo and in vitro. Mutational analysis has shown direct binding through a short sequence toward the N terminus of conserved region 2 of AdE1A, which encompasses the LXCXE motif, involved in interaction with the pRb family of proteins. In vivo, additional contacts are made between AdE1A and proteasomal components, as well as within the proteasome, such that deletion of the N-terminal region of E1A as well as part of conserved region 2 is required to completely disrupt S2 binding. Mutation of AdE1A, which disrupts complex formation with S2, results in the loss of its ability to stabilize the p53 protein. Similarly down-regulation of S2 expression using small interfering RNAs leads to the inhibition of p53 degradation. These effects were observed in normally growing cells and those subjected to UV irradiation. Furthermore, AdE1A had no effect on the Mdm2-mediated ubiquitination of p53. We suggest therefore that interaction of AdE1A with S2, as well as with the ATPases S4 and S8, directly causes inhibition of proteasomal activity and consequent increase in the protein levels of p53.

Avoiding proteasomal processing: the case of EBNA1

Ubiquitin/proteasome-dependent proteolysis is involved in the regulation of a large variety of cellular processes including cell cycle progression, tissue development and atrophy, flux of substrates through metabolic pathways, selective elimination of abnormal proteins and processing of intracellular antigens for major histocompatibility complex (MHC) class I-restricted T-cell responses. Many viruses tamper with this proteolytic machinery by encoding proteins that interact with various components of the pathway. A particularly interesting example of a viral protein that interferes with proteasomal processing is the Epstein-Barr virus (EBV) nuclear antigen-1 (EBNA1). EBNA1 contains an internal repeat exclusively composed of glycines and alanines that inhibits in cis the presentation of MHC class I-restricted T-cell epitopes and prevents ubiquitin/proteasome-dependent proteolysis in vitro and in vivo. The glycine-alanine repeat acts as a transferable element on a variety of proteasomal substrates and may therefore provide a new approach to the modification of cellular proteins for therapeutic purposes.

Transcriptional regulation of the human glycoprotein hormone common alpha subunit gene by cAMP-response-element-binding protein (CREB)-binding protein (CBP)/p300 and p53

We have investigated the functional interactions between adenovirus early region 1A (AdE1A) protein, the co-activators cAMP-response-element-binding protein (CREB)-binding protein (CBP)/p300 and SUG1, and the transcriptional repressor retinoblastoma (Rb) in mediating T3-dependent repression. Utilizing the human glycoprotein hormone common alpha-subunit (alpha-subunit) promoter and AdE1A mutants with selective binding capacity to these molecules we have determined an essential role for CBP/p300. In normal circumstances, wild-type 12 S AdE1A inhibited alpha-subunit activity. In contrast, adenovirus mutants that retain both the SUG1- and Rb-binding sites, but lack the CBP/p300-binding site, were unable to repress promoter activity. We have also identified a role for the tumour-suppressor gene product p53 in regulation of the alpha-subunit promoter. Akin to 12 S AdE1A, exogenous p53 expression repressed alpha-subunit activity. This function resided in the ability of p53 to interact with CBP/p300; an N-terminal mutant incapable of interacting with CBP/p300 did not inhibit alpha-subunit activity. Stabilization of endogenous p53 by UV irradiation also correlated positively with reduced alpha-subunit activity. Intriguingly, T3 stimulated endogenous p53 transcriptional activity, implicating p53 in T3-dependent signalling pathways. These data indicate that CBP/p300 and p53 are key regulators of alpha-subunit activity.

Acetylation of the adenovirus-transforming protein E1A determines nuclear localization by disrupting association with importin-alpha

Posttranslational modifications may alter the biochemical functions of a protein by modifying associations with other macromolecules, allosterically altering intrinsic catalytic activities, or determining subcellular localization. The adenovirus-transforming protein E1A is acetylated by its cellular targets, the co-activators CREB-binding protein, p300, and p300/CREB-binding protein-associated factor in vitro and also in vivo at a single lysine residue (Lys(239)) within a multifunctional carboxyl-terminal domain necessary for both nuclear localization and interaction with the transcriptional co-repressor carboxyl-terminal binding protein (CtBP). In contrast to a previous report, we demonstrate that acetylation of Lys(239) does not disrupt CtBP binding and that 12 S E1A-mediated repression of CREB-binding protein-dependent transcription does not require recruitment of CtBP. Instead we find that the cytoplasmic fraction of E1-transformed 293 cells is enriched for acetylated E1A with relative exclusion from the nuclear compartment. Whereas wild type 12 S E1A binds importin-alpha 3, binding affinity was markedly reduced both by single amino acid substitution mutations and acetylation at Lys(239). This is the first demonstration that acetylation may alter nuclear partitioning by direct interference with nuclear import receptor recognition. The finding that the cytoplasmic fraction of E1A is acetylated indicates that E1A may exert its pleiotropic effects on cellular transformation in part by affecting cytoplasmic processes.

MHC class I antigen processing regulated by cytosolic proteolysis-short cuts that alter peptide generation

Cytotoxic T lymphocyte (CTL)-mediated immune responses rely on the efficiency of MHC class I ligand generation and presentation by antigen presenting cells (APCs). Whereas the abnormal expression of MHC molecules and transporters associated with antigen processing (TAPs) are commonly discussed as factors that modulate antigen presentation, much less is known about possible regulatory mechanisms at the level of proteolysis responsible for the generation of antigenic peptides. The ubiquitin-proteasome system is recognized as the major component responsible for this process in the cytosol and its activity can be regulated by cytokines, such as IFN-gamma. However, new evidence suggests the involvement of other proteases that can contribute to cytosolic proteolysis and therefore, to the quality and quantity of antigen production. Here, we review recent findings on an increasing number of proteolytic enzymes linked to antigen presentation, and we discuss how regulation of cytosolic protease activities might have implications for immune escape mechanisms that could be used by tumor cells and pathogens.

Manipulation of immune responses by Epstein–Barr virus

Epstein-Barr virus (EBV) infects and persists for life in the majority of the human population. Persistence is achieved through a combination of strictly regulated programs of latent infection in B-cells and chronic reactivation of virus replication in lymphoid tissue and mucosal surfaces. The resulting multiple patterns of virus-host interaction have selected unique strategies of immune escape. T-cell mediated immunity plays a central role in the control of EBV latency and several immune escape mechanism that protect the virus at this stage of its life circle have been characterized in details. In contrast, the contribution of innate immunity and the immune regulation of productive infection are largely unexplored areas that may yield important clues on the establishment and maintenance of EBV persistence. This review summarizes well known and emerging mechanisms of EBV immune escape that may reveal new strategies of immunoregulation and promote new approaches to the prophylaxis and treatment of EBV associated diseases.

Physical association of the APIS complex and general transcription factors

It has recently been demonstrated that a fragment of the proteasome, called the APIS complex, plays an important role in RNA polymerase II-mediated transcription. Here, it is shown that the APIS complex is physically associated with many general transcription factors, including components of yeast FACT (Cdc68/Pob3), TFIID, TFIIH, and the RNA polymerase II holoenzyme. Depletion of this APIS transcription factor complex from a yeast whole cell extract resulted in reduced transcription, indicating that it is functionally relevant. The APIS/transcription factor complex does not include detectable levels of the 20S proteolytic sub-unit of the proteasome. Furthermore, immunopurified 26S proteasome contains little or no transcription factors, suggesting that transcription factors and the 20S bind competitively to the APIS complex. These data add to the growing body of evidence that the APIS complex has a role in transcription, independent of its role in proteolysis and, furthermore, argues that it functions in association with the general transcription complex.

Clastosome: a subtype of nuclear body enriched in 19S and 20S proteasomes, ubiquitin, and protein substrates of proteasome

Nuclear bodies represent a heterogeneous class of nuclear structures. Herein, we describe that a subset of nuclear bodies is highly enriched in components of the ubiquitin-proteasome pathway of proteolysis. We coined the term clastosome (from the Greek klastos, broken and soma, body) to refer to this type of nuclear body. Clastosomes contain a high concentration of 1) ubiquitin conjugates, 2) the proteolytically active 20S core and the 19S regulatory complexes of the 26S proteasome, and 3) protein substrates of the proteasome. Although detected in a variety of cell types, clastosomes are scarce under normal conditions; however, they become more abundant when proteasomal activity is stimulated. In contrast, clastosomes disappear when cells are treated with proteasome inhibitors. Protein substrates of the proteasome that are found concentrated in clastosomes include the short-lived transcription factors c-Fos and c-Jun, adenovirus E1A proteins, and the PML protein. We propose that clastosomes are sites where proteolysis of a variety of protein substrates is taking place.

Comparative sequence analysis of the largest E1A proteins of human and simian adenoviruses

The early region 1A (E1A) gene is the first gene expressed after infection with adenovirus and has been most extensively characterized in human adenovirus type 5 (hAd5). The E1A proteins interact with numerous cellular regulatory proteins, influencing a variety of transcriptional and cell cycle events. For this reason, these multifunctional proteins have been useful as tools for dissecting pathways regulating cell growth and gene expression. Despite the large number of studies using hAd5 E1A, relatively little is known about the function of the E1A proteins of other adenoviruses. In 1985, a comparison of E1A sequences from three human and one simian adenovirus identified three regions with higher overall levels of sequence conservation designated conserved regions (CR) 1, 2, and 3. As expected, these regions are critical for a variety of E1A functions. Since that time, the sequences of several other human and simian adenovirus E1A proteins have been determined. Using these, and two additional sequences that we determined, we report here a detailed comparison of the sequences of 15 E1A proteins representing each of the six hAd subgroups and several simian adenoviruses. These analyses refine the positioning of CR1, 2, and 3; define a fourth CR located near the carboxyl terminus of E1A; and suggest several new functions for E1A.

HBV X protein targets HIV Tat-binding protein 1

The HBV X protein (HBx) is implicated in infection and development of hepatocellular carcinoma. HBx has a pleiotropic effect on cells, suggesting multiple targets in the virus-host cell interaction. We employed the cytoplasmic-based two-hybrid screen and identified the HIV Tat-binding protein 1 (Tbp1) as a novel HBx interacting protein. Tbp1 interacts in vivo with HBx both in yeast and in animal cells. This interaction maps to the functionally important ATP-binding motif of Tbp1. Furthermore, HBx and Tbp1 interaction is functionally significant and regulates HBV transcription. Tbp1 homologues, such as Sug1, are known members of the proteasome 19S regulatory cap particle and have also been implicated in transcription coactivation. Remarkably, Tbp1 and Sug1 interact with multiple viral effector proteins including HIV Tat, SV40 large T antigen, and adenovirus E1A, establishing these proteins as important targets of the viral oncogenes.

Regulation of the 26S proteasome by adenovirus E1A

We have identified the N-terminus of adenovirus early region 1A (AdE1A) as a region that can regulate the 26S proteasome. Specifically, in vitro and in vivo co-precipitation studies have revealed that the 19S regulatory components of the proteasome, Sug1 (S8) and S4, bind through amino acids (aa) 4-25 of Ad5 E1A. In vivo expression of wild-type (wt) AdE1A, in contrast to the N-terminal AdE1A mutant that does not bind the proteasome, reduces ATPase activity associated with anti-S4 immunoprecipitates relative to mock-infected cells. This reduction in ATPase activity correlates positively with the ability of wt AdE1A, but not the N-terminal deletion mutant, to significantly reduce the ability of HPV16 E6 to target p53 for ubiquitin-mediated proteasomal degradation. AdE1A/proteasomal complexes are present in both the cytoplasm and the nucleus, suggesting that AdE1A interferes with both nuclear and cytoplasmic proteasomal degradation. We have also demonstrated that wt AdE1A and the N-terminal AdE1A deletion mutant are substrates for proteasomal-mediated degradation. AdE1A degradation is not, however, mediated through ubiquitylation, but is regulated through phosphorylation of residues within a C-terminal PEST region (aa 224-238).

cDNA cloning, expression, and functional characterization of PI31, a proline-rich inhibitor of the proteasome

The primary structure of PI31, a protein inhibitor of the 20 S proteasome, was deduced by cDNA cloning and sequencing. The human protein has a calculated molecular weight of 29,792, a value in excellent accord with 31,000, as estimated by SDS-polyacrylamide gel electrophoresis for purified bovine PI31, and is not similar to any other protein in current data bases. PI31 is a proline-rich protein, particularly within its carboxyl-terminal half where 26% of the amino acids are proline. Wild-type PI31 and various truncation mutants were expressed in Escherichia coli and purified to homogeneity. Recombinant wild-type PI31 displayed structural and functional properties similar to those of PI31 purified from bovine red blood cells and inhibited the hydrolysis of protein and peptide substrates by the 20 S proteasome. Analysis of truncation mutants demonstrated that proteasome inhibition was conferred by the carboxyl-terminal proline-rich domain of PI31, which appears to have an extended secondary structure. Inhibition of the 20 S proteasome by PI31 involved formation a proteasome-PI31 complex. In addition to its direct inhibition of the 20 S proteasome, PI31 inhibited the activation of the proteasome by each of two proteasome regulatory proteins, PA700 and PA28. These results suggest that PI31 plays an important role in control of proteasome function, including that in ubiquitin-dependent pathways of protein degradation.

Regulatory subunit interactions of the 26S proteasome, a complex problem

The 26S proteasome is the major non-lysosomal protease in eukaryotic cells. This multimeric enzyme is the integral component of the ubiquitin-mediated substrate degradation pathway. It consists of two subcomplexes, the 20S proteasome, which forms the proteolytic core, and the 19S regulator (or PA700), which confers ATP dependency and ubiquitinated substrate specificity on the enzyme. Recent biochemical and genetic studies have revealed many of the interactions between the 17 regulatory subunits, yielding an approximation of the 19S complex topology. Inspection of interactions of regulatory subunits with non-subunit proteins reveals patterns that suggest these interactions play a role in 26S proteasome regulation and localization.