Genetic analysis of polyomavirus large T nuclear localization: nuclear localization is required for productive association with pRb family members

Nuclear and Nucleolar Localization of Bovine Adenovirus-3 Protein V

The L2 region of bovine adenovirus-3 (BAdV-3) encodes a Mastadenovirus genus-specific protein, designated as pV, which is important for the production of progeny viruses. Here, we demonstrate that BAdV-3 pV, expressed as 55 kDa protein, localizes to the nucleus and specifically targets nucleolus of the infected cells. Analysis of deletion mutants of pV suggested that amino acids 81–120, 190–210, and 380–389 act as multiple nuclear localization signals (NLS), which also appear to serve as the binding sites for importin α-3 protein, a member of the importin α/β nuclear import receptor pathway. Moreover, pV amino acids 21–50 and 380–389 appear to act as nucleolar localization signals (NoLs). Interestingly, amino acids 380–389 appear to act both as NLS and as NoLS. The presence of NoLS is essential for the production of infectious progeny virions, as deletion of both NoLs are lethal for the production of infectious BAdV-3. Analysis of mutant BAV.pVd1d3 (isolated in pV completing CRL cells) containing deletion/mutation of both NoLS in non-complementing CRL cells not only revealed the altered intracellular localization of mutant pV but also reduced the expression of some late proteins. However, it does not appear to affect the incorporation of viral proteins, including mutant pV, in BAV.pVd1d3 virions. Further analysis of CsCl purified BAV.pVd1d3 suggested the presence of thermo-labile virions with disrupted capsids, which appear to affect the infectivity of the progeny virions. Our results suggest that pV contains overlapping and non-overlapping NoLS/NLS. Moreover, the presence of both NoLS appear essential for the production of stable and infectious progeny BAV.pVd1d3 virions.

Human polyomaviruses and cancer: an overview

The name of the family Polyomaviridae, derives from the early observation that cells infected with murine polyomavirus induced multiple (poly) tumors (omas) in immunocompromised mice. Subsequent studies showed that many members of this family exhibit the capacity of mediating cell transformation and tumorigenesis in different experimental models. The transformation process mediated by these viruses is driven by viral pleiotropic regulatory proteins called T (tumor) antigens. Similar to other viral oncoproteins T antigens target cellular regulatory factors to favor cell proliferation, immune evasion and downregulation of apoptosis. The first two human polyomaviruses were isolated over 45 years ago. However, recent advances in the DNA sequencing technologies led to the rapid identification of additional twelve new polyomaviruses in different human samples. Many of these viruses establish chronic infections and have been associated with conditions in immunosuppressed individuals, particularly in organ transplant recipients. This has been associated to viral reactivation due to the immunosuppressant therapy applied to these patients. Four polyomaviruses namely, Merkel cell polyomavirus (MCPyV), Trichodysplasia spinulosa polyomavirus (TSPyV), John Cunningham Polyomavirus (JCPyV) and BK polyomavirus (BKPyV) have been associated with the development of specific malignant tumors. However, present evidence only supports the role of MCPyV as a carcinogen to humans. In the present review we present a summarized discussion on the current knowledge concerning the role of MCPyV, TSPyV, JCPyV and BKPyV in human cancers.

Identification and characterization of nuclear and nucleolar localization signals in the adeno-associated virus serotype 2 assembly-activating protein

Assembly-activating protein (AAP) of adeno-associated virus serotype 2 (AAV2) is a nucleolar-localizing protein that plays a critical role in transporting the viral capsid VP3 protein to the nucleolus for assembly. Here, we identify and characterize AAV2 AAP (AAP2) nuclear (NLS) and nucleolar (NoLS) localization signals near the carboxy-terminal region of AAP2 (amino acid positions 144 to 184) (AAP2(144-184)). This region contains five basic-amino-acid-rich (BR) clusters, KSKRSRR (AAP2BR1), RRR (AAP2BR2), RFR (AAP2BR3), RSTSSR (AAP2BR4), and RRIK (AAP2BR5), from the amino terminus to the carboxy terminus. We created 30 AAP2BR mutants by arginine/lysine-to-alanine mutagenesis or deletion of AAP2BRs and 8 and 1 green fluorescent protein (GFP)-AAP2BR and β-galactosidase-AAP2BR fusion proteins, respectively, and analyzed their intracellular localization in HeLa cells by immunofluorescence microscopy. The results showed that AAP2(144-184) has redundant multipartite NLSs and that any combinations of 4 AAP2BRs, but not 3 or less, can constitute a functional NLS-NoLS; AAP2BR1 and AAP2BR2 play the most influential role for nuclear localization, but either one of the two AAP2BRs is dispensable if all 4 of the other AAP2BRs are present, resulting in 3 different, overlapping NLS motifs; and the NoLS is shared redundantly among the five AAP2BRs and functions in a context-dependent manner. AAP2BR mutations not only resulted in aberrant intracellular localization, but also attenuated AAP2 protein expression to various degrees, and both of these abnormalities have a significant negative impact on capsid production. Thus, this study reveals the organization of the intermingling NLSs and NoLSs in AAP2 and provides insights into their functional roles in capsid assembly.

IMPORTANCE

Adeno-associated virus (AAV) has become a popular and successful vector for in vivo gene therapy; however, its biology has yet to be fully understood. In this regard, the recent discovery of the assembly-activating protein (AAP), a nonstructural, nucleolar-localizing AAV protein essential for viral capsid assembly, has provided us a new opportunity to better understand the fundamental processes required for virion formation. Here, we identify clusters of basic amino acids in the carboxy terminus of AAP from AAV serotype 2 (AAV2) that act as nuclear and nucleolar localization signals. We also demonstrate their importance in maintaining AAP expression levels and efficient production of viral capsids. Insights into the functions of AAP can elucidate the requirements and process for AAV capsid assembly, which may lead to improved vector production for use in gene therapy. This study also contributes to the growing body of work on nuclear and nucleolar localization signals.

Backbone assignment of the N-terminal polyomavirus large T antigen

Polyoma Large T antigen (PyLT) is a viral oncoprotein that targets cell proteins important for growth regulation. PyLT has two functional domains. Here we report (1)H, (15)N, (13)C backbone and (13)C beta assignments of 76% of the residues of the polyomavirus large T antigen N-terminal domain (PyLTNT) that is sufficient to regulate cell phenotype. PyLTNT is substantially unfolded even in regions known to be critical for its biological function. The protein also includes a previously characterised J domain that although conformationally influenced by the residue extension, retains its folded state unlike the majority of the protein sequence.

Functional ablation of pRb activates Cdk2 and causes antiestrogen resistance in human breast cancer cells

Estrogens are required for the proliferation of hormone dependent breast cancer cells, making estrogen receptor (ER) positive tumors amenable to endocrine therapies such as antiestrogens. However, resistance to these agents remains a significant cause of treatment failure. We previously demonstrated that inactivation of the retinoblastoma protein (pRb) family tumor suppressors causes antiestrogen resistance in MCF-7 cells, a widely studied model of estrogen responsive human breast cancers. In this study, we investigate the mechanism by which pRb inactivation leads to antiestrogen resistance. Cdk4 and cdk2 are two key cell cycle regulators that can phosphorylate and inactivate pRb, therefore we tested whether these kinases are required in cells lacking pRb function. pRb family members were inactivated in MCF-7 cells by expressing polyomavirus large tumor antigen (PyLT), and cdk activity was inhibited using the cdk inhibitors p16^INK4A and p21^Waf1/Cip1. Cdk4 activity was no longer required in cells lacking functional pRb, while cdk2 activity was required for proliferation in both the presence and absence of pRb function. Using inducible PyLT cell lines, we further demonstrated that pRb inactivation leads to increased cyclin A expression, cdk2 activation and proliferation in antiestrogen arrested cells. These results demonstrate that antiestrogens do not inhibit cdk2 activity or proliferation of MCF-7 cells in the absence of pRb family function, and suggest that antiestrogen resistant breast cancer cells resulting from pRb pathway inactivation would be susceptible to therapies that target cdk2.

Role of the interaction between large T antigen and Rb family members in the oncogenicity of JC virus

Human polyomaviruses (JC virus, BK virus and simian virus 40) are causative agents of some human diseases and, interestingly, are involved in processes of cell transformation and oncogenesis. These viruses need the cell cycle machinery of the host cell to complete their replication; so they evolved mechanisms that can interfere with the growth control of infected cells and force them into DNA replication. The retinoblastoma family of proteins (pRb), which includes pRb/p105, p107 and pRb2/p130, acts as one of the most important regulators of the G1/S transition of the cell cycle. Rb proteins represent an important target for viral oncoproteins. Early viral T antigens can bind all members of the pRb family, promoting the activation of the E2F family of transcription factors, thus inducing the expression of genes required for the entry to the S phase. The interaction between early viral antigens and cell cycle regulators represents an important mechanism through which viruses deregulate cell cycle and lead to cell transformation. In this review, we will discuss the effects of the interaction between large T antigen and Rb proteins in JC virus-mediated oncogenesis.

Activation of CREB/ATF sites by polyomavirus large T antigen

Polyomavirus large T antigen (LT) has a direct role in viral replication and a profound effect on cell phenotype. It promotes cell cycle progression, immortalizes primary cells, blocks differentiation, and causes apoptosis. While much of large T function is related to its effects on tumor suppressors of the retinoblastoma susceptibility (Rb) gene family, we have previously shown that activation of the cyclin A promoter can occur through a non-Rb-dependent mechanism. Here we show that activation occurs via an ATF/CREB site. Investigation of the mechanism indicates that large T can synergize with CREB family members to activate transcription. Experiments with Gal4-CREB constructs show that synergy is independent of CREB phosphorylation by protein kinase A. Examination of synergy with Gal4-CREB deletion constructs indicates that large T acts on the constitutive activation domain of CREB. Large T can bind to CREB in vivo. Genetic analysis shows that the DNA-binding domain (residues 264 to 420) is sufficient to activate transcription when it is localized to the nucleus. Further analysis of the DNA-binding domain shows that while site-specific DNA binding is not required, non-site-specific DNA binding is important for the activation. Thus, CREB binding and DNA binding are both important for large T activation of CREB/ATF sites. In contrast to previous models where large T transactivation occurred indirectly, these results also suggest that large T can act directly at promoters to activate transcription.

Reversal of an antiestrogen-mediated cell cycle arrest of MCF-7 cells by viral tumor antigens requires the retinoblastoma protein-binding domain

Proliferation of MCF-7 cells is estrogen dependent and antiestrogen sensitive. In the absence of estrogens or presence of antiestrogens MCF-7 cells arrest in the G1 phase of the cell cycle, and this arrest is associated with an accumulation of the active, hypophosphorylated form of the retinoblastoma protein (pRb). Because active pRb negatively regulates passage from G1 to S phase, this suggests that pRb is a crucial target of estrogen action, and that its inactivation might lead to antiestrogen resistance. We tested this hypothesis by expressing viral tumor antigens (T antigens), which bind and inactivate pRb, in MCF-7 cells, and determining the effects on cell proliferation in the presence of antiestrogens. The results of these experiments demonstrate that T antigen expression confers antiestrogen resistance to MCF-7 cells. Using a panel of mutant T antigens, we further demonstrate that the pRb-binding, but not the p53 binding domain is required to confer antiestrogen resistance. Thus, pRb is an important target of estrogen action, and its inactivation can contribute to the development of antiestrogen resistance.

J domain-independent regulation of the Rb family by polyomavirus large T antigen

The ability of polyomavirus large T antigen (LT) to promote cell cycling, to immortalize primary cells, and to block differentiation has been linked to its effects on tumor suppressors of the retinoblastoma susceptibility (Rb) gene family. Our previous studies have shown that LT requires an intact N-terminal DnaJ domain, in addition to an Rb binding site, for activation of simple E2F-containing promoters and stimulation of cell cycle progression. Here we show that some LT effects dependent on interaction with the Rb family are largely DnaJ independent. In differentiating C2C12 myoblasts, overexpression of LT caused apoptosis. Although this activity of LT completely depended on Rb binding, LTs with mutations in the J domain remained able to kill. Comparisons of Rb(-) and J(-) LTs revealed additional differences. Wild-type but not Rb(-) LT activated the cyclin A promoter under serum starvation conditions. Genetic analysis of the promoter linked the Rb requirement to an E2F site in the promoter. LTs with mutations in the J domain were still able to activate the promoter. Finally, J mutant LTs caused changes in phosphorylation of both pRb and p130. In the case of p130, Thr-986 was shown to be a site that is regulated by J mutant LT. Taken together, these observations reveal that LT regulation of Rb function can be separated into both DnaJ-dependent and DnaJ-independent pathways.

The retinoblastoma protein alters the phosphorylation state of polyomavirus large T antigen in murine cell extracts and inhibits polyomavirus origin DNA replication

The retinoblastoma tumor suppressor protein (pRb) can associate with the transforming proteins of several DNA tumor viruses, including the large T antigen encoded by polyomavirus (Py T Ag). Although pRb function is critical for regulating progression from G1 to S phase, a role for pRb in S phase has not been demonstrated or excluded. To identify a potential effect of pRb on DNA replication, pRb protein was added to reaction mixtures containing Py T Ag, Py origin-containing DNA (Py ori-DNA), and murine FM3A cell extracts. We found that pRb strongly represses Py ori-DNA replication in vitro. Unexpectedly, however, this inhibition only partially depends on the interaction of pRb with Py T Ag, since a mutant Py T Ag (dl141) lacking the pRb interaction region was also significantly inhibited by pRb. This result suggests that pRb interferes with or alters one or more components of the murine cell replication extract. Furthermore, the ability of Py T Ag to be phosphorylated in such extracts is markedly reduced in the presence of pRb. Since cyclin-dependent kinase (CDK) phosphorylation of Py T Ag is required for its replication function, we hypothesize that pRb interferes with this phosphorylation event. Indeed, the S-phase CDK complex (cyclin A-CDK2), which phosphorylates both pRb and Py T Ag, alleviates inhibition caused by pRb. Moreover, hyperphosphorylated pRb is incapable of inhibiting replication of Py ori-DNA in vitro. We propose a new requirement for maintaining pRb phosphorylation in S phase, namely, to prevent deleterious effects on the cellular replication machinery.

Polyomavirus large T antigen induces alterations in cytoplasmic signalling pathways involving Shc activation

It has been extensively demonstrated that growth factors play a key role in the regulation of proliferation. Several lines of evidence support the hypothesis that for the induction of cell cycle progression in the absence of exogenous growth factors, oncogenes must either induce autocrine growth factor secretion or, alternatively, activate their receptors or their receptor substrates. Cells expressing polyomavirus large T antigen (PyLT) display reduced growth factor requirements, but the mechanisms underlying this phenomenon have yet to be explored. We conducted tests to see whether the reduction in growth factor requirements induced by PyLT was related to alterations of growth factor-dependent signals. To this end, we analyzed the phosphorylation status of a universal tyrosine kinase substrate, the transforming Shc adapter protein, in fibroblasts expressing the viral oncogene. We report that the level of Shc phosphorylation does not decrease in PyLT-expressing fibroblasts after growth factor withdrawal and that this PyLT-mediated effect does not require interaction with protein encoded by the retinoblastoma susceptibility gene. We also found that the chronic activation of the adapter protein is correlated with the binding of Shc to Grb-2 and with defects in the downregulation of mitogen-activated protein kinases. In fibroblasts expressing the nuclear oncoprotein, we also observed the formation of a PyLT-Shc complex that might be involved in constitutive phosphorylation of the adapter protein. Viewed comprehensively, these results suggest that the cell cycle progression induced by PyLT may depend not only on the direct inactivation of nuclear antioncogene products but also on the indirect induction, through the alteration of cytoplasmic pathways, of growth factor-dependent nuclear signals.

How do animal DNA viruses get to the nucleus?

Genome and pre-genome replication in all animal DNA viruses except poxviruses occurs in the cell nucleus (Table 1). In order to reproduce, an infecting virion enters the cell and traverses through the cytoplasm toward the nucleus. Using the cell's own nuclear import machinery, the viral genome then enters the nucleus through the nuclear pore complex. Targeting of the infecting virion or viral genome to the multiplication site is therefore an essential process in productive viral infection as well as in latent infection and transformation. Yet little is known about how infecting genomes of animal DNA viruses reach the nucleus in order to reproduce. Moreover, this nuclear locus for viral multiplication is remarkable in that the sizes and composition of the infectious particles vary enormously. In this article, we discuss virion structure, life cycle to reproduce infectious particles, viral protein's nuclear import signal, and viral genome nuclear targeting.

The DnaJ domain of polyomavirus large T antigen is required to regulate Rb family tumor suppressor function

Tumor suppressors of the retinoblastoma susceptibility gene family regulate cell growth and differentiation. Polyomavirus large T antigens (large T) bind Rb family members and block their function. Mutations of large T sequences conserved with the DnaJ family affect large T binding to a cellular DnaK, heat shock protein 70. The same mutations abolish large T activation of E2F-containing promoters and Rb binding-dependent large T activation of cell cycle progression. Cotransfection of a cellular DnaJ domain blocks wild-type large T action, showing that the connection between the chaperone system and tumor suppressors is direct. Although they are inactive in assays dependent on Rb family binding, mutants in the J region retain the ability to associate with pRb, p107, and p130. This suggests that binding of Rb family members by large T is not sufficient for their inactivation and that a functional J domain is required as well. This work connects the DnaJ and DnaK molecular chaperones to regulation of tumor suppressors by polyomavirus large T.

Phosphorylation sites in polyomavirus large T antigen that regulate its function in viral, but not cellular, DNA synthesis

Polyomavirus large T antigen (large T) is a highly phosphorylated protein that can be separated by proteolysis into two domains that have independent function. A cluster of phosphorylation sites was found in the protease-sensitive region connecting the N-terminal and C-terminal domains. Edman degradation of 32P-labeled protein identified serines 267, 271, and 274 and threonine 278 as sites of phosphorylation. Analysis of site-directed mutants confirmed directly that residues 271, 274, and 278 were phosphorylated. Threonine 278, shown here to be phosphorylated by cyclin/cyclin-dependent kinase activity, is required for viral DNA replication in either the full-length large T or C-terminal domain context. The serine phosphorylations are unimportant in the C-terminal domain context even though their mutations activates viral DNA replication in full-length large T. This finding suggests that these sites may function in relating the two domains to each other. Although the phosphorylation sites were involved in viral DNA replication, none was important for the ability of large T to drive cellular DNA replication as measured by bromodeoxyuridine incorporation, and they did not affect large T interactions with the Rb tumor suppressor family.