Simian virus 40 small-t antigen binds two zinc ions

Iron‑sulfur clusters in viral proteins: Exploring their elusive nature, roles and new avenues for targeting infections

Viruses have evolved complex mechanisms to exploit host factors for replication and assembly. In response, host cells have developed strategies to block viruses, engaging in a continuous co-evolutionary battle. This dynamic interaction often revolves around the competition for essential resources necessary for both host cell and virus replication. Notably, iron, required for the biosynthesis of several cofactors, including iron‑sulfur (FeS) clusters, represents a critical element in the ongoing competition for resources between infectious agents and host. Although several recent studies have identified FeS cofactors at the core of virus replication machineries, our understanding of their specific roles and the cellular processes responsible for their incorporation into viral proteins remains limited. This review aims to consolidate our current knowledge of viral components that have been characterized as FeS proteins and elucidate how viruses harness these versatile cofactors to their benefit. Its objective is also to propose that viruses may depend on incorporation of FeS cofactors more extensively than is currently known. This has the potential to revolutionize our understanding of viral replication, thereby carrying significant implications for the development of strategies to target infections.

Functional Domains of the Early Proteins and Experimental and Epidemiological Studies Suggest a Role for the Novel Human Polyomaviruses in Cancer

As their name indicates, polyomaviruses (PyVs) can induce tumors. Mouse PyV, hamster PyV and raccoon PyV have been shown to cause tumors in their natural host. During the last 30 years, 15 PyVs have been isolated from humans. From these, Merkel cell PyV is classified as a Group 2A carcinogenic pathogen (probably carcinogenic to humans), whereas BKPyV and JCPyV are class 2B (possibly carcinogenic to humans) by the International Agency for Research on Cancer. Although the other PyVs recently detected in humans (referred to here as novel HPyV; nHPyV) share many common features with PyVs, including the viral oncoproteins large tumor antigen and small tumor antigen, as their role in cancer is questioned. This review discusses whether the nHPyVs may play a role in cancer based on predicted and experimentally proven functions of their early proteins in oncogenic processes. The functional domains that mediate the oncogenic properties of early proteins of known PyVs, that can cause cancer in their natural host or animal models, have been well characterized and we examined whether these functional domains are conserved in the early proteins of the nHPyVs and presented experimental evidence that these conserved domains are functional. Furthermore, we reviewed the literature describing the detection of nHPyV in human tumors.

The Oncogenic Small Tumor Antigen of Merkel Cell Polyomavirus Is an Iron-Sulfur Cluster Protein That Enhances Viral DNA Replication

Merkel cell polyomavirus (MCPyV) plays an important role in Merkel cell carcinoma (MCC). MCPyV small T (sT) antigen has emerged as the key oncogenic driver in MCC carcinogenesis. It has also been shown to promote MCPyV LT-mediated replication by stabilizing LT. The importance of MCPyV sT led us to investigate sT functions and to identify potential ways to target this protein. We discovered that MCPyV sT purified from bacteria contains iron-sulfur (Fe/S) clusters. Electron paramagnetic resonance analysis showed that MCPyV sT coordinates a [2Fe-2S] and a [4Fe-4S] cluster. We also observed phenotypic conservation of Fe/S coordination in the sTs of other polyomaviruses. Since Fe/S clusters are critical cofactors in many nucleic acid processing enzymes involved in DNA unwinding and polymerization, our results suggested the hypothesis that MCPyV sT might be directly involved in viral replication. Indeed, we demonstrated that MCPyV sT enhances LT-mediated replication in a manner that is independent of its previously reported ability to stabilize LT. MCPyV sT translocates to nuclear foci containing actively replicating viral DNA, supporting a direct role for sT in promoting viral replication. Mutations of Fe/S cluster-coordinating cysteines in MCPyV sT abolish its ability to stimulate viral replication. Moreover, treatment with cidofovir, a potent antiviral agent, robustly inhibits the sT-mediated enhancement of MCPyV replication but has little effect on the basal viral replication driven by LT alone. This finding further indicates that MCPyV sT plays a direct role in stimulating viral DNA replication and introduces cidofovir as a possible drug for controlling MCPyV infection.

IMPORTANCE

MCPyV is associated with a highly aggressive form of skin cancer in humans. Epidemiological surveys for MCPyV seropositivity and sequencing analyses of healthy human skin suggest that MCPyV may represent a common component of the human skin microbial flora. However, much of the biology of the virus and its oncogenic ability remain to be investigated. In this report, we identify MCPyV sT as a novel Fe/S cluster protein and show that conserved cysteine clusters are important for sT's ability to enhance viral replication. Moreover, we show that sT sensitizes MCPyV replication to cidofovir inhibition. The discovery of Fe/S clusters in MCPyV sT opens new avenues to the study of the structure and functionality of this protein. Moreover, this study supports the notion that sT is a potential drug target for dampening MCPyV infection.

Update on human polyomaviruses and cancer

Over 50 years of polyomavirus research has produced a wealth of insights into not only general biologic processes in mammalian cells, but also, how conditions can be altered and signaling systems tweaked to produce transformation phenotypes. In the past few years three new members (KIV, WUV, and MCV) have joined two previously known (JCV and BKV) human polyomaviruses. In this review, we present updated information on general virologic features of these polyomaviruses in their natural host, concentrating on the association of MCV with human Merkel cell carcinoma. We further present a discussion on advances made in SV40 as the prototypic model, which has and will continue to inform our understanding about viruses and cancer.

Small tumor antigen of polyomaviruses: role in viral life cycle and cell transformation

The regulatory proteins of polyomaviruses, including small and large T antigens, play important roles, not only in the viral life cycle but also in virus-induced cell transformation. Unlike many other tumor viruses, the transforming proteins of polyomaviruses have no cellular homologs but rather exert their effects mostly by interacting with cellular proteins that control fundamental processes in the regulation of cell proliferation and the cell cycle. Thus, they have proven to be valuable tools to identify specific signaling pathways involved in tumor progression. Elucidation of these pathways using polyomavirus transforming proteins as tools is critically important in understanding fundamental regulatory mechanisms and hence to develop effective therapeutic strategies against cancer. In this short review, we will focus on the structural and functional features of one polyomavirus transforming protein, that is, the small t-antigen of the human neurotropic JC virus (JCV) and the simian virus, SV40.

Structural basis of PP2A inhibition by small t antigen

The SV40 small t antigen (ST) is a potent oncoprotein that perturbs the function of protein phosphatase 2A (PP2A). ST directly interacts with the PP2A scaffolding A subunit and alters PP2A activity by displacing regulatory B subunits from the A subunit. We have determined the crystal structure of full-length ST in complex with PP2A A subunit at 3.1 Å resolution. ST consists of an N-terminal J domain and a C-terminal unique domain that contains two zinc-binding motifs. Both the J domain and second zinc-binding motif interact with the intra-HEAT-repeat loops of HEAT repeats 3–7 of the A subunit, which overlaps with the binding site of the PP2A B56 subunit. Intriguingly, the first zinc-binding motif is in a position that may allow it to directly interact with and inhibit the phosphatase activity of the PP2A catalytic C subunit. These observations provide a structural basis for understanding the oncogenic functions of ST.

The study of how DNA tumor viruses induce malignant transformation has led to the identification of key pathways that also play a role in spontaneously arising cancers. One such virus, simian virus 40 (SV40), produces two proteins, the large T and small t antigens, that bind and inactivate tumor suppressor genes important for cell transformation. Specifically, SV40 small t antigen (ST) binds to and perturbs the function of the abundant protein phosphatase 2A (PP2A). PP2A is a family of heterotrimeric enzymes, composed of a structural A subunit, a catalytic C subunit, and one of several regulatory B subunits. Here we have determined the structure of SV40 ST in complex with the PP2A structural subunit Aα. SV40 ST consists of an N-terminal J domain and a C-terminal unique domain that contains two separate zinc-binding motifs. SV40 ST binds to the same region of PP2A as the regulatory subunit B56, which provides a structural explanation for the displacement of regulatory B subunits by SV40 ST. Taken together, these observations provide a structural basis for understanding the oncogenic functions of ST.

The crystal structure of full-length SV40 small t antigen (ST) in complex with the A subunit of its target, protein phosphatase 2A, contributes to our understanding of the oncogenic functions of ST.

SV40 reporter viruses

Three simian virus 40 (SV40) reporter viruses were constructed in this study. One expresses the green fluorescent protein (GFP) as a fusion protein with the first exon of large-T (LT) antigen and is useful for live-cell imaging. A second reporter virus has a FLAG epitope tag at the C-terminus of large-T antigen (vC-LT(FLAG)), and a third has the FLAG tag at the N-terminus of LT (vN-LT(FLAG)). The vC-LT(FLAG) construct grows to titers near those of wild-type (WT) virus and functions well as a reporter virus for SV40 infection. The vN-LT(FLAG) construct, while viable, has a defect in the production and spread of infectious particles. All three viruses are useful in detecting superinfecting virus in cells in which nuclear LT is already present, such as persistently infected human mesothelial cells.

Structural and biochemical insights into the regulation of protein phosphatase 2A by small t antigen of SV40

The small t antigen (ST) of DNA tumor virus SV40 facilitates cellular transformation by disrupting the functions of protein phosphatase 2A (PP2A) through a poorly defined mechanism. The crystal structure of the core domain of SV40 ST bound to the scaffolding subunit of human PP2A reveals that the ST core domain has a novel zinc-binding fold and interacts with the conserved ridge of HEAT repeats 3-6, which overlaps with the binding site for the B' (also called PR61 or B56) regulatory subunit. ST has a lower binding affinity than B' for the PP2A core enzyme. Consequently, ST does not efficiently displace B' from PP2A holoenzymes in vitro. Notably, ST inhibits PP2A phosphatase activity through its N-terminal J domain. These findings suggest that ST may function mainly by inhibiting the phosphatase activity of the PP2A core enzyme, and to a lesser extent by modulating assembly of the PP2A holoenzymes.

Molecular biology of malignant mesothelioma: a review

Malignant mesothelioma (MM) is an uncommon tumor with high mortality and morbidity rates. It arises from mesothelial cells that line the pleural, pericardial, peritoneal, and testicular cavities. This is a disease with an indolent course because tumors arise 20 to 40 years after exposure to an inciting agent. Extensive research has shown that mesothelial cells are transformed into MM cells through various chromosomal and cellular pathway defects. These changes alter the normal cells' ability to survive, proliferate, and metastasize. This article discusses the alterations that occur in transforming normal mesothelial cells into MM. It also details some of the signal transduction pathways that seem to be important in MM with the potential for novel targeted therapeutics.

Simian virus 40 small t antigen mediates conformation-dependent transfer of protein phosphatase 2A onto the androgen receptor

The tumor antigens simian virus 40 small t antigen (ST) and polyomavirus small and medium T antigens mediate cell transformation in part by binding to the structural A subunit of protein phosphatase 2A (PP2A). The replacement of B subunits by tumor antigens inhibits PP2A activity and prolongs phosphorylation-dependent signaling. Here we show that ST mediates PP2A A/C heterodimer transfer onto the ligand-activated androgen receptor (AR). Transfer by ST is strictly dependent on the agonist-activated conformation of AR, occurs within minutes of the addition of androgen to cells, and can occur in either the cytoplasm or the nucleus. The binding of ST changes the conformation of the A subunit, and ST rapidly dissociates from the complex upon PP2A A/C heterodimer binding to AR. PP2A is transferred onto the carboxyl-terminal half of AR, and the phosphatase activity is directed to five phosphoserines in the amino-terminal activation function region 1, with a corresponding reduction in transactivation. Thus, ST functions as a transfer factor to specify PP2A targeting in the cell and modulates the transcriptional activity of AR.

The role of the SV40 ST antigen in cell growth promotion and transformation

The simian virus 40 small-t (ST) antigen plays a key role in permissive and nonpermissive infections, increasing virus yields in lytic cycles of primate cells and enhancing the ability of large-T (LT) to transform rodent or even human cells. In the absence of ST, tumors in rodent model systems appear primarily in lymphoid and other proliferative tissues and transformation is reduced in several in vitro systems. The functions of ST largely reflect its binding and inhibition of protein phosphatase 2A, although a recently described dnaJ domain also contributes to its biology. The dnaJ domain is present in LT and a third early gene product, the 17kT protein, for which a potential role in transformation deserves further evaluation.

Binding specificity of protein phosphatase 2A core enzyme for regulatory B subunits and T antigens

The core enzyme of protein phosphatase 2A is composed of a regulatory subunit A and a catalytic subunit C. It is controlled by three types of regulatory B subunits (B, B', and B") and by tumor (T) antigens, which are unrelated by sequence but bind to overlapping regions on the A subunit. To find out whether the different B subunits and T antigens bind to identical or distinct amino acids of the A subunit, mutants were generated and their abilities to bind B subunits and T antigens were tested. We found that some amino acids are involved in the binding of all types of B subunits, whereas others are specifically involved in the binding of one or two types of B subunits. T-antigen-binding specificity does not correlate with that of a particular type of B subunit.

Identification of structural elements involved in the interaction of simian virus 40 small tumor antigen with protein phosphatase 2A

SV40 small tumor antigen (small-t) was used as a model to identify structural elements involved in the interactions between regulatory proteins and protein phosphatase 2A (PP2A). Using mutant proteins and synthetic peptides, we identified a small domain within small-t that is a major site for interaction with the dimeric form of PP2A. A series of small-t truncation mutants identified a region surrounding the first of two conserved cysteine clusters that was critical for interaction with PP2A. These mutants also identified additional regions of small-t that contribute to high affinity interaction. Deletion of residues 110-119, which encompass the first cysteine cluster, resulted in a protein that failed to bind to PP2A. Synthetic peptides that contained residues 105-122 of small-t blocked binding of small-t to PP2A. These peptides also inhibited the phosphatase activity of PP2A in a manner analogous to full-length small-t. The active small-t peptides adopt a beta-strand structure that was essential for high affinity interaction with the PP2A dimer. Based on circular dichroism measurements, the same cysteine cluster-containing peptides that bind to PP2A also interact with zinc. Interaction with zinc required the conserved cysteines but was not required for interaction with PP2A.

The amino-terminal transforming region of simian virus 40 large T and small t antigens functions as a J domain

Simian virus 40 (SV40) encodes two proteins, large T antigen and small t antigen that contribute to virus-induced tumorigenesis. Both proteins act by targeting key cellular regulatory proteins and altering their function. Known targets of the 708-amino-acid large T antigen include the three members of the retinoblastoma protein family (pRb, p107, and p130), members of the CBP family of transcriptional adapter proteins (cap-binding protein [CBP], p300, and p400), and the tumor suppressor p53. Small t antigen alters the activity of phosphatase pp2A and transactivates the cyclin A promoter. The first 82 amino acids of large T antigen and small t antigen are identical, and genetic experiments suggest that an additional target(s) important for transformation interacts with these sequences. This region contains a motif similar to the J domain, a conserved sequence found in the DnaJ family of molecular chaperones. We show here that mutations within the J domain abrogate the ability of large T antigen to transform mammalian cells. To examine whether a purified 136-amino-acid fragment from the T antigen amino terminus acts as a DnaJ-like chaperone, we investigated whether this fragment stimulates the ATPase activity of two hsc70s and discovered that ATP hydrolysis is stimulated four- to ninefold. In addition, ATPase-defective mutants of full-length T antigen, as well as wild-type small t antigen, stimulated the ATPase activity of hsc70. T antigen derivatives were also able to release an unfolded polypeptide substrate from an hsc70, an activity common to DnaJ chaperones. Because the J domain of T antigen plays essential roles in viral DNA replication, transcriptional control, virion assembly, and tumorigenesis, we conclude that this region may chaperone the rearrangement of multiprotein complexes.

A novel simian virus 40 early-region domain mediates transactivation of the cyclin A promoter by small-t antigen and is required for transformation in small-t antigen-dependent assays

At least three regions of the simian virus 40 small-t antigen (small-t) contribute to the protein's ability to enhance cellular transformation. As we showed previously for rat F111 cells, one region includes sequences from residues 97 to 103 that are involved in the binding and inhibition of protein phosphatase 2A. In the present study, the role of the protein phosphatase 2A binding region was confirmed in two additional small-t-dependent transformation systems. Second, small-t was found to provide a function previously identified as a large-T transformation domain. Mutations in residues 19 to 28 of large-T affected its transforming ability, but these mutations were complemented by a wild-type small-t. A third region of small-t was also required for efficient transformation. This region, the 42-47 region, is shared by large-T and small-t and contains a conserved HPDKGG hexapeptide. The 42-47 region function could be provided by either small-t or large-T in small-t-dependent systems. Mutations in the 42-47 region reduced the ability of small-t to transactivate the cyclin A promoter, of interest because small-t increased endogenous cyclin A mRNA levels in both human and monkey cells, as well as transactivating the promoter in transient assays.

Zinc-binding and protein-protein interactions mediated by the polyomavirus large T antigen zinc finger

Polyomavirus large tumor antigen (LT) contains a potential C2H2 zinc binding element between residues 452 and 472. LT also contains a third histidine in this region, conserved among the polyomavirus LTs. Synthetic peptides of this region bound a single atom of zinc, as determined by spectroscopic analysis. Blotting experiments also showed that fusion proteins containing the element, as well as full-length LT, bound 65Zn. Polyomavirus middle T and small T antigens also bound zinc in the blotting assay. Site-directed mutagenesis showed the importance of this element in LT. Point mutations in four of the conserved residues (C-452, C-455, H-465, and H-469) blocked the ability of LT to function in viral DNA replication, while mutation of H-472-->L decreased replication to 1/30th that of the wild type. Point mutations in intervening residues tested had little effect on replication. Mutants resulting from mutations in the conserved cysteine or histidine residues retained the ability to bind origin DNA. However, they did show a defect in self-association. Because double-hexamer formation is involved in DNA replication, this deficiency is sufficient to explain the defect in replication. Mutants created by point mutations of the coordinating residues were also deficient in replication-associated phosphorylations.

Mutations in a CCHC zinc-binding motif of the reovirus sigma 3 protein decrease its intracellular stability

It has been demonstrated that the sigma 3 protein of reovirus harbors a zinc-binding domain in its amino-terminal portion. A putative zinc finger in the CCHH form is located in this domain and was considered to be a good candidate for the zinc-binding motif. We performed site-directed mutagenesis to substitute amino acids in this region and demonstrated that many of these mutants, although expressed in COS cells, were unstable compared with the wild-type protein. Further analysis revealed that zinc-binding capability, as measured by retention on a zinc chelate affinity adsorbent, correlates with stability. These studies also allowed us to identify a CCHC box as the most probable zinc-binding motif.

Simian virus 40 small-t antigen stimulates viral DNA replication in permissive monkey cells

The simian virus 40 (SV40) large-T antigen is essential for SV40 DNA replication and for late viral gene expression, but the role of the SV40 small-t antigen in these processes is still unclear. We have previously demonstrated that small t inhibits SV40 DNA replication in vitro. In this study, we investigated the effect of small t on SV40 replication in cultured cells. CV1 monkey cell infection experiments indicated that mutant viruses that lack small t replicate less efficiently than the wild-type virus. We next microinjected CV1 cells with SV40 DNA with and without purified small-t protein and analyzed viral DNA replication efficiency by Southern blotting. Replication of either wild-type SV40 or small-t deletion mutant DNA was increased three- to fivefold in cells coinjected with purified small t. Thus, in contrast to our in vitro observation, small t stimulated viral DNA replication in vivo. This result suggests that small t has cellular effects that are not detectable in a reconstituted in vitro replication system. We also found that small t stimulated progression of permissive monkey cells--but not of nonpermissive rodent cells--from G0-G1 to the S phase of the cell cycle, possibly leading to an optimal intracellular environment for viral replication.

Mutations which affect the inhibition of protein phosphatase 2A by simian virus 40 small-t antigen in vitro decrease viral transformation

Three independent point mutations within residues 97 to 103 of the simian virus 40-small-t antigen (small-t) greatly reduced the ability of purified small-t to inhibit protein phosphatase 2A in vitro. These mutations affected the interaction of small-t antigen with the protein phosphatase 2A A subunit translated in vitro, and a peptide from the region identified by these mutations released the A subunit from immune complexes. When introduced into virus, the mutations eliminated the ability of small-t to enhance viral transformation of growth-arrested rat F111 cells. In contrast, the mutant small-t antigens were unimpaired in the transactivation of the adenovirus E2 promoter, an activity which was reduced by a double mutation in small-t residues 43 and 45.

The interaction of SV40 small tumor antigen with protein phosphatase 2A stimulates the map kinase pathway and induces cell proliferation

Interaction with SV40 small tumor antigen (small t) compromised the ability of multimeric protein phosphatase 2A to inactivate the mitogen-activated protein kinase ERK1 and the mitogen-activated protein kinase kinase MEK1. Transient expression of small t in CV-1 cells activated MEK and ERK but did not affect Raf activity. Small t stimulated the growth of quiescent CV-1 cells almost as effectively as did serum. Coexpression of kinase-deficient ERK2 blocked most, but not all, of the proliferation caused by small t. Activation of the mitogen-activated protein kinase pathway and stimulation of cell growth were dependent on the interaction of small t with protein phosphatase 2A. These findings indicate that SV40 small t is capable of inducing cell growth through blockade of protein phosphatase and deregulation of the mitogen-activated protein kinase cascade.