Characterization of cytopathic factors through genome-wide analysis of the Zika viral proteins in fission yeast.

The Zika virus (ZIKV) causes microcephaly and the Guillain-Barré syndrome. Little is known about how ZIKV causes these conditions or which ZIKV viral protein(s) is responsible for the associated ZIKV-induced cytopathic effects, including cell hypertrophy, growth restriction, cell-cycle dysregulation, and cell death. We used fission yeast for the rapid, global functional analysis of the ZIKV genome. All 14 proteins or small peptides were produced under an inducible promoter, and we measured the intracellular localization and the specific effects on ZIKV-associated cytopathic activities of each protein. The subcellular localization of each ZIKV protein was in overall agreement with its predicted protein structure. Five structural and two nonstructural ZIKV proteins showed various levels of cytopathic effects. The expression of these ZIKV proteins restricted cell proliferation, induced hypertrophy, or triggered cellular oxidative stress leading to cell death. The expression of premembrane protein (prM) resulted in cell-cycle G1 accumulation, whereas membrane-anchored capsid (anaC), membrane protein (M), envelope protein (E), and nonstructural protein 4A (NS4A) caused cell-cycle G2/M accumulation. A mechanistic study revealed that NS4A-induced cellular hypertrophy and growth restriction were mediated specifically through the target of rapamycin (TOR) cellular stress pathway involving Tor1 and type 2A phosphatase activator Tip41. These findings should provide a reference for future research on the prevention and treatment of ZIKV diseases.

Plant geminivirus rep protein induces rereplication in fission yeast.

The replication-associated protein (Rep) of geminiviruses, single-stranded DNA viruses of higher plants, is essential for virus replication. Since these viruses do not encode their own polymerases, Rep induces differentiated plant cells to reenter the cell cycle by interacting with the plant homologues of retinoblastoma proteins in order to activate the host DNA synthesis machinery. We have used fission yeast (Schizosaccharomyces pombe) as a model organism to analyze the impact of ectopically expressed African cassava mosaic virus Rep protein on the cell division cycle in closer detail. Upon expression, Rep showed its characteristic DNA cleavage activity, and about 10% of the cells exhibited morphological changes. They were elongated threefold, on average, and possessed a single but enlarged and less compact nucleus in comparison to noninduced or vector-only control cells. Flow cytometry of Rep-expressing cultures revealed a distinct subpopulation of Rep protein-containing cells with aberrant morphology. The other 90% of the cells were indistinguishable from control cells, and no Rep was detectable. Rep-expressing cells exhibited DNA contents beyond 2C, indicating ongoing replication without intervening mitosis. Because a second open reading frame (ORF), AC4, is present within the Rep gene, the role of AC4 was examined by destroying its start codon within the AC1 ORF. The results confirmed that Rep is necessary and sufficient to induce rereplication in fission yeast. The unique potential of this well-investigated model for dissecting the cell cycle control by geminiviral proteins is discussed.

Oxidative stress induced by HIV-1 F34IVpr in Schizosaccharomyces pombe is one of its multiple functions.

Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) exerts multiple effects on viral and host cellular activities during infection, including induction of cell cycle G(2) arrest and cell death in both human and the fission yeast Schizosaccharomyces pombe cells. In this study, a mutant derivative of Vpr (F34IVpr), which causes transient G2 arrest with little or no effect of cell killing, was used to study the molecular impact of Vpr on cellular oxidative stress responses in S. pombe. We demonstrated here that F34IVpr triggers low level of complex and atypical oxidative stress responses in comparison with its parental strain SP223 in early (14-h) and late (35-h) log phase cultures. Specifically, F34IVpr production in S. pombe causes significantly elevated levels of reactive oxygen species such as superoxide and peroxides; meanwhile, it also induces decreased levels of glutathione, hydroxyl radical concentrations and specific enzyme activities such as those of antioxidant enzymes including superoxide dismutases, catalase, glutathione peroxidase, glutathione reductase, glucose-6-phosphate dehydrogenase and glutathione transferase. These observations may provide functional insights into the significance of Vpr-induced oxidative stress as part of the multifaceted functions of Vpr, and contribute to the development of future new strategies aimed to reduce the adverse Vpr-mediated effects in HIV-infected patients.

The retrotransposon Tf1 assembles virus-like particles that contain excess Gag relative to integrase because of a regulated degradation process.

The retrotransposon Tf1, isolated from Schizosaccharomyces pombe, contains a single open reading frame with sequences encoding Gag, protease, reverse transcriptase, and integrase (IN). Tf1 has previously been shown to possess significant transposition activity. Although Tf1 proteins do assemble into virus-like particles, the assembly does not require readthrough of a translational reading frame shift or stop codon, common mechanisms used by retroelements to express Gag in molar excess of the polymerase proteins. This study was designed to determine if Tf1 particles contain equal amounts of Gag and polymerase proteins or whether they contain the typical molar excess of Gag. After using two separate methods to calibrate the strength of our antibodies, we found that both S. pombe extracts and partially purified Tf1 particles contained a 26-fold molar excess of Gag relative to IN. Knowing that Gag and IN are derived from the same Tf1 primary translation product, we concluded that the excess Gag most likely resulted from specific degradation of IN. We obtained evidence of regulated IN degradation in comparisons of Tf1 protein extracted from log-phase cells and that extracted from stationary-phase cells. The log-phase cells contained equal molar amounts of Gag and IN, whereas cells approaching stationary phase rapidly degraded IN, leaving an excess of Gag. Analysis of the reverse transcripts indicated that the bulk of reverse transcription occurred within the particles that possess a molar excess of Gag.

The dose-dependent H2O2 stress response promotes increased survival for Schizosaccharomyces pombe cells expressing HIV-1 Vpr.

Human immunodeficiency virus type 1 (HIV-1) viral protein R (Vpr) exerts multiple effects on viral and host cellular activities during infection, including induction of the cell cycle G2 arrest, and cell death in both human cells and the fission yeast Schizosaccharomyces pombe. We show that treament of exponential-phase wild-type Vpr-expressing S. pombe cells with a low, subinhibitory concentration (0.15 mmol/L) of hydrogen peroxide and 0.1 mmol/L thiamine significantly increased both cell proliferation and survival rates and decreased the number of elongated G2-arrested cells. Short-term, H2O2-induced adaptive stress increased the survival of the cells while acute stress conditions interrupted the Vpr-mediated death of the cells; however, no changes in cell length or cell phase were detected. The results suggest the importance of the oxidative status of the cells in Vpr-mediated processes. Our findings contribute to the development of a new approach via which to investigate the contribution of Vpr to HIV pathogenesis and to reduce the Vpr-mediated effects in HIV-infected patients.

Translation, modification and cellular distribution of two AC4 variants of African cassava mosaic virus in yeast and their pathogenic potential in plants.

Plant infecting geminiviruses encode a small (A)C4 protein within the open reading frame of the replication-initiator protein. In African cassava mosaic virus, two in-frame start codons may be used for the translation of a longer and a shorter AC4 variant. Both were fused to green fluorescent protein or glutathione-S-transferase genes and expressed in fission yeast. The longer variant accumulated in discrete spots in the cytoplasm, whereas the shorter variant localized to the plasma membrane. A similar expression pattern was found in plants. A myristoylation motif may promote a targeting of the shorter variant to the plasma membrane. Mass spectrometry analysis of the yeast-expressed shorter variant detected the corresponding myristoylation. The biological relevance of the second start codon was confirmed using mutated infectious clones. Whereas mutating the first start codon had no effect on the infectivity in Nicotiana benthamiana plants, the second start codon proved to be essential.

Expression of human papillomavirus 16 E2 protein in Schizosaccharomyces pombe delays the initiation of mitosis.

Infection by some types of human papillomavirus (HPV) is associated with the development of cervical cancer. Analysis of viral DNA from cervical tumours shows that the E2 gene is frequently disrupted during integration into the host cell's DNA. It has therefore been suggested that loss of E2p is an important step in malignant transformation. Expression of E2p in the fission yeast Schizosaccharomyces pombe retards the G2-M transition, by delaying activation of Cdc2p kinase. In contrast, S phase progression, and commitment to cell division in late G1 are not affected. The delay is independent of the transcriptional trans-activation function of E2p, and does not result from E2p DNA binding mimicking DNA damage. Increased expression of E2p also delays mitotic initiation in mammalian cells. S. pombe may thus provide a simple model for the analysis of E2p function.

[Studies on HIV/AIDS using the fission yeast Schizosaccharomyces pombe].

Human immunodeficiency virus (HIV) is a causative agent of acquired immunodeficiency syndrome (AIDS) and a member of Retrovirus family. The name of "retrovirus" is said to be derived from "reverse-transcribing oncogenic virus." Living up to its name, retrovirus has contributed to oncology, especially in the field of cancer pathogenesis. Retrovirus research has led to discovery of a number of oncogenes as well. Since the discovery of HIV in 1983, however, retrovirus has also been considered as an important etiologic agent that could incapacitate the cells involved in immune responses. As of the end of 2004, the number of people living with HIV/AIDS is estimated to be as large as 40 million. Every year, nearly 5 million people are newly infected with HIV, and 3 million people die of AIDS in the world, mainly in Africa and southeastern and southern Asia. Despite extensive studies, detailed mechanisms of HIV pathogenesis are still unclear, and efforts are being made to clarify functions of various HIV proteins and identify the cellular factors that could interact with the HIV proteins. One of the HIV accessory proteins, Vpr, causes the host cell cycle arrest at G2 phase, which may play an important pathogenic role in AIDS induction. Exploiting the fission yeast Schizosaccharomyces pombe useful for cell cycle studies, I've been trying to elucidate the mechanism by which Vpr induces the G2 arrest as presented in this review.

The RXL motif of the African cassava mosaic virus Rep protein is necessary for rereplication of yeast DNA and viral infection in plants.

Geminiviruses, single-stranded DNA plant viruses, encode a replication-initiator protein (Rep) that is indispensable for virus replication. A potential cyclin interaction motif (RXL) in the sequence of African cassava mosaic virus Rep may be an alternative link to cell cycle controls to the known interaction with plant homologs of retinoblastoma protein (pRBR). Mutation of this motif abrogated rereplication in fission yeast induced by expression of wildtype Rep suggesting that Rep interacts via its RXL motif with one or several yeast proteins. The RXL motif is essential for viral infection of Nicotiana benthamiana plants, since mutation of this motif in infectious clones prevented any symptomatic infection. The cell-cycle link (Clink) protein of a nanovirus (faba bean necrotic yellows virus) was investigated that activates the cell cycle by binding via its LXCXE motif to pRBR. Expression of wildtype Clink and a Clink mutant deficient in pRBR-binding did not trigger rereplication in fission yeast.

The application of a homologous recombination assay revealed amino acid residues in an LTR-retrotransposon that were critical for integration.

Retroviruses and their relatives, the LTR-retrotransposons, possess an integrase protein (IN) that is required for the insertion of reverse transcripts into the genome of host cells. Schizosaccharomyces pombe is the host of Tf1, an LTR-retrotransposon with integration activity that can be studied by using techniques of yeast genetics. In this study, we sought to identify amino acid substitutions in Tf1 that specifically affected the integration step of transposition. In addition to seeking amino acid substitutions in IN, we also explored the possibility that other Tf1 proteins contributed to integration. By comparing the results of genetic assays that monitored both transposition and reverse transcription, we were able to seek point mutations throughout Tf1 that blocked transposition but not the synthesis of reverse transcripts. These mutant versions of Tf1 were candidates of elements that possessed defects in the integration step of transposition. Five mutations in Tf1 that resulted in low levels of integration were found to be located in the IN protein: two substitutions in the N-terminal Zn domain, two in the catalytic core, and one in the C-terminal domain. These results suggested that each of the three IN domains was required for Tf1 transposition. The potential role of these five amino acid residues in the function of IN is discussed. Two of the mutations that reduced integration mapped to the RNase H (RH) domain of Tf1 reverse transcriptase. The Tf1 elements with the RH mutations produced high levels of reverse transcripts, as determined by recombination and DNA blot analysis. These results indicated that the RH of Tf1 possesses a function critical for transposition that is independent of the accumulation of reverse transcripts.

A general purification protocol for E7 proteins from "high- and low-risk" human papillomavirus types expressed in the yeast Schizosaccharomyces pombe.

A purification protocol was developed to obtain human papillomavirus (HPV) type 16 E7 protein expressed in the yeast Schizosaccharomyces pombe. Only three chromatographic steps were necessary to purify the unfused HPV 16 E7 protein to homogeneity (95-99%) as shown by silver staining after polyacrylamide gel electrophoresis. Approximately 0.8 mg of highly purified E7 was obtained from 5 x 10(10) yeast cells. The purified HPV 16 E7 phosphoprotein (Ser 31/32) was refolded and assayed for functionality. Binding to the proteins Rb1 and p107 in vitro and induction of DNA synthesis after microinjection into serum-deprived NIH 3T3 cells suggest that the E7 protein retains some of its biological activities. Most importantly, the purification strategy is also applicable for different HPV 16 E7 mutants and for E7 proteins from other HPV types such as HPV 18 and 11.