Department of Molecular and Cellular Biology, Centro Nacional de Biotecnología, Consejo Superior de Investigaciones Científicas (CSIC), Campus Universidad Autónoma, 28049 Madrid, Spain1
Laboratorio de Inmunopatología del SIDA, Centro de Biología Fundamental, Instituto de Salud Carlos III, Carretera de Majadahonda a Pozuelo km.2, 28220, Majadahonda, Madrid, Spain2
Author for correspondence: Mariano Esteban. Fax +34 91 585 4506. e-mail mesteban{at}cnb.uam.es
![]() |
Abstract |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Molluscum contagiosum virus (MCV) is a member of the poxvirus family, provoking benign skin tumours in humans (Buller, 1999 ). Despite extensive homologies, with regard to structural proteins, with the most characterized member of the poxvirus family, VV, factors involved in modulation of host responses to infection are not conserved between MCV and VV (Senkevich et al., 1996
, 1997
). In particular, the sequence of MCV contains neither an E3L nor a K3L homologue. In contrast, MCV encodes a set of regulators of host response (for a review see Moss et al., 2000
), such as a glutathione peroxidase, involved in prevention of ultraviolet-induced cell death (MC066L; Shisler et al., 1998
), a protein involved in inhibiting chemokine action (MC148R; Krathwohl et al., 1997
; Damon et al., 1998
), another protein that modulates the action of steroid receptors (Chen et al., 2000
) and at least three genes belonging to a family of IL-18 binding proteins (Xiang & Moss, 1999a
, b
). In addition, MCV encodes two proteins, MC159L and MC160L, containing death effector domains (DEDs), which belong to a family of cellular and viral FLIP (vFLIP)-like proteins (Bertin et al., 1997
; Hu et al., 1997
; Thome et al., 1997
). Although proteins MC159L and MC160L have a similar structure, only MC159L has been shown to inhibit apoptosis induced in response to death receptor FADD (Bertin et al., 1997
; Hu et al., 1997
; Thome et al., 1997
). MC159L physically interacts with FADD (Bertin et al., 1997
; Hu et al., 1997
; Thome et al., 1997
), thus explaining its physiological function. In addition, recent work (Chaudhary et al., 1999
) has suggested that this FLIP-like viral protein also inhibits NF-
B induction, triggered by activation of caspase 8 and its homologues. Since PKR is able to induce NF-
B activation and to trigger a translational block through phosphorylation of the small subunit of the initiation factor eIF-2 (eIF-2
), in this investigation we have examined the ability of MC159L to modulate PKR-induced pathways. We have evaluated whether MC159L directly interacts with PKR acting as a general inhibitor of PKR or whether its action is limited to a more specific interference with PKR-triggered pathways downstream of PKR activation.
![]() |
Methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Plasmids.
The DNA from the MC159L protein of MCV was amplified by PCR from DNA obtained from a patient with MCV lesions, and cloned in the VV insertional vector pHLZ as previously described (Gil & Esteban, 2000 ).
Cells and viruses.
African green monkey kidney cells BSC-40 (ATCC CCL-26) were grown in Dulbeccos modified Eagles medium (DMEM) supplemented with 10% heat-inactivated newborn calf serum (NCS). HeLa cells (ECACC 85060701) were grown in DMEM supplemented with 10% NCS. After mock inoculation or virus adsorption, cells were maintained in DMEM supplemented with 2% NCS or 2% FCS.
The recombinant VV (rVV) expressing IPTG-inducible PKR (VV PKR) has been previously described (Lee & Esteban, 1993 ). VV MC159L, a rVV constitutively expressing an Xpress epitope N-terminal-tagged MC159L coding fragment, and VV PKR-MC159L, which expresses IPTG-inducible PKR and MC159L constitutively, have been described before (Gil & Esteban, 2000
).
Gel retardation assay.
Nuclear extracts were prepared as previously described (Arenzana-Seisdedos et al., 1995 ). Three µg of nuclear extracts was analysed using [
-32P]dCTP-labelled double-stranded synthetic wild-type human immunodeficiency virus enhancer oligonucleotide 5' AGC TTA CAA GGG ACT TTC CGC TGG GGA CTT TCC AGG GA 3' containing the two
B consensus motifs.
Measurement of the extent of apoptosis.
The cell death detection ELISA kit (Roche) was used according to the manufacturers instructions. This assay is based on the quantitative enzyme sandwich immunoassay principle and uses mouse monoclonal antibodies directed against DNA and histones to estimate the amount of cytoplasmic histone-associated DNA.
Caspase 3 activity assay.
3x106 cells were collected in lysis buffer (150 mM KCl, 10% glycerol, 1 mM dithiothreitol, 5 mM magnesium acetate, 0·5% Nonidet P-40) and clarified by centrifugation. Equal amounts of supernatant and 2x reaction buffer (100 mM HEPES pH 7·5, 20% glycerol, 5 mM dithiothreitol, 0·5 mM EDTA) were mixed and assayed for caspase 3 activity using as substrate 200 µM DEVD-pNA from Calbiochem. Free pNA produced by caspase activity was determined by measuring absorbance at 405 nm.
In vitro analyses of PKRMC159L interaction.
PKR0/0 cells infected for 20 h with VV PKR or with control VV were immunoprecipitated using a PKR antiserum. Washed immunocomplexes were then incubated for 1 h at 25 °C with 35S-labelled proteins obtained from in vitro transcription/translations of plasmids carrying MC159L and E3L cDNAs under the transcriptional control of a T7 promoter using the TNT T7 Quickcoupled system (Promega). Upon incubation, resins were extensively washed and bound proteins analysed by SDSPAGE followed by autoradiography.
Measurement of
-galactosidase activity.
Confluent BSC-40 cells seeded in 24-well plates were infected with 5 p.f.u. per cell of the indicated viruses. After 1 h of virus adsorption 5 mM IPTG was added to induce PKR expression. Cells were collected at indicated times, resuspended in 100 µl of 0·25 M Tris, pH 7·8, and lysed by three freezethaw cycles. Lysis extracts were diluted to 1 ml with water, centrifuged and 10 µl of supernatant was used for -galactosidase determination. Cell lysate supernatants (10 µl) were mixed with 150 µl of CPRG solution (1 mM MgCl2, 45 mM
-mercaptoethanol, 0·1 M sodium phosphate, pH 7·5, 5 mM CPRG) in a 96-well plate, incubated at 37 °C for 1 h and absorbance at 540 nm was determined.
Immunoblotting.
For immunoblot analysis, total cell extracts were fractionated by SDSPAGE and proteins were transferred to nitrocellulose paper. Filters were incubated with antiserum overnight at 4 °C, then incubated with secondary antibody and reactivity was detected using ECL Western blotting reagents (Amersham). Exposure of filters to Kodak X-OMAT films was performed for the necessary time.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
|
|
|
|
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Since MCV cannot be grown in vitro and consequently cannot be genetically modified, it becomes difficult to study the function of genes involved in the virus life-cycle, replication and interaction with the host. The use of rVV is an effective approach to this problem. To investigate the relationship of MC159L with PKR, we used VV recombinants that singly express PKR or MC159L and doubly express PKR and MC159L. Expression of PKR was under an IPTG-inducible promoter, while MC159L was expressed constitutively. Through various assays measuring apoptosis, NF-B activation, proteinprotein interaction and antiviral effects, we present evidence suggesting that the MC159L protein interferes with PKR action.
We found that the MC159L protein from MCV regulates PKR action at two different levels, inhibiting apoptosis and blocking NF-B activation (Figs 1C
and 2
). The effect of MC159L on PKR-induced apoptosis is especially interesting because MCV infection persists for months and sometimes for years in patients. Therefore, inhibition of PKR-induced cell death could be a key mechanism for virus persistence (Kaufman, 1999
; Yeung et al., 1999
). Disruption of death receptor-induced apoptosis by MC159L and other vFLIP-like proteins has been associated with their ability to interact with FADD, thus blocking caspase 8 activation. Since PKR induces apoptosis through FADD-mediated activation of caspase 8 (Gil & Esteban, 2000
), this pathway probably represents the mechanism of inhibition of PKR-triggered cell death by MC159L.
We have also found that MC159L is able to inhibit NF-B activation induced by PKR. Although FLIP, the cellular homologue of MC159, is not required for NF-
B activation in response to TNF-
treatment, as shown in knockout studies (Yeh et al., 2000
), when it is overexpressed it is able to activate NF-
B and Erk pathways through recruitment of adapter molecules (Kataoka et al., 2000
). In a similar fashion, some vFLIP proteins have been shown to induce NF-
B (Chaudhary et al., 1999
). Our data show that MC159L has the ability to counteract NF-
B activation triggered by PKR. Other investigators had attributed to MC159L a role in the inhibition of NF-
B in response to caspase 8 activation (Chaudhary et al., 1999
), but this is the first evidence showing that MC159L interferes with activation of NF-
B in response to PKR.
A major role has been attributed to eIF-2 phosphorylation in mediating PKR-induced effects, but there is increasing evidence which assigns to PKR a more important role in the regulation of different transcriptional pathways, in particular the one involving NF-
B. Thus, NF-
B activation triggered by PKR is involved in apoptosis induction (Gil et al., 1999
) and in imposing an antiviral state through the production of IFN and other pro-inflammatory cytokines (Chu et al., 1999
). Moreover, since NF-
B has been allocated as a key regulator of the transcription of multiple genes involved in inflammation, we should not underestimate the regulatory effects exerted by MC159L over the PKR-triggered NF-
B pathway as an anti-inflammatory mechanism. Interestingly, the ability of MC159L to inhibit NF-
B activation has been associated to sequestration of IKK activators, such as RIP or TRAFs (Chaudhary et al., 1999
). Thus, it seems possible that PKR could activate IKK through some common molecule upstream of the IKK complex, thus explaining why MC159L inhibits the process. The explanation of this observation will emerge from a better knowledge of the transduction pathways triggered by PKR.
Herpesviruses encode for a protein that, although is not a direct PKR inhibitor, targets the eIF-2 phosphorylation pathway through recruitment and activation of a phosphatase with the ability to revert the translational block imposed by PKR (He et al., 1997
). Thus, the effect of MC159L on PKR could be explained in a similar fashion, not by directly inhibiting PKR but by blocking several PKR-dependent pathways and, hence, neutralizing to some extent PKR biological effects such as apoptosis induction. Recently the role of other vFLIPs has been analysed through the genetic modification of herpesvirus saimiri (Glykofrydes et al., 2000
). Similar to the results that we report here, where MC159L is not capable of overriding PKR antiviral effects, the deletion of vFLIP from herpesvirus saimiri was not an obstacle to virus replication. These observations show that modulation of host responses to these viruses is carried out by the combined actions exerted by different proteins.
In conclusion, in this investigation we have made an in depth analysis of the relationship between MC159L and PKR. We have confirmed the importance of MC159L in regulating PKR-dependent apoptosis and the induction of the transcription factor NF-B by this kinase. Although these actions do not correlate with alteration in the antiviral state imposed by PKR, these results suggest that some of the biological effects exerted by MC159L could be mediated through the regulation of PKR-triggered pathways. The facts that there is no physical interaction between PKR and the MC159L open reading frame product, and phosphorylation of the well characterized PKR substrate, eIF-2
, is not affected suggest that cells may contain functionally distinct pools of PKR. This is the first demonstration of the existence of PKR regulatory proteins encoded by the human poxvirus MCV.
![]() |
Acknowledgments |
---|
![]() |
Footnotes |
---|
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Balachandran, S., Roberts, P. C., Brown, L. E., Truong, H., Pattnaik, A. K., Archer, D. R. & Barber, G. N. (2000). Essential role for the dsRNA-dependent protein kinase PKR in innate immunity to viral infection. Immunity 13, 129-141.[Medline]
Bertin, J., Armstrong, R. C., Ottlie, S., Martin, D. A., Wang, Y., Banks, S., Wang, G. H., Senkevich, T. G., Alnemri, E. S., Moss, B., Lenardo, M. J., Tomaselli, K. J. & Cohen, J. I. (1997). DED-containing herpesvirus and poxvirus proteins inhibit both Fas- and TNFR1-induced apoptosis. Proceedings of the National Academy of Sciences, USA 94, 1172-1176.
Black, T. L., Safer, B., Hovanessian, A. & Katze, M. G. (1989). The cellular 68,000 Mr protein kinase is highly autophosphorylated and activated yet significantly degraded during poliovirus infection: implications for translational regulation. Journal of Virology 63, 2244-2251.[Medline]
Buller, R. M. L. (1999). Infectious Diseases, pp. 8-7.18-7.6. Edited by D. Armstrong & J. Cohen. St Louis: Mosby.
Chaudhary, P. M., Jasmin, A., Eby, M. T. & Hood, L. (1999). Modulation of the NF-B pathway by virally encoded death effector domains containing proteins. Oncogene 18, 5738-5746.[Medline]
Chen, N., Baudino, T., MacDonald, P. N., Green, M., Kelley, W. L., Burnett, J. W. & Buller, R. M. L. (2000). Selective inhibition of nuclear steroid receptor function by a protein from a human tumorigenic poxvirus. Virology 274, 17-25.[Medline]
Chong, K. L., Feng, L., Schappert, K., Meurs, E., Donahue, T. F., Friesen, J. D., Hovanessian, A. G. & Williams, B. R. (1992). Human p68 kinase exhibits growth suppression in yeast and homology to the translational regulator GCN2. EMBO Journal 11, 1553-1562.[Abstract]
Chu, W. M., Ostertag, D., Li, Z. W., Chang, L., Chen, Y., Hu, Y., Williams, B., Perrault, J. & Karin, M. (1999). JNK2 and IKKbeta are required for activating the innate response to viral infection. Immunity 11, 721-731.[Medline]
Clemens, M. J. & Elia, A. (1997). The double-stranded RNA-dependent protein kinase PKR: structure and function. Journal of Interferon and Cytokine Research 17, 503-524.[Medline]
Damon, I., Murphy, P. M. & Moss, B. (1998). Broad spectrum chemokine antagonistic activity of a human poxvirus chemokine homolog. Proceedings of the National Academy of Sciences, USA 95, 6403-6407.
Davies, M. V., Chang, H. W., Jacobs, B. L. & Kaufman, R. J. (1993). The E3L and K3L vaccinia virus gene products stimulate translation through inhibition of the double-stranded RNA-dependent protein kinase by different mechanisms. Journal of Virology 66, 1688-1692.[Abstract]
Dever, T. E., Sripriya, R., McLachlin, J. R., Lu, J., Fabian, J. R., Kimball, S. R. & Miller, L. K. (1998). Disruption of cellular translational control by a viral truncated eukaryotic translation initiation factor 2alpha kinase homolog. Proceedings of the National Academy of Sciences, USA 95, 4164-4169.
Gale, M.Jr & Katze, M. G. (1998). Molecular mechanisms of interferon resistance mediated by viral-directed inhibition of PKR, the interferon-induced protein kinase. Pharmacology & Therapeutics 78, 29-46.[Medline]
Gale, M.Jr, Korth, M. J., Tang, N. M., Tan, S. L., Hopkins, D. A., Dever, T. E., Polyak, S. J., Gretch, D. R. & Katze, M. G. (1997). Evidence that hepatitis C virus resistance to interferon is mediated through repression of the PKR protein kinase by the nonstructural 5A protein. Virology 230, 217-227.[Medline]
Gale, M.Jr, Blakely, C. M., Kwieciszewski, B., Tan, S. L., Dossett, M., Tang, N. M., Korth, M. J., Polyak, S. J., Gretch, D. R. & Katze, M. G. (1998). Control of PKR protein kinase by hepatitis C virus nonstructural 5A protein: molecular mechanisms of kinase regulation. Molecular and Cellular Biology 18, 5208-5218.
Gil, J. & Esteban, M. (2000). FADD-mediated activation of caspase 8 by a mechanism independent of Fas and TNFR is involved in PKR-induced apoptosis. Oncogene 19, 3665-3674.[Medline]
Gil, J., Alcamí, J. & Esteban, M. (1999). Induction of apoptosis by double-stranded-RNA-dependent protein kinase (PKR) involves the subunit of eukaryotic translation initiation factor 2 and NF-
B. Molecular and Cellular Biology 19, 4653-4663.
Gil, J., Alcamí, J. & Esteban, M. (2000a). Activation of NF-B by the dsRNA dependent protein kinase, PKR involves the I
B kinase complex. Oncogene 19, 1369-1378.[Medline]
Gil, J., Esteban, M. & Roth, D. (2000b). In vivo regulation of the dsRNA-dependent protein kinase PKR by the cellular glycoprotein p67. Biochemistry 26, 16016-16025.
Glykofrydes, D., Niphuis, H., Kuhn, E. M., Rosenwirth, B., Heeney, J. L., Bruder, J., Niedobitek, G., Fleckenstein, I. M., Fleckenstein, B. & Ensser, A. (2000). Herpesvirus saimiri vFLIP provides an antiapoptotic function but is not essential for viral replication, transformation, or pathogenicity. Journal of Virology 74, 11919-11927.
He, B., Gross, M. & Roizman, B. (1997). The 134.5 protein of herpes simplex virus 1 complexes with protein phosphatase 1
to dephosphorylate the
subunit of the eukaryotic translation initiation factor 2 and preclude the shutoff of protein synthesis by double-stranded RNA-activated protein kinase. Proceedings of the National Academy of Sciences, USA 94, 843-848.
Hu, S., Vincenz, C., Buller, R. M. L. & Dixit, V. M. (1997). A novel family of viral effector domain-containing molecules that inhibit both CD-95- and TNFR1-induced apoptosis. Journal of Biological Chemistry 272, 9261-9264.
Jacobs, B. L. & Langland, J. O. (1996). When two strands are better than one: the mediators and modulators of the cellular responses to double-stranded RNA. Virology 219, 339-349.[Medline]
Kataoka, T., Budd, R. C., Holler, N., Thome, M., Martinon, F., Irmler, M., Burns, K., Hahne, M., Kennedy, N., Kovacsovics, M. & Tschopp, J. (2000). The caspase-8 inhibitor FLIP promotes activation of NF-B and Erk signalling pathways. Current Biology 10, 640-648.[Medline]
Katze, M. G., Decorato, D., Safer, B., Galabru, J. & Hovanessian, A. G. (1987). Adenovirus VAI RNA complexes with the 68,000 Mr protein kinase to regulate its autophosphorylation and activity. EMBO Journal 6, 689-697.[Abstract]
Kaufman, R. J. (1999). Double-stranded RNA-activated protein kinase mediates virus-induced apoptosis: a new role for an old actor. Proceedings of the National Academy of Sciences, USA 96, 11693-11695.
Koromilas, A. E., Galabru, J., Barber, G. N., Katze, M. G. & Sonenberg, N. (1992). Malignant transformation by a mutant of the IFN-inducible dsRNA-dependent protein kinase. Science 257, 1685-1689.[Medline]
Krathwohl, M. D., Hromas, R., Brown, D. R., Broxmeyer, H. E. & Fife, K. H. (1997). Functional characterization of C-C chemokine-like molecules encoded by Molluscum contagiosum virus types 1 and 2. Proceedings of the National Academy of Sciences, USA 94, 9875-9880.
Kumar, A., Haque, J., Lacoste, J., Hiscott, J. & Williams, B. R. G. (1994). Double-stranded RNA-dependent protein kinase activates transcription factor NF-B by phosphorylating I
B. Proceedings of the National Academy of Sciences, USA 91, 6288-6292.[Abstract]
Lee, S. B. & Esteban, M. (1993). The interferon-induced double-stranded RNA-activated human p68 protein kinase inhibits the replication of vaccinia virus. Virology 193, 1037-1041.[Medline]
Lee, S. B. & Esteban, M. (1994). The interferon-induced double-stranded RNA-activated protein kinase induces apoptosis. Virology 199, 491-496.[Medline]
Lee, S. B., Bablanian, R. & Esteban, M. (1996). Regulated expression of the interferon-induced protein kinase p68 (PKR) by vaccinia virus recombinants inhibits the replication of vesicular stomatitis virus but not that of poliovirus. Journal of Interferon and Cytokine Research 16, 1073-1078.[Medline]
Lloyd, R. M. & Shatkin, A. J. (1993). Translational stimulation by reovirus polypeptide sigma 3: substitution for VAI RNA and inhibition of phosphorylation of the alpha subunit of eukaryotic initiation factor 2. Journal of Virology 66, 6878-6884.[Abstract]
Lu, Y., Wambach, M., Katze, M. G. & Krug, R. M. (1995). Binding of the influenza virus NS1 protein to double-stranded RNA inhibits the activation of the protein kinase that phosphorylates the elF-2 translation initiation factor. Virology 214, 222-228.[Medline]
McInnes, C. J., Wood, A. R. & Mercer, A. A. (1998). Orf virus encodes a homolog of the vaccinia virus interferon-resistance gene E3L. Virus Genes 17, 107-115.[Medline]
Meurs, E. F., Chong, K., Galabru, J., Thomas, N. S., Kerr, I. M., Williams, B. R. & Hovanessian, A. G. (1990). Molecular cloning and characterization of the human double-stranded RNA-activated protein kinase induced by interferon. Cell 62, 379-390.[Medline]
Meurs, E., Galabru, J., Barber, G. N., Katze, M. G. & Hovanessian, A. G. (1993). Tumor suppressor function of the interferon-induced double-stranded RNA-activated protein kinase. Proceedings of the National Academy of Sciences, USA 90, 232-236.[Abstract]
Moss, B., Shisler, J. L., Xiang, Y. & Senkevich, T. G. (2000). Immune-defense molecules of Molluscum contagiosum virus, a human poxvirus. Trends in Microbiology 8, 473-477.[Medline]
Petryshyn, R., Chen, J. J. & London, I. M. (1984). Growth-related expression of a double-stranded RNA-dependent protein kinase in 3T3 cells. Journal of Biological Chemistry 259, 14736-14742.
Rivas, C., Gil, J., Melkova, Z., Esteban, M. & Diaz-Guerra, M. (1998). Vaccinia virus E3L protein is an inhibitor of the interferon (IFN)-induced 25A synthetase enzyme. Virology 243, 406-414.[Medline]
Romano, P. R., Zhang, F., Tan, S. L., Garcia Barrio, M., Katze, M. G., Dever, T. E. & Hinnebusch, A. G. (1998). Inhibition of double-stranded RNA-dependent protein kinase PKR by vaccinia virus E3: role of complex formation and the E3 N-terminal domain. Molecular and Cellular Biology 18, 7304-7316.
Senkevich, T. G., Bugert, J., Shisler, J. R., Koonin, E. V., Darai, G. & Moss, B. (1996). Genome sequence of a human tumorigenic poxvirus: prediction of specific host response-evasion genes. Science 273, 813-816.[Abstract]
Senkevich, T. G., Koonin, E. V., Bugert, J., Darai, G. & Moss, B. (1997). The genome of Molluscum contagiosum virus: analysis and comparison with other poxviruses. Virology 233, 19-42.[Medline]
Shchelkunov, S. N., Resenchuk, S. M., Totmenin, A. V., Blinov, V. M., Marennikova, S. S. & Sandakhchiev, L. S. (1993). Comparison of the genetic maps of variola and vaccinia viruses. FEBS Letters 327, 321-324.[Medline]
Shisler, J. L., Senkevich, T. G., Berry, M. J. & Moss, B. (1998). Ultraviolet-induced cell death blocked by a selenoprotein from a human dermotropic poxvirus. Science 279, 102-105.
Smith, G. L., Symons, J. A. & Alcamí, A. (1998). Poxviruses: interfering with interferon. Seminars in Virology 8, 409-418.
Taylor, D. R., Shi, S. T., Romano, P. R., Barber, G. N. & Lai, M. M. (1999). Inhibition of the interferon-inducible protein kinase PKR by HCV E2 protein. Science 285, 107-110.
Thome, M., Schneider, P., Hofmann, K., Fickenscher, H., Meinl, E., Neipel, F., Mattmann, C., Burns, K., Bodmer, J. L., Schroter, M., Scaffidi, C., Krammer, P. H., Peter, M. E. & Tschopp, J. (1997). Viral FLICE-inhibitory proteins (FLIPs) prevent apoptosis induced by death receptors. Nature 386, 517-521.[Medline]
Xiang, Y. & Moss, B. (1999a). Identification of human and mouse homologs of the MC51L-53L-54L family of secreted glycoproteins encoded by the Molluscum contagiosum poxvirus. Virology 257, 297-302.[Medline]
Xiang, Y. & Moss, B. (1999b). IL-18 binding and inhibition of interferon induction by a human poxvirus encoded protein. Proceedings of the National Academy of Sciences, USA 96, 11537-11542.
Yeh, W., Itie, A., Elia, A. J., Ng, M., Shu, H., Wakeham, A., Mirtsos, C., Suzuki, N., Bonnard, M., Goeddel, D. V. & Mak, T. W. (2000). Requirement for Casper (c-FLIP) in regulation of death receptor-induced apoptosis and embryonic development. Immunity 12, 633-642.[Medline]
Yeung, M. C., Chang, D. L., Camatingue, R. E. & Lau, A. S. (1999). Inhibitory role of the host apoptogenic gene PKR in the establishment of persistent infection by encephalomyocarditis virus in U937 cells. Proceedings of the National Academy of Sciences, USA 96, 11860-11865.
Received 22 May 2001;
accepted 8 August 2001.