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. 2007 Sep;81(18):10123-36.
doi: 10.1128/JVI.01009-07. Epub 2007 Jul 11.

Recruitment of human cytomegalovirus immediate-early 2 protein onto parental viral genomes in association with ND10 in live-infected cells

Affiliations

Recruitment of human cytomegalovirus immediate-early 2 protein onto parental viral genomes in association with ND10 in live-infected cells

George Sourvinos et al. J Virol. 2007 Sep.

Abstract

The human cytomegalovirus (HCMV) immediate-early 2 (IE2) transactivator has previously been shown to form intranuclear, dot-like accumulations in association with subnuclear structures known as promyelocytic leukemia protein (PML) nuclear bodies or ND10. We recently observed that IE2 can form dot-like structures even after infection of PML knockdown cells, which lack genuine ND10. To further analyze the determinants of IE2 subnuclear localization, a recombinant HCMV expressing IE2 fused to the enhanced green fluorescent protein was constructed. We infected primary human fibroblasts expressing Sp100 fused to the autofluorescent protein mCherry while performing live-cell imaging experiments. These experiments revealed a very dynamic association of IE2 dots with ND10 structures during the first hours postinfection: juxtaposed structures rapidly fused to precise co-localizations, followed by segregation, and finally, the dispersal of ND10 accumulations. Furthermore, by infecting PML knockdown cells we determined that the number of IE2 accumulations was dependent on the multiplicity of infection. Since time-lapse microscopy in live-infected cells revealed that IE2 foci developed into viral replication compartments, we hypothesized that viral DNA could act as a determinant of IE2 accumulations. Direct evidence that IE2 molecules are associated with viral DNA early after HCMV infection was obtained using fluorescence in situ hybridization. Finally, a DNA-binding-deficient IE2 mutant could no longer be recruited into viral replication centers, suggesting that the association of IE2 with viral DNA is mediated by a direct DNA contact. Thus, we identified viral DNA as an important determinant of IE2 subnuclear localization, which suggests that the formation of a virus-induced nucleoprotein complex and its spatial organization is likely to be critical at the early stages of a lytic infection.

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Figures

FIG. 1.
FIG. 1.
Generation of a recombinant HCMV expressing an EGFP-tagged version of the viral transactivator protein IE2. The EGFP sequence was fused to the 3′ end of the open reading frame UL122 (exon 5 of the IE1/2 gene region), and an exon 5-EGFP hybrid gene was integrated into the HCMV genome via homologous recombination in E. coli, using an HCMV BAC lacking exon 5 of the MIE gene locus. (A) Schematic representation of the HCMV genomic region where the exon 5-EGFP fusion gene was inserted. To check for the integrity of the obtained recombinant BACs, BamHI digestion and subsequent Southern blot analysis were performed. The BamHI restriction sites and the probe used for Southern blotting are indicated in the diagram. (B) Wild-type AD169 and recombinant AD169/IE2-EGFP BACs were digested with BamHI, and the fragments were separated by agarose gel electrophoresis. Due to an additional BamHI restriction site inserted together with the EGFP sequence, the wt 5.2-kb fragment (indicated by a circle in lane 1) was digested into a 4.3-kb fragment for successful insertion of the IE2-EGFP fusion gene (indicated by a triangle in lane 3). (C) Southern blot analysis of BamHI digests of BAC DNAs using an IE2 cDNA probe.
FIG. 2.
FIG. 2.
Determination of the particle-to-PFU ratio followed by multistep growth curve analysis of wt HCMV AD169 and the recombinant AD169/IE2-EGFP virus. (A) Results of quantitative real-time PCR for evaluation of the input viral DNA load after infection with wt AD169 or AD169/IE2-EGFP at an MOI of 1. DNA, extracted at 14 hpi from infected cells, was used for real-time PCR in order to determine the number of input HCMV genome copies. (B) Western blot for detection of pp65 (UL83) protein levels in cells harvested at 2 hpi. (C) Western blot for detection of IE1p72 protein levels in cells harvested at 48 hpi. (B and C) Lanes: 1, mock infection; 2, infection with wt AD169; 3, infection with AD169/IE2-EGFP. Infection was performed at an MOI of 1; detection of actin by Western blot analysis (see lower panels of B and C) served as a loading control. (D and E) HFF cells were infected in parallel at a high (panel D; MOI = 1) or low (panel E; MOI = 0.1) MOI with viral inocula of wt AD169 (filled diamonds) and AD169/IE2-EGFP (filled rectangles) which were standardized for equal IE1p72 expression at 24 hpi. The virus progeny in the supernatants of infected cell cultures were harvested at different time points postinfection as indicated, followed by quantification of the viral load via IE1p72 fluorescence. Error bars indicate the standard deviation derived from three independent experiments.
FIG. 3.
FIG. 3.
Comparison of the protein expression kinetics of wt HCMV AD169 and the recombinant virus AD169/IE2-EGFP. HFF cells were infected in parallel with AD169 and AD169/IE2-EGFP at an MOI of 1 (A) or 0.1 (B). At the indicated times postinfection, total cell lysates were prepared, subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and subsequently transferred to a nitrocellulose membrane. Immediate-early (IE2p86, IE1p72), early-late (UL84, UL44, UL69), and true late (UL86, UL99) protein levels were analyzed by Western blotting as described in Materials and Methods. Cellular actin levels served as controls for equal protein loading.
FIG. 4.
FIG. 4.
Generation of primary human fibroblast cells stably expressing an mCherry-Sp100 (mCh-Sp100) autofluorescent fusion protein. Primary human fibroblast cells stably expressing mCherry-Sp100 were generated via retroviral transduction using plasmid pHM2397, followed by blasticidin selection. Indirect immunofluorescence analysis revealed colocalization of stably expressed mCh-Sp100 with both endogenously expressed PML (subpanels a to d) and hDaxx (subpanels e to m).
FIG. 5.
FIG. 5.
Live-cell imaging of the association of IE2p86 and ND10 in transfected or infected cells during the immediate-early phase of the replication cycle. Although IE2p86 precisely colocalizes with ND10 after transient expression of the IE2-EGFP protein in mCh-Sp100 fibroblasts (A), it is either associated with or juxtaposed to Sp100 during the course of an HCMV infection (panel B, MOI = 0.5; panel C, MOI = 2.5). Images were obtained from live, transfected or infected (4 hpi) mCh-Sp100 fibroblasts. The inset images show examples of association but not precise colocalization of IE2-EGFP and Sp100.
FIG. 6.
FIG. 6.
The number of IE2 foci increases with the multiplicity of infection. The number of IE2-EGFP foci is dependent on the MOI of the infection but it is independent of the ND10 status of the cells. Both HFFs and PML-kd cells were infected with AD169/IE2-EGFP either at an MOI of 0.5 (panel A, subpanels a and c; panel B), or at an MOI of 2.5 (panel A, subpanels b and d; panel B). The graph in panel B shows mean values for 50 analyzed cells; standard deviations are indicated.
FIG. 7.
FIG. 7.
In vivo dynamics of IE2 in relation to that of ND10. mCh-Sp100 cells live-infected with AD169/IE2-EGFP at an MOI of 0.5 were monitored at immediate-early times after infection. Presented are selected images that illustrate a dynamic pattern of association where, within minutes, Sp100 and IE2 dots, which are in the first instance next to each other, show perfect colocalization and then become juxtaposed until Sp100 is dispersed. The inset images show two regions of the cell at a higher magnification.
FIG. 8.
FIG. 8.
Recruitment of IE2 into foci, some of which later develop into replication compartments. (A) HFF cells were infected with AD169/IE2-EGFP at an MOI of 0.5 and analyzed by live-cell microscopy. The localization of IE2p86 was detected via EGFP fluorescence. Times after addition of the virus are indicated in each panel. (B) Subnuclear colocalization of IE2-EGFP and ppUL44 was revealed after indirect immunofluorescence analysis with the antibody BS510 against ppUL44, confirming the development of the IE2p86 foci into viral replication compartments 30 hpi.
FIG. 9.
FIG. 9.
HCMV genomes as determinants of IE2p86 subnuclear localization. (A) Combined in situ hybridization of HCMV DNA and immunostaining of IE2 in HCMV-infected HFFs at an MOI of 0.5 shows an association between them 12 hpi or at later times during infection. Note the larger structures of HCMV DNA 24 hpi, indicating active replication of the viral genomes. (B) HFFs were transfected with plasmid pEGFP-IE2 and superinfected with HCMV AD169 (subpanels a to d). Images were obtained at different time points postinfection, demonstrating the progressive recruitment of IE2 into developing viral replication compartments. This was not the case when plasmid pEGFP-ΔIE2, encoding a DNA-binding-deficient mutant of IE2 fused to EGFP, was transfected instead, when IE2 failed to form foci even as late as 40 hpi (subpanel f) although viral replication had occurred, as indicated by the recruitment of ppUL44 (subpanel g). Subpanel h shows the two channels merged (ppUL44, red; IE2, green). Subpanel e shows DAPI staining.

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