Research in the last 2 decades has demonstrated that a specific organelle of the cell nucleus, termed PML nuclear body (PML-NB) or nuclear domain 10 (ND10), is frequently modified during viral infection. This correlates with antagonization of a direct repressive function of individual PML-NB components, such as the PML, hDaxx, Sp100, or ATRX protein, that are able to act as cellular restriction factors. Recent studies now reveal an emerging role of PML-NBs as coregulatory structures of both type I and type II interferon responses. This emphasizes that targeting of PML-NBs by viral regulatory proteins has evolved as a strategy to compromise intrinsic antiviral defense and innate immune responses. P romyelocytic leukemia (PML) protein, a member of the tripartite motif (TRIM) protein family, is the key component of subnuclear structures known as PML nuclear bodies (PML-NBs) or nuclear domains 10 (ND10). PML-NBs are dynamic foci that consist of numerous permanently or transiently associated proteins and, consequently, have been implicated in the regulation of diverse cellular functions, including the cell cycle, apoptosis, senescence, stress, and DNA damage responses (reviewed in reference 1). Although the underlying biochemical function of these subnuclear structures is still unclear, three models can be found in the literature, proposing PML-NBs to be nuclear protein depots, sites of nuclear activities (e.g., transcription), or hotspots for posttranslational modifications. In particular, since nearly all PMLassociated proteins are modified by SUMO and SUMOylation of PML is essential for the integrity of PML-NBs, these structures may form "catalytic surfaces" for SUMOylation (1). This is of importance since recent studies suggest that the SUMO pathway is required for the regulation of innate immune signaling and intrinsic immunity during viral infection (reviewed in reference 2). The observation that PML-NBs are targeted and modified by many viruses during infection set off a longstanding debate as to whether PML-NBs exert a pro-or antiviral function and led to a fruitful area of virology research over the past 20 years. Interestingly, these studies revealed that viruses, even representatives of the same virus family, trigger diverse modifications of PML-NBs during infection, ranging from proteasomal degradation of NB components by herpes simplex virus type 1 (HSV-1), to dispersal of PML-NBs by human cytomegalovirus (HCMV), to a rearrangement of PML-NB foci into nuclear track-like structures by adenoviruses or a relocalization of PML into cytoplasmic bodies by HIV-1 (3-5). While there are a few examples of viral factors that undergo interactions with PML-NBs in order to exploit these structures for the benefit of the virus, the main body of evidence supports a role of PML-NBs as components of the antiviral defense against a variety of DNA and RNA viruses (reviewed in reference 3). PML-NBs AND INTRINSIC IMMUNITYThe role of PML-NBs in intrinsic immunity, which represents the first line of intracellular defense agains...

PML nuclear bodies (PML-NBs) are enigmatic structures of the cell nucleus that act as key mediators of intrinsic immunity against viral pathogens. PML itself is a member of the E3-ligase TRIM family of proteins that regulates a variety of innate immune signaling pathways. Consequently, viruses have evolved effector proteins to modify PML-NBs; however, little is known concerning structure-function relationships of viral antagonists. The herpesvirus human cytomegalovirus (HCMV) expresses the abundant immediate-early protein IE1 that colocalizes with PML-NBs and induces their dispersal, which correlates with the antagonization of NB-mediated intrinsic immunity. Here, we delineate the molecular basis for this antagonization by presenting the first crystal structure for the evolutionary conserved primate cytomegalovirus IE1 proteins. We show that IE1 consists of a globular core (IE1CORE) flanked by intrinsically disordered regions. The 2.3 Å crystal structure of IE1CORE displays an all α-helical, femur-shaped fold, which lacks overall fold similarity with known protein structures, but shares secondary structure features recently observed in the coiled-coil domain of TRIM proteins. Yeast two-hybrid and coimmunoprecipitation experiments demonstrate that IE1CORE binds efficiently to the TRIM family member PML, and is able to induce PML deSUMOylation. Intriguingly, this results in the release of NB-associated proteins into the nucleoplasm, but not of PML itself. Importantly, we show that PML deSUMOylation by IE1CORE is sufficient to antagonize PML-NB-instituted intrinsic immunity. Moreover, co-immunoprecipitation experiments demonstrate that IE1CORE binds via the coiled-coil domain to PML and also interacts with TRIM5α We propose that IE1CORE sequesters PML and possibly other TRIM family members via structural mimicry using an extended binding surface formed by the coiled-coil region. This mode of interaction might render the antagonizing activity less susceptible to mutational escape.

In recent studies, the nuclear domain 10 (ND10) components PML and hDaxx were identified as cellular restriction factors that inhibit the initiation of human cytomegalovirus (HCMV) replication. The antiviral function of ND10, however, is antagonized by the IE1 protein, which induces ND10 disruption. Here we show that IE1 not only de-SUMOylates PML immediately upon infection but also directly targets Sp100. IE1 expression alone was sufficient to downregulate endogenous Sp100 independently of the presence of PML. Moreover, cotransfection experiments revealed that IE1 negatively interferes with the SUMOylation of all Sp100 isoforms. The modulation of Sp100 at immediate-early (IE) times of infection, indeed, seemed to have an in vivo relevance for HCMV replication, since knockdown of Sp100 resulted in more cells initiating the viral gene expression program. In addition, we observed that Sp100 was degraded in a proteasome-dependent manner at late times postinfection, suggesting that Sp100 may play an additional antiviral role during the late phase. Infection experiments conducted with Sp100 knockdown human foreskin fibroblasts (HFFs) confirmed this hypothesis: depletion of Sp100 resulted in augmented release of progeny virus particles compared to that from control cells. Consistent with this observation, we noted increased amounts of viral late gene products in the absence of Sp100. Importantly, this elevated late gene expression was not dependent on enhanced viral IE gene expression. Taken together, our data provide evidence that Sp100 is the first ND10-related factor identified that not only possesses the potential to restrict the initial stage of infection but also inhibits HCMV replication during the late phase.

Promyelocytic leukemia nuclear bodies, also termed nuclear domain 10 (ND10), have emerged as nuclear protein accumulations mediating an intrinsic cellular defense against viral infections via chromatin-based mechanisms, however, their contribution to the control of herpesviral latency is still controversial. In this study, we utilized the monocytic cell line THP-1 as an in vitro latency model for human cytomegalovirus infection (HCMV). Characterization of THP-1 cells by immunofluorescence and Western blot analysis confirmed the expression of all major ND10 components. THP-1 cells with a stable, individual knockdown of PML, hDaxx or Sp100 were generated. Importantly, depletion of the major ND10 proteins did not prevent the terminal cellular differentiation of THP-1 monocytes. After construction of a recombinant, endotheliotropic human cytomegalovirus expressing IE2-EYFP, we investigated whether the depletion of ND10 proteins affects the onset of viral IE gene expression. While after infection of differentiated, THP-1-derived macrophages as well as during differentiation-induced reactivation from latency an increase in the number of IE-expressing cells was readily detectable in the absence of the major ND10 proteins, no effect was observed in non-differentiated monocytes. We conclude that PML, hDaxx and Sp100 primarily act as cellular restriction factors during lytic HCMV replication and during the dynamic process of reactivation but do not serve as key determinants for the establishment of HCMV latency.

PML is the organizer of cellular structures termed nuclear domain 10 (ND10) or PML-nuclear bodies (PML-NBs) that act as key mediators of intrinsic immunity against human cytomegalovirus (HCMV) and other viruses. The antiviral function of ND10 is antagonized by viral regulatory proteins such as the immediate early protein IE1 of HCMV. IE1 interacts with PML through its globular core domain (IE1 CORE ) and induces ND10 disruption in order to initiate lytic HCMV infection. Here, we investigate the consequences of a point mutation (L174P) in IE1 CORE , which was shown to abrogate the interaction with PML, for lytic HCMV infection. We found that a recombinant HCMV encoding IE1-L174P displays a severe growth defect similar to that of an IE1 deletion virus. Bioinformatic modeling based on the crystal structure of IE1 CORE suggested that insertion of proline into the highly alpha-helical domain severely affects its structural integrity. Consistently, L174P mutation abrogates the functionality of IE1 CORE and results in degradation of the IE1 protein during infection. In addition, our data provide evidence that IE1 CORE as expressed by a recombinant HCMV encoding IE1 1-382 not only is required to antagonize PML-mediated intrinsic immunity but also affects a recently described function of PML in innate immune signaling. We demonstrate a coregulatory role of PML in type I and type II interferon-induced gene expression and provide evidence that upregulation of interferon-induced genes is inhibited by IE1 CORE . In conclusion, our data suggest that targeting PML by viral regulatory proteins represents a strategy to antagonize both intrinsic and innate immune mechanisms. IMPORTANCEPML nuclear bodies (PML-NBs), which represent nuclear multiprotein complexes consisting of PML and additional proteins, represent important cellular structures that mediate intrinsic resistance against many viruses, including human cytomegalovirus (HCMV). During HCMV infection, the major immediate early protein IE1 binds to PML via a central globular domain (IE1 CORE ), and we have shown previously that this is sufficient to antagonize intrinsic immunity. Here, we demonstrate that modification of PML by IE1 CORE not only abrogates intrinsic defense mechanisms but also attenuates the interferon response during infection. Our data show that PML plays a novel coregulatory role in type I as well as type II interferon-induced gene expression, which is antagonized by IE1 CORE . Importantly, our finding supports the view that targeting of PML-NBs by viral regulatory proteins has evolved as a strategy to inhibit both intrinsic and innate immune defense mechanisms. H uman cytomegalovirus (HCMV), a member of the ␤-subgroup of herpesviruses, is a widespread human pathogen of high clinical relevance that can cause life-threatening diseases in newborns and people with compromised immune system such as AIDS, transplantation, or malignancy patients. The lytic replication cycle of HCMV is characterized by three sequential phases of viral gene expression, te...