Human cytomegalovirus carries serine/threonine protein phosphatases PP1 and a host-cell derived PP2A

Spatially resolved protein map of intact human cytomegalovirus virions

Herpesviruses assemble large enveloped particles that are difficult to characterize structurally due to their size, fragility and complex multilayered proteome with partially amorphous nature. Here we used crosslinking mass spectrometry and quantitative proteomics to derive a spatially resolved interactome map of intact human cytomegalovirus virions. This enabled the de novo allocation of 32 viral proteins into four spatially resolved virion layers, each organized by a dominant viral scaffold protein. The viral protein UL32 engages with all layers in an N-to-C-terminal radial orientation, bridging nucleocapsid to viral envelope. We observed the layer-specific incorporation of 82 host proteins, of which 39 are selectively recruited. We uncovered how UL32, by recruitment of PP-1 phosphatase, antagonizes binding to 14-3-3 proteins. This mechanism assures effective viral biogenesis, suggesting a perturbing role of UL32-14-3-3 interaction. Finally, we integrated these data into a coarse-grained model to provide global insights into the native configuration of virus and host protein interactions inside herpesvirions.

Protein Phosphatase 1 Regulates Human Cytomegalovirus Protein Translation by Restraining AMPK Signaling

Human cytomegalovirus (HCMV) carries the human protein phosphatase 1 (PP1) and other human proteins important for protein translation in its tegument layer for a rapid supply upon infection. However, the biological relevance behind PP1 incorporation and its role during infection is unclear. Additionally, PP1 is a difficult molecular target due to its promiscuity and similarities between the catalytic domain of multiple phosphatases. In this study, we circumvented these shortcomings by using 1E7-03, a small molecule protein–protein interaction inhibitor, as a molecular tool of noncatalytic PP1 inhibition. 1E7-03 treatment of human fibroblasts severely impaired HCMV replication and viral protein translation. More specifically, PP1 inhibition led to the deregulation of metabolic signaling pathways starting at very early time points post-infection. This effect was at least partly mediated by the prevention of AMP-activated protein kinase dephosphorylation, leading to elongation factor 2 hyperphosphorylation and reduced translation rates. These findings reveal an important mechanism of PP1 for lytic HCMV infection.

Mass Spectrometry-Based Characterization of the Virion Proteome, Phosphoproteome, and Associated Kinase Activity of Human Cytomegalovirus

The assembly of human cytomegalovirus (HCMV) virions is an orchestrated process that requires, as an essential prerequisite, the complex crosstalk between viral structural proteins. Currently, however, the mechanisms governing the successive steps in the constitution of virion protein complexes remain elusive. Protein phosphorylation is a key regulator determining the sequential changes in the conformation, binding, dynamics, and stability of proteins in the course of multiprotein assembly. In this review, we present a comprehensive map of the HCMV virion proteome, including a refined view on the virion phosphoproteome, based on previous publications supplemented by new results. Thus, a novel dataset of viral and cellular proteins contained in HCMV virions is generated, providing a basis for future analyses of individual phosphorylation steps and sites involved in the orchestrated assembly of HCMV virion-specific multiprotein complexes. Finally, we present the current knowledge on the activity of pUL97, the HCMV-encoded and virion-associated kinase, in phosphorylating viral and host proteins.

HSC70 is required for infectious bursal disease virus (IBDV) infection in DF-1 cells

Background

Infectious bursal disease (IBD) is a highly contagious infectious disease that causes severe immunosuppression and damage to the bursa of Fabricius in chickens. Several proteins involved in IBD virus (IBDV) infection, such as surface immunoglobulin M, integrin, annexin A2 and chicken heat shock protein 90, have been identified. However, the main protein that plays key roles in virus infection has not yet been confirmed.

Methods

DF-1 cell line was transfected with the pcDNA-VP2 plasmid and analyzed by immunofluorescence assay. The proteins reacted with VP2 of IBDV in DF-1 cells were pulldown with the monoclonal antibody and identified by mass spectrometry. Heat shock cognate protein 70 (HSC70), one of these proteins, was selected to be investigated in the function in IBDV infection by specific antibody and its inhibitor.

Results

The DF-1 cell line was transfected with the pcDNA-VP2 plasmid, and expression of IBDV VP2 in DF-1 cells was confirmed by immunofluorescence assays. Heat shock cognate protein 70 (HSC70) was one of the proteins identified by coimmunoprecipitation using a monoclonal antibody (2H11) against VP2 and mass spectrometry analysis. IBDV infection in DF-1 cells was strongly inhibited by both an anti-HSC70 antibody and a HSC70 inhibitor (VER155008).

Conclusion

These results suggest that HSC70 may be an essential factor for IBDV infection.

Dynamic regulatory interaction between cytomegalovirus major tegument protein pp65 and protein kinase pUL97 in intracellular compartments, dense bodies and virions

Human cytomegalovirus (HCMV) is a ubiquitous pathogen of considerable clinical importance. Understanding the processes that are important for viral replication is essential for the development of therapeutic strategies against HCMV infection. The HCMV-encoded protein kinase pUL97 is an important multifunctional regulator of viral replication. Several viral and cellular proteins are phosphorylated by pUL97. The phosphoprotein pp65 is one important substrate of pUL97. It is the most abundant tegument protein of HCMV virions, mediating the upload of other virion constituents and contributing to particle integrity. Further to that, it interferes with host innate immune defences, thereby enabling efficient viral replication. By applying different approaches, we characterized the pp65-pUL97 interaction in various compartments. Specifically, the pUL97 interaction domain of pp65 was defined (282-415). A putative cyclin bridge that enhances pUL97-pp65 interaction was identified. The impact of pUL97 mutation on virion and dense body morphogenesis was addressed using pUL97 mutant viruses. Alterations in the proteome of viral particles were seen, especially with mutant viruses expressing cytoplasmic variants of pUL97. On the basis of these data we postulate a so far poorly recognized functional relationship between pp65 and pUL97, and present a refined model of pp65-pUL97 interaction.

Human cytomegalovirus phosphoproteins are hypophosphorylated and intrinsically disordered

Protein phosphorylation has important regulatory functions in cell homeostasis and is tightly regulated by kinases and phosphatases. The tegument of human cytomegalovirus (CMV) contains not only several proteins reported to be extensively phosphorylated but also cellular protein phosphatases (PP1 and PP2A). To investigate this apparent inconsistency, we evaluated the phosphorylation status of the tegument proteins pUL32 and pp65 by enzymatic dephosphorylation and MS. Enzymatic dephosphorylation with bacterial λ phosphatase, but not with PP1, shifted the pUL32-specific signal on reducing SDS-PAGE from ~150 to ~148 kDa, a mass still much larger than the ~118 kDa obtained from our diffusion studies and from the calculated protein mass of ~113 kDa. Remarkably, inhibition of phosphatases through treatment with the phosphatase inhibitors calyculin A and okadaic acid resulted in a shift to ~190 or ~180 kDa, respectively, indicating that a considerable number of potential phosphorylated residues on pUL32 are not phosphorylated under normal conditions. MS revealed a general state of hypophosphorylation of CMV phosphoproteins with only 17 phosphorylated residues detected on pUL32 and 19 on pp65, respectively. Moreover, bioinformatics analysis shows that the C-terminal two-thirds of pUL32 are intrinsically disordered and that most phosphorylations map to this region. In conclusion, we show that important CMV tegument proteins are indeed phosphorylated, though to a lesser extent than previously reported, and the difference in mobility on SDS-PAGE and calculated mass of pUL32 may not be attributed to phosphorylation but more likely due to the partially intrinsically disordered nature of pUL32.

The tegument protein pp65 of human cytomegalovirus acts as an optional scaffold protein that optimizes protein uploading into viral particles

The mechanisms that lead to the tegumentation of herpesviral particles are only poorly defined. The phosphoprotein 65 (pp65) is the most abundant constituent of the virion tegument of human cytomegalovirus (HCMV). It is, however, nonessential for virion formation. This seeming discrepancy has not met with a satisfactory explanation regarding the role of pp65 in HCMV particle morphogenesis. Here, we addressed the question of how the overall tegument composition of the HCMV virion depended on pp65 and how the lack of pp65 influenced the packaging of particular tegument proteins. To investigate this, we analyzed the proteomes of pp65-positive (pp65pos) and pp65-negative (pp65neg) virions by label-free quantitative mass spectrometry and determined the relative abundances of tegument proteins. Surprisingly, only pUL35 was elevated in pp65neg virions. As the abundance of pUL35 in the HCMV tegument is low, it is unlikely that it replaced pp65 as a structural component in pp65neg virions. A subset of proteins, including the third most abundant tegument protein, pUL25, as well as pUL43, pUL45, and pUL71, were reduced in pp65neg or pp65low virions, indicating that the packaging of these proteins was related to pp65. The levels of tegument components, like pp28 and the capsid-associated tegument proteins pp150, pUL48, and pUL47, were unaffected by the lack of pp65. Our analyses demonstrate that deletion of pp65 is not compensated for by other viral proteins in the process of virion tegumentation. The results are concordant with a model of pp65 serving as an optional scaffold protein that facilitates protein upload into the outer tegument of HCMV particles.

IMPORTANCE

The assembly of the tegument of herpesviruses is only poorly understood. Particular proteins, like HCMV pp65, are abundant tegument constituents. pp65 is thus considered to play a major role in tegument assembly in the process of virion morphogenesis. We show here that deletion of the pp65 gene leads to reduced packaging of a subset of viral proteins, indicating that pp65 acts as an optional scaffold protein mediating protein upload into the tegument.

Role of human cytomegalovirus tegument proteins in virion assembly

Like other herpesviruses, human cytomegalovirus (HCMV) contains a unique proteinaceous layer between the virion envelope and capsid, termed the tegument. Upon infection, the contents of the tegument layer are delivered to the host cell, along with the capsid and the viral genome, where they facilitate the initial stages of virus replication. The tegument proteins also play important roles in virion assembly and this dual nature makes them attractive potential targets for antiviral therapies. While our knowledge regarding tegument protein function during the initiation of infection has been the subject of intense study, their roles in assembly are much less well understood. In this review, we will focus on recent studies that highlight the functions of HCMV tegument proteins during assembly, and pose key questions for further investigation.

Proteomic analyses of human cytomegalovirus strain AD169 derivatives reveal highly conserved patterns of viral and cellular proteins in infected fibroblasts

Human cytomegalovirus (HCMV) particle morphogenesis in infected cells is an orchestrated process that eventually results in the release of enveloped virions. Proteomic analysis has been employed to reveal the complexity in the protein composition of these extracellular particles. Only limited information is however available regarding the proteome of infected cells preceding the release of HCMV virions. We used quantitative mass spectrometry to address the pattern of viral and cellular proteins in cells, infected with derivatives of the AD169 laboratory strain. Our analyses revealed a remarkable conservation in the patterns of viral and of abundant cellular proteins in cells, infected for 2 hours, 2 days, or 4 days. Most viral proteins increased in abundance as the infection progressed over time. Of the proteins that were reliably detectable by mass spectrometry, only IE1 (pUL123), pTRS1, and pIRS1 were downregulated at 4 days after infection. In addition, little variation of viral proteins in the virions of the different viruses was detectable, independent of the expression of the major tegument protein pp65. Taken together these data suggest that there is little variation in the expression program of viral and cellular proteins in cells infected with related HCMVs, resulting in a conserved pattern of viral proteins ultimately associated with extracellular virions.

Human cytomegalovirus tegument protein pp150 acts as a cyclin A2-CDK-dependent sensor of the host cell cycle and differentiation state

Upon cell entry, herpesviruses deliver a multitude of premade virion proteins to their hosts. The interplay between these incoming proteins and cell-specific regulatory factors dictates the outcome of infections at the cellular level. Here, we report a unique type of virion-host cell interaction that is essential for the cell cycle and differentiation state-dependent onset of human cytomegalovirus (HCMV) lytic gene expression. The major tegument 150-kDa phosphoprotein (pp150) of HCMV binds to cyclin A2 via a functional RXL/Cy motif resulting in its cyclin A2-dependent phosphorylation. Alanine substitution of the RXL/Cy motif prevents this interaction and allows the virus to fully escape the cyclin-dependent kinase (CDK)-mediated block of immediate early (IE) gene expression in S/G2 phase that normally restricts the onset of the HCMV replication cycle to G0/G1. Furthermore, the cyclin A2-CDK-pp150 axis is also involved in the establishment of HCMV quiescence in NTera2 cells, showing the importance of this molecular switch for differentiation state-dependent regulation of IE gene expression. Consistent with the known nucleocapsid-binding function of pp150, its RXL/Cy-dependent phosphorylation affects gene expression of the parental virion only, suggesting a cis-acting, virus particle-associated mechanism of control. The pp150 homologs of other primate and mammalian CMVs lack an RXL/Cy motif and accordingly even the nearest relative of HCMV, chimpanzee CMV, starts its lytic cycle in a cell cycle-independent manner. Thus, HCMV has evolved a molecular sensor for cyclin A2-CDK activity to restrict its IE gene expression program as a unique level of self-limitation and adaptation to its human host.

PP2A targeting by viral proteins: a widespread biological strategy from DNA/RNA tumor viruses to HIV-1

Protein phosphatase 2A (PP2A) is a large family of holoenzymes that comprises 1% of total cellular proteins and accounts for the majority of Ser/Thr phosphatase activity in eukaryotic cells. Although initially viewed as constitutive housekeeping enzymes, it is now well established that PP2A proteins represent a family of highly and sophistically regulated phosphatases. The past decade, multiple complementary studies have improved our knowledge about structural and functional regulation of PP2A holoenzymes. In this regard, after summarizing major cellular regulation, this review will mainly focus on discussing a particulate biological strategy, used by various viruses, which is based on the targeting of PP2A enzymes by viral proteins in order to specifically deregulate, for their own benefit, cellular pathways of their hosts. The impact of such PP2A targeting for research in human diseases, and in further therapeutic developments, is also discussed.

Synthesis, transport and antiviral activity of Ala-Ser and Val-Ser prodrugs of cidofovir

We report the synthesis and biological evaluation of Ala-(Val-)l-Ser-CO(2)R prodrugs of 1, where a dipeptide promoiety is conjugated to the P(OH)(2) group of cidofovir (1) via esterification by the Ser side chain hydroxyl group and an ethyl group (4 and 5) or alone (6 and 7). In a murine model, oral administration of 4 or 5 did not significantly increase total cidofovir species in the plasma compared to 1 or 2, but 7 resulted in a 15-fold increase in a rat model and had an in vitro EC(50) value against human cytomegalovirus comparable to 1. Neither 6 nor 7 exhibited toxicity up to 100 μM in KB or HFF cells.

Identification of replication-competent HSV-1 Cgal+ strain signaling targets in human hepatoma cells by functional organelle proteomics

In the present work, we have attempted a comprehensive analysis of cytosolic and microsomal proteomes to elucidate the signaling pathways impaired in human hepatoma (Huh7) cells upon herpes simplex virus type 1 (HSV-1; Cgal(+)) infection. Using a combination of differential in-gel electrophoresis and nano liquid chromatography/tandem mass spectrometry, 18 spots corresponding to 16 unique deregulated cellular proteins were unambiguously identified, which were involved in the regulation of essential processes such as apoptosis, mRNA processing, cellular structure and integrity, signal transduction, and endoplasmic-reticulum-associated degradation pathway. Based on our proteomic data and additional functional studies target proteins were identified indicating a late activation of apoptotic pathways in Huh7 cells upon HSV-1 Cgal(+) infection. Additionally to changes on RuvB-like 2 and Bif-1, down-regulation of Erlin-2 suggests stimulation of Ca(2+)-dependent apoptosis. Moreover, activation of the mitochondrial apoptotic pathway results from a time-dependent multi-factorial impairment as inferred from the stepwise characterization of constitutive pro- and anti-apoptotic factors. Activation of serine-threonine protein phosphatase 2A (PP2A) was also found in Huh7 cells upon HSV-1 Cgal(+) infection. In addition, PP2A activation paralleled dephosphorylation and inactivation of downstream mitogen-activated protein (MAP) kinase pathway (MEK(1/2), ERK(1/2)) critical to cell survival and activation of proapoptotic Bad by dephosphorylation of Ser-112. Taken together, our results provide novel molecular information that contributes to define in detail the apoptotic mechanisms triggered by HSV-1 Cgal(+) in the host cell and lead to the implication of PP2A in the transduction of cell death signals and cell survival pathway arrest.

Cellular serine/threonine phosphatase activity during human cytomegalovirus infection

While the importance of cellular and viral kinases in HCMV replication has been demonstrated, relatively little is known about the activity of cellular phosphatases. We conducted a series of experiments designed to investigate the effect of HCMV infection on cellular serine/threonine phosphatase activity. We found that the abundance of two major cellular serine/threonine phosphatases, PP1 and PP2A, increases during HCMV infection. This was associated with an increase in threonine phosphatase activity in HCMV-infected cells. HCMV infection conferred resistance to the effects of the phosphatase inhibitors calyculin A (CA) and okadaic acid with regards to global protein hyperphosphorylation and the shutoff of protein synthesis. The protective effect of HCMV infection could be overcome at a high concentration of CA, suggesting that cellular phosphatase activity is required for critical cellular processes during HCMV infection. Specifically, phosphatase activity was required to limit the accumulation of phospho-eIF2alpha, but not phospho-PKR, during HCMV infection.

Human cytomegalovirus modulation of signal transduction

An upregulation of cellular signaling pathways is observed in multiple cell types upon human cytomegalovirus (HCMV) infection, suggesting that a global feature of HCMV infection is the activation of the host cell. HCMV initiates and maintains cellular signaling through a multitiered process that is dependent on a series of events: (1) the viral glycoprotein ligand interacts with its cognate receptor, (2) cellular enzymes and viral tegument proteins present in the incoming virion are released and (3) a variety of viral gene products are expressed. Viral-mediated cellular modification has differential outcomes depending on the cell type infected. In permissive cell types, such as diploid fibroblasts, the upregulation of cellular signaling pathways following infection can initiate the viral gene cascade and promote the efficient transcription of multiple viral gene classes. In other cell types, such as endothelial cells and monocytes/macrophages, the upregulation of cellular pathways initiates functional host changes that allow viral spread to multiple organ systems. Together, the modification of signaling processes appears to be part of a thematic strategy deployed by the virus to direct the required functional changes in target cells that ultimately promote viral survival and persistence in the host.

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.

Nuclear trafficking of the human cytomegalovirus pp71 (ppUL82) tegument protein

The human cytomegalovirus tegument protein pp71 localizes to the nucleus immediately upon infection, and functions to initiate viral gene expression. Analysis of a series of random insertion mutations revealed that sequences within the mid region (MR) of pp71 are important for localization to the nucleus. Fusion of MR sequences with eGFP revealed that amino acids 94 to 300 were sufficient to target proteins to the nucleus. Random substitution mutagenesis within this domain resulted in two double substitution mutants, pp71P203T/T223M and pp71T228M/L275Q, with a predominantly cytoplasmic localization. Disruption of nuclear targeting resulted in relocalization of the fusion proteins to a distinct perinuclear region. Using tandem mass spectrometry, we determined that threonine 223 can be phosphorylated. Mutation of this residue to a phosphomimetic amino acid resulted in abrogation of nuclear targeting. These results strongly suggest that the intracellular trafficking of pp71 is regulated by phosphorylation.

Application of proteomics technology for analyzing the interactions between host cells and intracellular infectious agents

Host–pathogen interactions involve protein expression changes within both the host and the pathogen. An understanding of the nature of these interactions provides insight into metabolic processes and critical regulatory events of the host cell as well as into the mechanisms of pathogenesis by infectious microorganisms. Pathogen exposure induces changes in host proteins at many functional levels including cell signaling pathways, protein degradation, cytokines and growth factor production, phagocytosis, apoptosis, and cytoskeletal rearrangement. Since proteins are responsible for the cell biological functions, pathogens have evolved to manipulate the host cell proteome to achieve optimal replication. Intracellular pathogens can also change their proteome to adapt to the host cell and escape from immune surveillance, or can incorporate cellular proteins to invade other cells. Given that the interactions of intracellular infectious agents with host cells are mainly at the protein level, proteomics is the most suitable tool for investigating these interactions. Proteomics is the systematic analysis of proteins, particularly their interactions, modifications, localization and functions, that permits the study of the association between pathogens with their host cells as well as complex interactions such as the host–vector–pathogen interplay. A review on the most relevant proteomic applications used in the study of host–pathogen interactions is presented.

Dephosphorylation of JC virus agnoprotein by protein phosphatase 2A: inhibition by small t antigen

Previous studies have demonstrated that the JC virus (JCV) late regulatory protein agnoprotein is phosphorylated by the serine/threonine-specific protein kinase-C (PKC) and mutants of this protein at the PKC phosphorylation sites exhibit defects in the viral replication cycle. We have now investigated whether agnoprotein phosphorylation is regulated by PP2A, a serine/threonine-specific protein phosphatase and whether JCV small t antigen (Sm t-Ag) is involved in this regulation. Protein-protein interaction studies demonstrated that PP2A associates with agnoprotein and dephosphorylates it at PKC-specific sites. Sm t-Ag was also found to interact with PP2A and this interaction inhibited the dephosphorylation of agnoprotein by PP2A. The interaction domains of Sm t-Ag and agnoprotein with PP2A were mapped, as were the interaction domains of Sm t-Ag with agnoprotein. The middle portion of Sm t-Ag (aa 82-124) was found to be critical for the interaction with both agnoprotein and PP2A and the N-terminal region of agnoprotein for interaction with Sm t-Ag. To further understand the role of Sm t-Ag in JCV regulation, a stop codon was introduced at Ser90 immediately after splice donor site of the JCV early gene and the functional consequences of this mutation were investigated. The ability of this mutant virus to replicate was substantially reduced compared to WT. Next, the functional significance of PP2A in JCV replication was examined by siRNA targeting. Downregulation of PP2A caused a significant reduction in the level of JCV replication. Moreover, the impact of Sm t-Ag on agnoprotein phosphorylation was investigated by creating a double mutant of JCV, where Sm t-Ag stop codon mutant was combined with an agnoprotein triple phosphorylation mutant (Ser7, Ser11 and Thr21 to Ala). Results showed that double mutant behaves much like the triple phosphorylation mutant of agnoprotein during viral replication cycle, which suggests that agnoprotein might be an important target of Sm t-Ag with respect to the regulation of its phosphorylation. Collectively, these results suggest that there is an interplay between agnoprotein, Sm t-Ag and PP2A with respect to the regulation of JCV life cycle and this could be important for the progression of the JCV-induced disease, PML.

D1 dopamine receptor hyperphosphorylation in renal proximal tubules in hypertension

A defect in the coupling of the D(1) receptor (D(1)R) to its G protein/effector complex in renal proximal tubules plays a role in the pathogenesis of spontaneous hypertension. As there is no mutation of the D(1)R gene in the spontaneously hypertensive rat (SHR), we tested the hypothesis that the coupling defect is associated with constitutive desensitization/phosphorylation of the D(1)R. The following experiments were performed: (1) Cell culture and membrane preparations from rat kidneys and immortalized rat renal proximal tubule cells (RPTCs); (2) immunoprecipitation and immunoblotting; (3) cyclic adenosine 3',5' monophosphate and adenylyl cyclase assays; (4) immunofluorescence and confocal microscopy; (5) biotinylation of cell surface proteins; and (6) in vitro enzyme dephosphorylation. Basal serine-phosphorylated D(1)Rs in renal proximal tubules, brush border membranes, and membranes from immortalized RPTCs were greater in SHRs (21.0+/-1.5 density units, DU) than in normotensive rats (7.4+/-2.9 DU). The increased basal serine phosphorylation of D(1)Rs in SHRs was accompanied by decreased expression of D(1)R at the cell surface, and decreased ability of a D(1)-like receptor agonist (fenoldopam) to stimulate cyclic adenosine 3',5' monophosphate (cAMP) production. Increasing protein phosphatase 2A activity with protamine enhanced the ability of fenoldopam to stimulate cAMP accumulation (17+/-4%) and alter D(1)R cell surface expression in intact cells from SHRs. Alkaline phosphatase treatment of RPTC membranes decreased D(1)R phosphorylation and enhanced fenoldopam stimulation of adenylyl cyclase activity (26+/-6%) in SHRs. Uncoupling of the D(1)R from its G protein/effector complex in renal proximal tubules in SHRs is caused, in part, by increased D(1)R serine phosphorylation.

Human herpesvirus 6 open reading frame U14 protein and cellular p53 interact with each other and are contained in the virion

A mass spectroscopic analysis of proteins from human herpesvirus 6 (HHV-6)-infected cells showed that the HHV-6 U14 protein coimmunoprecipitated with the tumor suppressor p53. The binding of U14 to p53 was verified by coimmunoprecipitation experiments in both Molt-3 cells infected with HHV-6 and 293 cells cotransfected with U14 and p53 expression vectors. Indirect immunofluorescence assays (IFAs) showed that by 18 h postinfection (hpi) U14 localized to the dot-like structures observed in both the nucleus and cytoplasm where p53 was partly accumulated. Despite Northern blotting evidence that U14 follows late kinetics, the U14 protein was detected immediately after infection (at 3 hpi) by IFA. In addition, by Western blotting, U14 was detected at 0 hpi or in the presence of cycloheximide which completely abolished the expression of IE1 protein. In addition to U14, p53 was detected at 0 hpi although it was not detected in mock-infected cells. Furthermore, both U14 and p53 were clearly detected in the viral particles by Western blotting and immunoelectron microscopy, supporting the idea that U14 and p53 are incorporated into virions. Our study provides the first evidence of the incorporation of cellular p53 into viral particles and suggests that p53 may play an important role in viral infection.

Identification of proteins in human cytomegalovirus (HCMV) particles: the HCMV proteome

Human cytomegalovirus (HCMV), a member of the herpesvirus family, is a large complex enveloped virus composed of both viral and cellular gene products. While the sequence of the HCMV genome has been known for over a decade, the full set of viral and cellular proteins that compose the HCMV virion are unknown. To approach this problem we have utilized gel-free two-dimensional capillary liquid chromatography-tandem mass spectrometry (MS/MS) and Fourier transform ion cyclotron resonance MS to identify and determine the relative abundances of viral and cellular proteins in purified HCMV AD169 virions and dense bodies. Analysis of the proteins from purified HCMV virion preparations has indicated that the particle contains significantly more viral proteins than previously known. In this study, we identified 71 HCMV-encoded proteins that included 12 proteins encoded by known viral open reading frames (ORFs) previously not associated with virions and 12 proteins from novel viral ORFs. Analysis of the relative abundance of HCMV proteins indicated that the predominant virion protein was the pp65 tegument protein and that gM rather than gB was the most abundant glycoprotein. We have also identified over 70 host cellular proteins in HCMV virions, which include cellular structural proteins, enzymes, and chaperones. In addition, analysis of HCMV dense bodies indicated that these viral particles are composed of 29 viral proteins with a reduced quantity of cellular proteins in comparison to HCMV virions. This study provides the first comprehensive quantitative analysis of the viral and cellular proteins that compose infectious particles of a large complex virus.

Human cytomegalovirus carries a cell-derived phospholipase A2 required for infectivity

Human cytomegalovirus (HCMV) is known to carry host cell-derived proteins and mRNAs whose role in cell infection is not understood. We have identified a phospholipase A2 (PLA2) activity borne by HCMV by using an assay based on the hydrolysis of fluorescent phosphatidylcholine. This activity was found in all virus strains analyzed and in purified strains. It was calcium dependent and was sensitive to inhibitors of cytosolic PLA2 (cPLA2) but not to inhibitors of soluble PLA2 or calcium-independent PLA2. No other phospholipase activity was detected in the virus. Purified virus was found to contain human cellular cPLA2alpha, as detected by monoclonal antibody. No homology with PLA2 was found in the genome of HCMV, indicating that HCMV does not code for a PLA2. Decreased de novo expression of immediate-early proteins 1 and 2 (IE1 and IE2), tegument phosphoprotein pp65, and virus production was observed when HCMV was treated with inhibitors of cPLA2. Cell entry of HCMV was not altered by those inhibitors, suggesting the action of cPLA2 was postentry. Together, our results indicate a selective sorting of a cell-derived cPLA2 during HCMV maturation, which is further required for infectivity.

Requirement of multiple cis-acting elements in the human cytomegalovirus major immediate-early distal enhancer for viral gene expression and replication

We have shown previously that the human cytomegalovirus (HCMV) major immediate-early (MIE) distal enhancer is needed for MIE promoter-dependent transcription and viral replication at low multiplicities of infection (MOI). To understand how this region works, we constructed and analyzed a series of HCMVs with various distal enhancer mutations. We show that the distal enhancer is composed of at least two parts that function independently to coordinately activate MIE promoter-dependent transcription and viral replication. One such part is contained in a 47-bp segment that has consensus binding sites for CREB/ATF, SP1, and YY1. At low MOI, these working parts likely function in cis to directly activate MIE gene expression, thus allowing viral replication to ensue. Three findings support the view that these working parts are likely cis-acting elements. (i) Deletion of either part of a bisegmented distal enhancer only slightly alters MIE gene transcription and viral replication. (ii) Reversing the distal enhancer's orientation largely preserves MIE gene transcription and viral replication. (iii) Placement of stop codons at -300 or -345 in all reading frames does not impair MIE gene transcription and viral replication. Lastly, we show that these working parts are dispensable at high MOI, partly because of compensatory stimulation of MIE promoter activity and viral replication that is induced by a virion-associated component(s) present at a high viral particle/cell ratio. We conclude that the distal enhancer is a complex multicomponent cis-acting region that is required to augment both MIE promoter-dependent transcription and HCMV replication.

Molecular pathways in virus-induced cytokine production

Virus infections induce a proinflammatory response including expression of cytokines and chemokines. The subsequent leukocyte recruitment and antiviral effector functions contribute to the first line of defense against viruses. The molecular virus-cell interactions initiating these events have been studied intensively, and it appears that viral surface glycoproteins, double-stranded RNA, and intracellular viral proteins all have the capacity to activate signal transduction pathways leading to the expression of cytokines and chemokines. The signaling pathways activated by viral infections include the major proinflammatory pathways, with the transcription factor NF-kappaB having received special attention. These transcription factors in turn promote the expression of specific inducible host proteins and participate in the expression of some viral genes. Here we review the current knowledge of virus-induced signal transduction by seven human pathogenic viruses and the most widely used experimental models for viral infections. The molecular mechanisms of virus-induced expression of cytokines and chemokines is also analyzed.

Protein phosphatase 2A: the Trojan Horse of cellular signaling

Dynamic phosphorylation and dephosphorylation of proteins are fundamental mechanisms utilized by cells to transduce signals. Whereas transduction by protein kinases has been a major focus of studies in the last decade, protein phosphatase 2A (PP2A) enzymes emerge in this millenium as the most fashionable players in cellular signaling. Viral proteins target specific PP2A enzymes in order to deregulate chosen cellular pathways in the host and promote viral progeny. The observation that a variety of viruses utilize PP2A to alienate cellular behavior emphasizes the fundamental importance of PP2A in signal transduction. This review will primarily focus on discussing the uniqueness of PP2A regulation and uncovering the critical role played by protein-protein interactions in the modulation of PP2A signaling. Moreover, the place of PP2A in signaling pathways and its functional significance for human diseases will be discussed.

Type 2A protein phosphatase, the complex regulator of numerous signaling pathways

Type 2A protein phosphatase (PP2A) comprises a diverse family of phosphoserine- and phosphothreonine-specific enzymes ubiquitously expressed in eukaryotic cells. Common to all forms of PP2A is a catalytic subunit (PP2Ac) which can form two distinct complexes, one with a structural subunit termed PR65/A and another with an alpha4 protein. The PR65/A-PP2Ac dimer may further associate with a regulatory subunit and form a trimeric holoenzyme. To date, three distinct families of regulatory subunits, which control substrate selectivity and phosphatase activity and target PP2A holoenzymes to their substrates, have been identified. Other molecular mechanisms that regulate PP2Ac function include phosphorylation, carboxyl methylation, inhibition by intracellular protein inhibitors (I(1)(PP2A) and I(2)(PP2A)), and stimulation by ceramide. PP2A dephosphorylates many proteins in vitro, but in vivo protein kinases and transcription factors appear to represent two major sets of substrates. Several natural compounds can inhibit PP2A activity and are used to study its function. Mutations in genes encoding PR65/A subunits have been identified in several different human cancers and the PP2A inhibitor, termed fostriecin, is being tested as an anticancer drug. Thus, a more thorough understanding of PP2A structure and function may lead to the development of novel strategies against human diseases.

Protein phosphatase 2A: a definite player in viral and parasitic regulation

Cells use phosphorylation/dephosphorylation mechanisms to regulate the activity of several proteins required to transmit information from the cell surface to the nucleus. Recent studies have significantly increased our knowledge regarding the structure/function of one major regulator of cell phosphorylation: protein phosphatase 2A (PP2A). This review will discuss the role of PP2A in virology and parasitology.

Activation of the mitogen-activated protein kinase p38 by human cytomegalovirus infection through two distinct pathways: a novel mechanism for activation of p38

Recent evidence indicates activated mitogen-activated protein kinase (MAPK) p38 has a critical function in human cytomegalovirus (HCMV) viral DNA replication in infected human fibroblasts. To elucidate the mechanism of HCMV-mediated p38 activation, we have performed a detailed analysis of p38 activation and the kinases associated with this activation at different times postinfection. We demonstrate that p38 kinase activity is strongly increased following viral infection. Inhibition of this activity significantly inhibited HCMV-induced hyperphosphorylation of pRb and phosphorylation of heat shock protein 27, suggesting that p38 activation is involved in virus-mediated changes in host cell metabolism throughout the course of infection. We then provide evidence that p38 activation is mediated by different mechanisms at early times versus later times of infection. At early times of infection (8 to 14 h postinfection [hpi]), when p38 activation is first observed, no significant activation of the three kinases which can directly phosphorylate p38 (namely, MKK3, MKK6, and MKK4) is detected. Using vectors which express dominant negative proteins, we demonstrate that basal MKK6 kinase activity is necessary for HCMV-mediated p38 activation at these early times of infection (12 hpi). Then, we use ATP depletion to show that at 12 hpi, HCMV inhibits dephosphorylation of activated p38. These two experiments suggest that HCMV activates p38 by inhibition of dephosphorylation of p38. In contrast to early times of infection, at later times of infection (48 to 72 hpi), increased MKK3/6, but not MKK4, activity is observed. These results indicate that at early times of HCMV infection, increased steady-state levels of activated p38 is mediated at least in part by inhibition of dephosphorylation of p38, while at later times of infection p38 activation is due to increased activity of the upstream kinases MKK3 and MKK6. These findings indicate that HCMV has developed multiple mechanisms to ensure activation of the MAPK p38, a kinase critical to viral infection.

Activation of herpesvirus gene expression by the human cytomegalovirus protein pp71

The activation of gene expression by the human cytomegalovirus (HCMV) particle was investigated. The HCMV major immediate-early (IE) promoter was cloned upstream of the Escherichia coli lacZ coding sequences, and the resulting cassette was introduced into the genome of a herpes simplex virus type 1 (HSV-1) mutant lacking functional VP16. Upon infection with the HSV-1 recombinant in the presence of cycloheximide, to block de novo protein synthesis, expression of lacZ-specific transcripts was increased by fivefold when HCMV was included in the inoculum. Accumulation of HSV-1 IE RNAs was also stimulated by coinfection with HCMV, as was expression of the adenovirus 5 VAI transcript when the VAI gene was cloned into the HSV-1 genome. Coinfection with HCMV did not alter mRNA stability or uncoating of the HSV-1 genome. The coding sequences for the HCMV phosphoprotein pp71, controlled by the HCMV IE promoter, were cloned into an HSV-1 recombinant impaired for the production of the three major transactivators (VP16, ICP0, and ICP4) to yield a recombinant (in1324) which expressed pp71 but did not cause significant cytotoxicity. Infection with in1324 resulted in stimulation of HCMV IE, HSV-1 IE, and VAI expression, demonstrating that pp71 is responsible for the effects we observed when using the entire HCMV particle. Therefore, HCMV pp71 exhibits novel properties in its ability to stimulate gene expression from a range of promoters present in a herpesvirus genome.

Vimentin dephosphorylation by protein phosphatase 2A is modulated by the targeting subunit B55

The intermediate filament protein vimentin is a major phosphoprotein in mammalian fibroblasts, and reversible phosphorylation plays a key role in its dynamic rearrangement. Selective inhibition of type 2A but not type 1 protein phosphatases led to hyperphosphorylation and concomitant disassembly of vimentin, characterized by a collapse into bundles around the nucleus. We have analyzed the potential role of one of the major protein phosphatase 2A (PP2A) regulatory subunits, B55, in vimentin dephosphorylation. In mammalian fibroblasts, B55 protein was distributed ubiquitously throughout the cytoplasm with a fraction associated to vimentin. Specific depletion of B55 in living cells by antisense B55 RNA was accompanied by disassembly and increased phosphorylation of vimentin, as when type 2A phosphatases were inhibited using okadaic acid. The presence of B55 was a prerequisite for PP2A to efficiently dephosphorylate vimentin in vitro or to induce filament reassembly in situ. Both biochemical fractionation and immunofluorescence analysis of detergent-extracted cells revealed that fractions of PP2Ac, PR65, and B55 were tightly associated with vimentin. Furthermore, vimentin-associated PP2A catalytic subunit was displaced in B55-depleted cells. Taken together these data show that, in mammalian fibroblasts, the intermediate filament protein vimentin is dephosphorylated by PP2A, an event targeted by B55.

Rationale for a vaccine using cellular-derived epitope presented by HIV isolates

It has been clearly demonstrated that cellular antigens (HLA, beta 2-microglobulin) are incorporated at the virion surface. The same epitope derived from beta 2-microglobulin is presented on all virus isolates. The peptide was identified by blocking the neutralizing capacity of a monoclonal antibody directed to R7V epitope: using this peptide for developing an ELISA, we have detected antibodies in nonprogressor patients with neutralizing property to laboratory strains and primary isolates. Purified anti-R7V antibodies immunoprecipitate all HIV isolates at concentration dependent. R7V is immunogenic after rabbit immunization and induces HIV immunoprecipitating and neutralizing antibodies. The patient's as well as the immunized rabbit antibodies did not bind to any cell. No autoimmune disease is found in nonprogressor patients. For all these reasons, R7V is a good candidate for an universal AIDS vaccine.

Polo-like kinase 1 as a target for human cytomegalovirus pp65 lower matrix protein

Human cytomegalovirus (HCMV) pp65 protein is the major constituent of viral dense bodies but is dispensable for viral growth in vitro. pp65 copurifies with a S/T kinase activity and has been implicated in phosphorylation of HCMV IE1 immediate-early protein and its escape from major histocompatibility complex 1 presentation. Furthermore, the presence of pp65 correlates with a virion-associated kinase activity. To clarify the role of pp65, yeast two-hybrid system (THS) screening was performed to identify pp65 cellular partners. A total of 18 out of 48 yeast clones harboring cDNAs for putative pp65 binding proteins encoded the Polo-like kinase 1 (Plk1) C-terminal domain. Plk1 behaved as a bona fide pp65 partner in THS control crosses, and the interaction was confirmed by in vitro binding experiments. Endogenous Plk1 was coimmunoprecipitated with pp65 from transiently transfected COS7 cells. In infected fibroblasts, Plk1 was coimmunoprecipitated with pp65 at late infection stages. Furthermore, Plk1 was detected within wild-type HCMV particles but not within the particles of a pp65-negative mutant (RVAd65). The hydrophilic region of pp65 was phosphorylated in vitro by Plk1. These results suggest that one function of pp65 may be to capture a cell kinase, perhaps in order to alter its activity, nucleotide preference, substrate specificity, or subcellular localization to the advantage of HCMV.

Identification of positive and negative regulatory regions involved in regulating expression of the human cytomegalovirus UL94 late promoter: role of IE2-86 and cellular p53 in mediating negative regulatory function

The human cytomegalovirus (HCMV) UL94 gene product is a herpesvirus-common virion protein that is expressed with true late kinetics. To identify the important cis- and trans-acting factors which contribute to UL94 transcriptional regulation, we have cloned, sequenced, and analyzed UL94 promoter function by transient transfection analysis. Transfection of UL94 promoter-reporter gene constructs into permissive human fibroblasts or U373(MG) cells indicated that promoter activity was detected following infection with HCMV. Point mutations within a TATA-like element located upstream of the RNA start site significantly reduced UL94 promoter activity. Deletion mutagenesis of the promoter indicated that a positive regulatory element (PRE) was likely to exist downstream of the UL94 mRNA start site, while a negative regulatory element (NRE) was present upstream of the TATA box. At late times of infection, the PRE appeared to have a dominant effect over the NRE to stimulate maximum levels of UL94 promoter activity, while at earlier times of infection, no activity associated with the PRE could be detected. The NRE, however, appeared to cause constitutive down-regulation of UL94 promoter activity. Binding sites for the cellular p53 protein located within the NRE appeared to contribute to NRE function, and NRE function could be recapitulated in cotransfection assays by concomitant expression of p53 and HCMV IE2-86 protein. Our results suggest a novel mechanism by which the cellular protein p53, which is involved in both transcriptional regulation and progression of cellular DNA synthesis, plays a central role in the regulation of a viral promoter which is not activated prior the onset of viral DNA replication.

Modulation of RANTES production by human cytomegalovirus infection of fibroblasts

Chemokines play a major role in inflammatory responses and affect hematopoiesis both negatively and positively. We show that fresh isolates and laboratory strains (Towne and Ad-169) of human cytomegalovirus (HCMV) induce production of the CC chemokine RANTES in fibroblasts. Induction of extracellular RANTES production occurred as early as 8 h after infection, peaked around 24 h after infection, and was almost undetectable by 48 and 72 h. Upregulation occurred in the absence of viral DNA synthesis, suggesting that it was due to immediate-early-early HCMV gene expression. CMV infection stimulated RANTES transcription, since reverse transcription-PCR detected a sharp increase in RANTES RNA which persisted even when extracellular RANTES was no longer detected. Induction of RANTES in fibroblasts was not due to prior induction of tumor necrosis factor alpha or interleukin 1 beta. Down-regulation required an active viral genome. Decrease of RANTES in culture supernatants may be associated with the appearance of the HCMV CC chemokine receptor US28, since we show that this gene is transcribed as early as 8 h after infection. Modulation of CC chemokine production early during CMV infection might have a regulatory effect on viral replication, as well as affect immune surveillance.

Host cell-dependent alterations in envelope components of human immunodeficiency virus type 1 virions

In addition to gp41 and gp120, an array of cell adhesion molecules is present on the envelope of human immunodeficiency virus type 1 (HIV-1). To examine the role of the host cell in the acquisition of these molecules by virions, both laboratory-adapted and primary isolates were sequentially passaged into different host cells. Viruses obtained from the various host cells were examined for the presence of 10 different cell-derived molecules by a virus binding enzyme-linked immunosorbent assay. Virus progeny raised in peripheral blood mononuclear cells expressed most of the adhesion molecules tested, with the level of LFA-1 being the highest. When viruses were passaged into CEM-SS or SupT1 cells, the expression of most of the adhesion molecules on the virus envelope was lost. In contrast, when viruses were passaged into MT2 cells, the virus progeny bore high levels of LFA-3, ICAM-1, and major histocompatibility complex classes I and II. These studies demonstrate for the first time the host cell dependence of the adhesion molecule profile present on the envelope of primary isolates of HIV-1. The presence of several adhesion molecules that have not previously been identified as components of the envelope of either laboratory or primary isolates is also described. In addition, we show that the adhesion molecule profile of the virions is acquired, or lost, within one passage and is maintained with subsequent passages in the same cell type.

The effect of human cytomegalovirus on selected functions of peripheral blood monocytes

The effect of in vitro infection of human cytomegalovirus (HCMV) on various monocyte functions relevant to antimicrobial defence mechanisms has been investigated: the phagocytic activity of monocytes, the release of lysozyme and intracellular concentration of acid phosphatase, and the release of the cytokines interleukin-1 (IL-1), IL-6, and tumour necrosis factor-alpha (TNF-alpha). HCMV significantly inhibited the release of lysozyme and intracellular concentration of acid phosphatase. Regarding the phagocytic activity and the release of cytokines, there was considerable variation in the HCMV effect among the different blood donors tested. There was no clear tendency in the observed results; both stimulation and inhibition were seen. The HCMV-specific pp65 was detected in the nucleus of about 1% of the monocytes 3 h after infection and HCMV-specific IE antigens were found in about 0.1% of the monocytes 2 days postinfection. No E- or L-gene expression was observed and no infectious virus was produced in the monocytes. Our results indicate that HCMV infection may influence monocyte functions in spite of no productive infection of these cells.