Visualization of the dynamic multimerization of human Cytomegalovirus pp65 in punctuate nuclear foci

The Aβ42 Peptide and IAPP Physically Interact in a Yeast-Based Assay

Numerous studies have demonstrated that people with type 2 diabetes mellitus (associated with IAPP peptide aggregation) show an increased incidence of Alzheimer's disease (associated with Aβ aggregation), but the mechanism responsible for this correlation is presently unknown. Here, we applied a yeast-based model to study the interactions of IAPP with PrP (associated with TSEs) and with the Aβ42 peptide. We demonstrated that fluorescently tagged IAPP forms detergent-resistant aggregates in yeast cells. Using the FRET approach, we showed that IAPP and Aβ aggregates co-localize and physically interact in yeast cells. We also showed that this interaction is specific and that there is no interaction between IAPP and PrP in the yeast system. Our data confirmed a direct physical interaction between IAPP and Aβ42 aggregates in a living cell. Based on these findings, we hypothesize that this interaction may play a crucial role in seeding Aβ42 aggregation in T2DM patients, thereby promoting the development of AD.

Human Cytomegalovirus Primary Infection and Reactivation: Insights From Virion-Carried Molecules

Human cytomegalovirus (HCMV), a ubiquitous beta-herpesvirus, is able to establish lifelong latency after initial infection. Periodical reactivation occurs after immunosuppression, remaining a major cause of death in immunocompromised patients. HCMV has to reach a structural and functional balance with the host at its earliest entry. Virion-carried mediators are considered to play pivotal roles in viral adaptation into a new cellular environment upon entry. Additionally, one clear difference between primary infection and reactivation is the idea that virion-packaged factors are already formed such that those molecules can be used swiftly by the virus. In contrast, virion-carried mediators have to be transcribed and translated; thus, they are not readily available during reactivation. Hence, understanding virion-carried molecules helps to elucidate HCMV reactivation. In this article, the impact of virion-packaged molecules on viral structure, biological behavior, and viral life cycle will be reviewed.

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.

Crystal Structure of Human Herpesvirus 6B Tegument Protein U14

The tegument protein U14 of human herpesvirus 6B (HHV-6B) constitutes the viral virion structure and is essential for viral growth. To define the characteristics and functions of U14, we determined the crystal structure of the N-terminal domain of HHV-6B U14 (U14-NTD) at 1.85 Å resolution. U14-NTD forms an elongated helix-rich fold with a protruding β hairpin. U14-NTD exists as a dimer exhibiting broad electrostatic interactions and a network of hydrogen bonds. This is first report of the crystal structure and dimerization of HHV-6B U14. The surface of the U14-NTD dimer reveals multiple clusters of negatively- and positively-charged residues that coincide with potential functional sites of U14. Three successive residues, L424, E425 and V426, which relate to viral growth, reside on the β hairpin close to the dimer's two-fold axis. The hydrophobic side-chains of L424 and V426 that constitute a part of a hydrophobic patch are solvent-exposed, indicating the possibility that the β hairpin region is a key functional site of HHV-6 U14. Structure-based sequence comparison suggests that U14-NTD corresponds to the core fold conserved among U14 homologs, human herpesvirus 7 U14, and human cytomegalovirus UL25 and UL35, although dimerization appears to be a specific feature of the U14 group.

Human herpesvirus 6B (HHV-6B), a causative agent of exanthema subitum for children and immunocompromised adults, encodes numerous tegument proteins that constitute the viral matrix. HHV-6B U14 is a tegument protein essential for viral propagation, and additionally it interacts with host factors such as tumor suppressor p53 and cellular protein EDD, thereby regulating host cell responses. Here, we report the molecular structure of HHV-6B U14 at an atomic resolution. The N-terminal domain of U14 (U14-NTD) adopts an elongated, helix-rich fold without any significant overall similarity to known structures. U14-NTD forms a 100 kDa homodimer through electrostatic interactions and a wide hydrogen bond network. The U14-NTD homodimer displays four clusters of electrostatic potential with deep grooves, implying multiple binding sites for other viral or host proteins. U14-NTD corresponds to the core fold shared by homologous proteins of human herpesvirus 7 (HHV-7) and of human cytomegalovirus, although dimerization seems to be specific to HHV-6 and HHV-7. The U14-NTD structure provides clues to promote further analysis on the role and behavior of U14 in the pathogenesis of HHV-6. It also leads to a comprehensive understanding of the U14 homologs in beta herpesviruses, and furthermore contributes to the overall knowledge about tegument proteins in herpesviruses.

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.

Fragment of tegument protein pp65 of human cytomegalovirus induces autoantibodies in BALB/c mice

Introduction

Human cytomegalovirus (HCMV) infection has been implicated in the development of autoimmunity, including systemic lupus erythematosus (SLE). Previously we reported that HCMV phosphoprotein 65 (pp65) could induce early onset of autoantibody and glomerulonephritis on lupus-prone NZB/W mice. This study further examined whether the B cell epitope(s) in pp65 is able to drive the development of autoantibody.

Methods

Sera from SLE patients or HCMVpp65-immunized mice were analyzed for anti-nuclear antibody by immunoblotting, enzyme-linked immunosorbent assay (ELISA), immunofluorescent stain and Crithidia luciliae stain. The deposition of immunoglobulin to the kidney was also examined by immunofluorescent stain. The interactions between pp65 sub-fragment to cellular proteins were revealed by yeast two-hybrid analyses.

Results

Our results showed that most SLE patients possessed antibodies to the C-terminal half of the HCMVpp65 antigen. Of these positive sera, 73% were also positive to the pp65_336-439 sub-fragment. The immunization of pp65_336-439 induced formation of multiple anti-nuclear antibodies, including anti-chromatin, anti-centriole, anti-mitotic spindle type I/II (MSA I/II) and a significant elevation of anti-double-stranded DNA (anti-dsDNA) antibodies on BALB/c mice. Yeast two-hybrid analyses revealed the binding of pp65_336-439 sub-fragment to cellular proteins. Immunoglobulin deposition on glomeruli was also detected on pp65_336-439-immunized mice.

Conclusions

Our data suggested that HCMVpp65_336-439 sub-fragment may induce cross-reactive antibodies to several nuclear antigens, which could contribute to the development of autoimmunity in genetic-suspected individuals.

Interacting domains of P14-3-3 and actin involved in protein-protein interactions of living cells

14-3-3 proteins are conserved regulatory proteins present in all eukaryotic cells that control numerous cellular activities via targeted protein interactions. To elucidate the interaction between P14-3-3 from Physarum polycephalum and actin in living cells, PCR and DNA recombination were used to generate various P14-3-3 and actin constructs. Yeast two-hybrid assay and FRET were employed to characterize the interaction between P14-3-3 and actin. The two-hybrid assay indicated that P14-3-3 N-terminal 76-108 amino acids and the C-terminal 207-216 amino acids played an important role in mediating interactions with actin, and the actin N-terminal 1-54 amino acids and the C-terminal 326-376 amino acids are also crucial in the interactions with the mPa, a P14-3-3 with mutations at Ser62 (Ser62 → Gly62). Mutations to potential phosphorylation sites did not affect interactions between P14-3-3 and actin. FRET results demonstrated that P14-3-3 co-localized with actin with a FRET efficiency of 22.2% and a distance of 7.4 nm and that P14-3-3 N-terminal 76-108 and C-terminal 207-216 amino acids were important in mediating this interaction, the truncated actin peptides without either the N-terminal 1-54 or C-terminal 326-376 amino acids interacted with P14-3-3, consistent with the results obtained from the yeast two-hybrid assay. Based on data obtained, we identified critical actin and P14-3-3 contact regions.

Modification of the major tegument protein pp65 of human cytomegalovirus inhibits virus growth and leads to the enhancement of a protein complex with pUL69 and pUL97 in infected cells

The tegument protein pp65 of human cytomegalovirus (HCMV) is abundant in lytically infected human foreskin fibroblasts (HFF), as well as in virions and subviral dense bodies (DB). Despite this, we showed previously that pp65 is dispensable for growth in HFF. In the process of refining a DB-based vaccine candidate, different HCMV mutants were generated, expressing a dominant HLA-A2-presented peptide of the IE1 protein fused to pp65. One of the mutant viruses (RV-VM1) surprisingly showed marked impairment in virus release from HFF. We hypothesized that analysis of the phenotypic alterations of RV-VM1 would provide insight into the functions of pp65, poorly defined thus far. RV-VM1 infection resulted in nuclear retention of the fusion protein and reorganization of nuclear inclusion bodies. Coimmunoprecipitation experiments suggested that wild-type (wt) pp65 and pp65-VM1 were substrates of the viral pUL97 kinase in vitro and formed a complex with the viral RNA-export protein pUL69 and with pUL97 in lysates of infected cells. No evidence for an impairment of pUL97 within this complex was found. However, RV-VM1 replication in infected cells was resistant to a pUL97 inhibitor, and pUL97 inhibitors mimicked the mutant in terms of pp65 being retained in the nucleus. The results suggest that the life cycle of RV-VM1 was impeded at the stages of early-late transcription, RNA export or capsid maturation. wt-pp65 may play a role at these stages of infection, and complex formation with pUL69 and pUL97 may be important for that function.