Speckled 100 kDa gene in pigs: Alternative splicing, subcellular localization, and response to interferon-α stimulation

Speckled 100 kDa (Sp100) plays an important role in the antiviral immune response, however, little is known about porcine Sp100. In this study, porcine Sp100 was cloned and its response to interferon (IFN) α was identified. We obtained the cDNA (V1) of the gene, SP100, and seven alternative splicing variants (V2-8). Isoform V1 encoded a 386 amino acid protein and contained a homogeneously-staining region (HSR) domain. Isoforms V3, 4, 6 and 7 were deletion/insertion variants and contained HSR domain as V1. The splicing of porcine SP100 was very complicated and many transcripts existed as revealed by cloning and minigene analyses. Using GFP-fusion constructs isoforms V1, 3, 4, 6 and 7 were localized to nucleus and the nuclear localization signal was identified as PSNRKRR at positions 331-337 of V1. Porcine SP100 was unevenly distributed in all tissues studied and differentially expressed between pigs with different disease-resistance/susceptibilities. Porcine SP100 was strongly increased by IFNα due to the existence of an IFN-stimulated response element in the promoter. A single nucleotide - 70A > C polymorphism enhanced promoter activity. The results provided the basis for determining the role of Sp100 in antiviral responses and may assist in breeding pigs with high disease-resistance.

Manipulation of Promyelocytic Leukemia Protein Nuclear Bodies by Marek's Disease Virus Encoded US3 Protein Kinase

Promyelocytic leukemia protein nuclear bodies (PML-NBs) are dynamic nuclear structures, shown to be important for herpesvirus replication; however, their role in regulating Marek’s disease virus (MDV) infection has not been studied. MDV is an oncogenic alphaherpesvirus that causes lymphoproliferative disease in chickens. MDV encodes a US3 serine/threonine protein kinase that is important for MDV replication and gene expression. In this study, we studied the role of MDV US3 in regulating PML-NBs. Using an immunofluorescence assay, we found that MDV US3 disrupts PML and SP100 in a kinase dependent manner. In addition, treatment with MG-132 (a proteasome inhibitor) could partially restore the levels of PML and SP100, suggesting that a cellular proteasome dependent degradation pathway is involved in MDV US3 induced disruption of PML and SP100. These findings provide the first evidence for the interplay between MDV proteins and PML-NBs.

The Speckled Protein (SP) Family: Immunity's Chromatin Readers

Chromatin 'readers' are central interpreters of the epigenome that facilitate cell-specific transcriptional programs and are therapeutic targets in cancer and inflammation. The Speckled Protein (SP) family of chromatin 'readers' in humans consists of SP100, SP110, SP140, and SP140L. SPs possess functional domains (SAND, PHD, bromodomain) that dock to DNA or post-translationally modified histones and a caspase activation and recruitment domain (CARD) to promote multimerization. Mutations within immune expressed SPs associate with numerous immunological diseases including Crohn's disease, multiple sclerosis, chronic lymphocytic leukemia, veno-occlusive disease with immunodeficiency, as well as Mycobacterium tuberculosis infection, underscoring their importance in immune regulation. In this review, we posit that SPs are central chromatin regulators of gene silencing that establish immune cell identity and function.

Early Nuclear Events after Herpesviral Infection

Herpesviruses are important pathogens that can cause significant morbidity and mortality in the human population. Herpesviruses have a double-stranded DNA genome, and viral genome replication takes place inside the nucleus. Upon entering the nucleus, herpesviruses have to overcome the obstacle of cellular proteins in order to enable viral gene expression and genome replication. In this review, we want to highlight cellular proteins that sense incoming viral genomes of the DNA-damage repair (DDR) pathway and of PML-nuclear bodies (PML-NBs) that all can act as antiviral restriction factors within the first hours after the viral genome is released into the nucleus. We show the function and significance of both nuclear DNA sensors, the DDR and PML-NBs, and demonstrate for three human herpesviruses of the alpha-, beta- and gamma-subfamilies, HSV-1, HCMV and KSHV respectively, how viral tegument proteins antagonize these pathways.

Mitogen-activated protein kinase-mediated licensing of interferon regulatory factor 3/7 reinforces the cell response to virus

The induction of the intrinsic antiviral defense in mammals relies on the accumulation of foreign genetic material. As such, complete engagement of this response is limited to replication-competent viruses. Interferon regulatory factors (IRFs) are mediators of this defense with shared enhancer elements but display a spectrum of transcriptional potential. Here we describe a mechanism designed to enhance this response should a pathogen not be successfully inhibited. We find that activation of IRF7 results in the induction of MAP3K8 and restructuring of the antiviral transcriptome. MAP3K8 mediates the phosphorylation and repression of IRF3 homodimers to promote greater transcriptional activity through utilization of IRF3:IRF7 heterodimers. Among the genes influenced by the MAP3K8/IRF7 signaling axis are members of the SP100 gene family that serve as general transcriptional enhancers of the antiviral defense. We propose that this feed forward loop serves to reinforce the cellular response and is reserved for imminent threats to the host.

L2, the minor capsid protein of papillomavirus

The capsid protein L2 plays major roles in both papillomavirus assembly and the infectious process. While L1 forms the majority of the capsid and can self-assemble into empty virus-like particles (VLPs), L2 is a minor capsid component and lacks the capacity to form VLPs. However, L2 co-assembles with L1 into VLPs, enhancing their assembly. L2 also facilitates encapsidation of the ∼8 kbp circular and nucleosome-bound viral genome during assembly of the non-enveloped T=7d virions in the nucleus of terminally differentiated epithelial cells, although, like L1, L2 is not detectably expressed in infected basal cells. With respect to infection, L2 is not required for particles to bind to and enter cells. However L2 must be cleaved by furin for endosome escape. L2 then travels with the viral genome to the nucleus, wherein it accumulates at ND-10 domains. Here, we provide an overview of the biology of L2.