EDITORIAL
Journal of Virology (JVI) marks its 50th year of publishing in 2017. To highlight particularly noteworthy JVI articles from over the years, 2017 issues are featuring Classic Spotlights selected from the archives by the editors. These Classic Spotlights are appearing chronologically, and in this issue, we have selected articles from 2004 and 2005.
Identification of Proteins in Human Cytomegalovirus (HCMV) Particles: the HCMV Proteome
Human cytomegalovirus (HCMV) is a betaherpesvirus that encodes over 200 proteins. The HCMV virion is composed of an icosahedral protein capsid that contains a linear ∼230-kbp double-stranded DNA genome and an outer layer of proteins called the tegument, which is surrounded by a cellular lipid bilayer containing viral glycoproteins. While some of the viral and cellular proteins that constitute the infectious HCMV virion were identified by biochemical approaches, the complete protein composition of the HCMV virion had not been elucidated. Varnum et al. (S. M. Varnum et al., J Virol 78:10960–10966, 2004, https://doi.org/10.1128/JVI.78.20.10960-10966.2004) used mass spectrometry (MS)-based proteomic approaches and identified 71 HCMV-encoded proteins that included 12 proteins encoded by known viral open reading frames (ORFs), which were not known previously to be associated with virions. In addition, 12 proteins from novel viral ORFs were identified. The relative abundance of HCMV proteins was analyzed and the predominant virion protein was the pp65 tegument protein. Glycoprotein M (gM) was the most abundant glycoprotein. Over 70 host cellular proteins were identified in HCMV virions, including cellular structural proteins, enzymes, and chaperones. This study provided the first comprehensive quantitative analysis of the viral and cellular proteins that comprise infectious particles of a large complex virus.
Adenovirus VA1 Noncoding RNA Can Inhibit Small Interfering RNA and MicroRNA Biogenesis
RNA interference (RNAi) is a mechanism of posttranscriptional gene silencing that relies on short noncoding RNAs. There are two distinct types, small interfering RNAs (siRNAs) and microRNAs (miRNAs), and both are incorporated into a large protein complex, the RNA-induced silencing complex (RISC), where they guide RISC to complementary mRNA targets. RISC binding can lead to mRNA degradation for a fully complementary mRNA target, or translation inhibition for mRNA targets with a partial mismatch. Lu and Cullen (S. Lu and B. R. Cullen, J Virol 78:12868–12876, 2004, https://doi.org/10.1128/JVI.78.23.12868-12876.2004) reported that the highly structured ∼160-nucleotide adenoviral VA1 noncoding RNA can inhibit RNAi at physiological levels of expression. VA1 is expressed at very high levels in adenovirus-infected cells; however, VA1 was able to inhibit RNAi even at levels significantly below those seen in virus-infected cells. VA1 RNA inhibits the biogenesis of miRNAs and siRNAs by inhibiting nuclear export of the pre-miRNA and short hairpin RNA (shRNA) precursors for mature miRNAs and siRNAs. VA1 binds Dicer, inhibiting Dicer function. These findings identified the first gene product encoded by a human virus that is able to inhibit RNAi in human cells in culture.
Inhibition of Alpha/Beta Interferon Signaling by the NS4B Protein of Flaviviruses
Arthropod-borne flaviviruses are positive-strand RNA viruses that are important human pathogens and which include dengue (DEN), West Nile, and yellow fever viruses. The positive-strand RNA genome is translated into a polyprotein precursor, which is co- and posttranslationally processed by cellular and viral proteases to yield three structural and seven nonstructural proteins. Muñoz-Jordán et al. (J. L. Muñoz-Jordán, G. G. Sánchez-Burgos, M. Laurent-Rolle, and A. García-Sastre, Proc Natl Acad Sci U S A 100:14333–14338, 2003, https://doi.org/10.1073/pnas.2335168100) previously analyzed the ability of DEN type 2 to block the interferon (IFN) system and reported that expression of the DEN nonstructural protein NS4B partially blocked the IFN-induced signal transduction cascade by interfering with STAT1 phosphorylation. Muñoz-Jordán et al. (J. L. Muñoz-Jordán et al., J Virol 79:8004–8013, 2005, https://doi.org/10.1128/JVI.79.13.8004-8013.2005) later reported that this function of NS4B is conserved in West Nile and yellow fever viruses. Deletion analysis showed that the first 125 amino acids of DEN NS4B were sufficient for inhibition of alpha/beta IFN (IFN-α/β) signaling. Coexpression of DEN NS4B and NS4A, another nonstructural protein, resulted in enhanced inhibition of interferon-stimulated response element (ISRE) promoter activation in response to IFN-α/β. Expression of the precursor NS4A/B fusion protein did not inhibit IFN signaling. When the fusion protein was cleaved by the viral peptidase NS2B/NS3, inhibition occurred, indicating that viral polyprotein processing is required for the anti-IFN function.
Cloning and Identification of a MicroRNA Cluster within the Latency-Associated Region of Kaposi's Sarcoma-Associated Herpesvirus
MicroRNAs (miRNAs) are small noncoding regulatory RNA molecules that bind to 3′ untranslated regions (UTRs) of mRNAs to either prevent their translation or induce their degradation. Over 200 miRNAs have been identified in human cells, and miRNAs have been identified and isolated in the gammaherpesvirus Epstein-Barr virus. Kaposi's sarcoma-associated herpesvirus (KSHV) is a gammaherpesvirus that is associated with KS and two lymphoproliferative diseases: primary effusion lymphomas (PEL) and multicentric Castleman's disease. Samols et al. (M. A. Samols, J. Hu, R. L. Skalsky, and R. Renne, J Virol 79:9301–9305, 2005, https://doi.org/10.1128/JVI.79.14.9301-9305.2005) cloned small RNAs from KSHV-positive PEL-derived cells and endothelial cells. Eleven RNAs of 19 to 23 bases aligned entirely with KSHV sequences. All candidate miRNAs mapped to a single genomic locale within the latency-associated region of KSHV, suggesting that viral and cellular gene expression may be regulated by miRNAs during latent and lytic KSHV replication.