Virion-associated viral proteins of a Chinese giant salamander (Andrias davidianus) iridovirus (genus Ranavirus) and functional study of the major capsid protein (MCP)

Highlights

• First proteomics of an amphibian-like ranavirus (ALRV) in the genus Ranavirus.

• A total of 40 CGSIV viral proteins were identified.

• Thirty-seven of 40 viral proteins are classified into three temporal kinetic classes.

• Major capsid protein was confirmed to be essential for virus production in in vitro.

Abstract

Chinese giant salamander iridovirus (CGSIV) is the emerging causative agent to farmed Chinese giant salamanders in nationwide China. CGSIV is a member of the common midwife toad ranavirus (CMTV) subset of the amphibian-like ranavirus (ALRV) in the genus Ranavirus of Iridoviridae family. However, viral protein information on ALRV is lacking. In this first proteomic analysis of ALRV, 40 CGSIV viral proteins were detected from purified virus particles by liquid chromatography-tandem mass spectrometry analysis. The transcription products of all 40 identified virion proteins were confirmed by reverse transcription polymerase chain reaction analysis. Temporal expression pattern analysis combined with drug inhibition assay indicated that 37 transcripts of the 40 virion protein genes could be classified into three temporal kinetic classes, namely, 5 immediate early, 12 delayed early, and 20 late genes. The presence of major capsid proteins (MCP, ORF019L) and a proliferating cell nuclear antigen (ORF025L) was further confirmed by Western blot analysis. The functions of MCP were also determined by small interfering RNA (siRNA)-based knockdown assay and anti-recombinant MCP serum-based neutralization testing. At low dosages of CGSIV, siRNA-based knockdown of the MCP gene effectively inhibited CGSIV replication in fathead minnow cells. The antiviral effect observed in the anti-MCP serum-based neutralization test confirms the crucial function of the MCP gene in CGSIV replication. Taken together, detailed information on the virion-associated viral proteins of ALRV is presented for the first time. Our results also provide evidence that MCP is essential for CGSIV replication in vitro.

Introduction

Piscine iridovirus is a large double-stranded DNA virus, which mainly comprises the genera Ranavirus, Lymphocystivirus, and Megalocytivirus in the Iridoviridae family; this virus can infect a wide range of cold-blooded low vertebrates (Jancovich et al., 2012). Among piscine iridoviruses, lymphocystivirus and megalocytivirus can only infect certain kinds of bony fishes, whereas ranavirus has a broader and global range of low vertebrate host species, including fishes, amphibians, and reptiles (Chinchar and Waltzek, 2014, Whittington et al., 2010). As of this writing, well-known ranaviruses, such as frog virus 3 (FV3) (Tan et al., 2004), Ambystoma tigrinum virus (ATV) (Jancovich et al., 2003), tiger frog virus (TFV) (He et al., 2002), soft-shelled turtle iridovirus (STIV) (Huang et al., 2009), Rana grylio virus (RGV) (Lei et al., 2012), epizootic hematopoietic necrosis virus (EHNV) (Jancovich et al., 2010), European sheatfish ranavirus (ESV/ECV) (Mavian et al., 2012b), Singapore grouper iridovirus (SGIV) (Song et al., 2004), and grouper iridovirus (GIV) (Tsai et al., 2005), have been associated with mortality events in ecologically and economically important bony fishes, amphibians, and reptiles; these viruses have been fully sequenced and widely studied (Chinchar et al., 2011).

Recently, a new common midwife toad ranavirus (CMTV) reportedly causes fetal diseases in several amphibian species across Europe (Mavian et al., 2012a). Mavian et al. proposed that CMTV represents an intermediate in the amphibian-like ranavirus (ALRV) evolution. In the meantime, CMTV-close ranaviruses have also been isolated and identified from infected farmed Chinese giant salamanders (Andrias davidianus) in different areas in China and were designated as Chinese giant salamander virus (CGSV) (Geng et al., 2011), A. davidianus iridovirus (Jiang et al., 2011), and A. davidianus ranavirus (Chen et al., 2013). Clinical signs of infection include skin ulceration, anorexia, lethargy, occasionally edema, petechiae, erythema, toe necrosis, friable and gray–black liver, and friable lesions of the kidney and spleen. Morbidity could be higher than 95% in some affected areas (Dong et al., 2011b). In 2011, we isolated a Chinese giant salamander iridovirus (CGSIV strain HN11) from diseased farmed Chinese giant salamanders in Zhangjiajie National Forest Park, Hunan Province, China, and determined and annotated the complete genome sequence of CGSIV-HN11 (Accession number: KF512820).

Ranaviruses can be subdivided into two distinct groups according to 26 conserved core iridoviral proteins-based phylogenetic analyses and genome collinearity; these groups include GIV-like ranaviruses (GLRV) and ALRV (Jancovich et al., 2010). The GLRV group includes GIV and SGIV, which specifically infect marine-cultured groupers (Epinephelus sp.), as reported in Singapore and Chinese Taiwan (Song et al., 2004, Tsai et al., 2005). ALRVs mainly comprise well-known viruses such as FV3, ATV, TFV, RGV, CMTV, EHNV, ESV, and STIV; the natural hosts of these viruses include a wide range of cold-blooded vertebrates, such as fish, reptiles, amphibians, and turtles (Mavian et al., 2012a). Within the ALRV group, three different subsets can be distinguished: EHNV/ATV/ESV, FV3/TFV/STIV, and CMTV (Mavian et al., 2012a). Conserved core iridoviral protein-based phylogenetic analyses and genome collinearity analysis show that CGSIV-HN11 is a member of the CMTV subset in ALRV (data not shown).

Viral structural proteins have crucial functions in viral bioprocesses, including structure formation and scaffolding of virus particles, virus–host interactions, initial steps in virus infection, transcription of viral genes, early-stage DNA replication, and host shifts (Whitley et al., 2010). As of this writing, 22 piscine iridoviruses, including 13 ranaviruses, 7 megalocytiviruses, and 2 lymphocystiviruses, have been fully sequenced. However, the virion proteins of only one GLRV-like ranavirus and two megalocytiviruses have been comprehensively determined and reported (Dong et al., 2011a, Shuang et al., 2013, Song et al., 2006). The functions of most viral proteins remain unclear, which greatly limits the understanding of the viral biological process in piscine iridovirus (Chinchar et al., 2011).

In this work, we report the proteomic analysis of CGSIV-HN11 virions. The functional profiles of MCP in in vitro cell models of Epithelioma papulosum cyprini (EPC) cells and fathead minnow muscle (FHM) cells were evaluated. A better understanding of structural proteins and their localization in the virions will contribute to future studies on ranavirus assembly and infection pathways and the discovery of vaccine candidates for CGSIV and other piscine iridoviruses.