Infectious myonecrosis virus has a totivirus-like, 120-subunit capsid, but with fiber complexes at the fivefold axes

Abstract

Infectious myonecrosis virus (IMNV) is an emerging pathogen of penaeid shrimp in global aquaculture. Tentatively assigned to familyTotiviridae, it has a nonsegmented dsRNA genome of 7,560 bp and an isometric capsid of the 901-aa major capsid protein. We used electron cryomicroscopy and 3D image reconstruction to examine the IMNV virion at 8.0-Å resolution. Results reveal a totivirus-like, 120-subunit T ‫؍‬ 1 capsid, 450 Å in diameter, but with fiber complexes protruding a further 80 Å at the fivefold axes. These protrusions likely mediate roles in the extracellular transmission and pathogenesis of IMNV, capabilities not shared by most other totiviruses. The IMNV structure is also notable in that the genome is centrally organized in five or six concentric shells. Within each of these shells, the densities alternate between a dodecahedral frame that connects the threefold axes vs. concentration around the fivefold axes, implying certain regularities in the RNA packing scheme.dsRNA virus ͉ electron cryomicroscopy ͉ nonenveloped virus ͉ penaeid shrimp ͉ Totiviridae I nfectious myonecrosis virus (IMNV) was first isolated from whiteleg shrimp, Litopenaeus vannamei, from aquaculture farms in northeast Brazil (1, 2). The associated disease was characterized by skeletal muscle necrosis, most markedly in distal abdomen and tail, with mortality over the harvest cycle nearing 70%. Purified IMNV virions reproduce this disease in pathogen-free L. vannamei (2). Diagnostics are available using reverse transcription and PCR to distinguish IMNV from other RNA viruses of shrimp (3, 4), and a survey from Pernambuco, Brazil, detected IMNV in 9 of 11 farms (5). IMNV has also been detected in L. vannamei from Indonesian farms, probably after transfer of aquaculture stocks (4).The genome sequence and many features of IMNV have been reported (2). Negatively stained virions exhibit an isometric capsid with a diameter of Ϸ400 Å. The virions contain a major capsid protein (MCP) of relative molecular weight (M r ) 106,000 and an N terminus of sequence IVSMENQSEID as shown by Edman degradation. The genome, a single molecule of 7,560-bp dsRNA, contains two extended ORFs in different frames of the plus strand: ORF1 in frame 1 (nucleotides 136-4953) and ORF2 in frame 3 (nucleotides 5241-7451). ORF1 encodes a 1,605-aa protein that includes the N-terminal sequence of the MCP starting at amino acid 705. The protein spanning amino acids 705-1605 have a sequencepredicted mass of 99 kDa, consistent with the M r of the MCP. The MCP thus seems to be cleaved from a larger precursor. A 60-aa region at the N terminus of ORF1 shares sequence similarities with dsRNA-binding proteins. ORF2 encodes a 736-aa protein that contains typical motifs of an RNA-dependent RNA polymerase (RdRp). Proteins representing ORF2 and the first 704 aa of ORF1 have yet to be identified, although candidate minor proteins have been seen in denaturing gels of IMNV virions. Phylogenetic analyses link IMNV to members of the family Totiviridae of nonsegmented dsRNA viruses with isomet...

Molecular characterization of infectious myonecrosis virus (IMNV) isolated from the shrimp Litopenaeus vannamei farmed in Ceara State, Brazil

“…The presence of two protein bands can be observed with apparent molecular masses of 96 and 106 kDa, corresponding to the major capsid protein (MCP) and the protein corresponding to the fusion of the MCP with RdRp or with other viral capsid fragments (data not shown) in accord with and Tang et al (2008).…”

The T=1 Capsid Protein of Penicillium chrysogenum Virus Is Formed by a Repeated Helix-Rich Core Indicative of Gene Duplication

“…Another notable example is infectious myonecrosis virus (IMNV), a shrimp pathogen with a totivirus-like 120-subunit capsid. The IMNV capsid surface has fibers at the 5-fold axis, which are probably involved in cell entry, as this virus is transmitted extracellularly (67).…”

“…These capsids are an extended feature of dsRNA viruses (Fig. 1); they are described (i) in members of the family Reoviridae (37,44,69,71), which infect mostly higher eukaryotic systems, (ii) in viruses of the family Picobirnaviridae (18), a recently established taxonomic family whose members infect humans and other vertebrates, (iii) in bacteriophages of the family Cystoviridae, which infect the prokaryote Pseudomonas syringae (26,28), (iv) and in viruses of the families Totiviridae (12,13,67) and Partitiviridae (48), which mostly infect unicellular and simple eukaryotes (fungi and protozoa). A notable exception is the Penicillium chrysogenum virus (PcV) of the family Chrysoviridae, which has an authentic Tϭ1 capsid formed by just 60 copies of one polypeptide (11).…”

Bacillus NP5 Improves Growth Performance and Resistance Against Infectious Myonecrosis Virus in White Shrimp (Litopenaeus vannamei) (Bacillus NP5 Meningkatkan Pertumbuhan dan Ketahanan Terhadap Infeksi Virus Myonecrosis pada Udang Putih (L. vannamei)

“…It attacks white shrimp which is caused by infectious myonecrosis virus (IMNV) and causes high mortality in brackishwater ponds in Indonesia (Senapin et al, 2007). IMNV is a dsRNA virus from the Totiviridae family (Poulos et al, 2006;Tang et al, 2008). It is a non-envelope virus and is a 40 nm icosahedralshaped virion (Senapin et al, 2007).…”