White spot syndrome virus envelope protein VP28 is involved in the systemic infection of shrimp

Inactivation of White Spot Syndrome Virus (WSSV) by normal rabbit serum: implications for the role of the envelope protein VP28 in WSSV infection of shrimp

White Spot Syndrome Virus (WSSV) is a highly pathogenic and prevalent virus affecting crustacea. A number of WSSV envelope proteins, including vp28, have been proposed to be involved in viral infectivity based on the ability of specific antibodies to attenuate WSSV-induced mortality in vivo. In the present study, a series of monoclonal and polyclonal antibodies targeting vp28 were tested for their ability to neutralize WSSV infectivity, with the purpose of identifying epitopes potentially involved in vp28-mediated infection of shrimp. Surprisingly, when used as protein A-purified immunoglobulin, none of the antibodies tested were capable of inhibiting WSSV infectivity. This included one polyclonal preparation that has been previously shown to inactivate WSSV, when used as whole rabbit serum. Moreover, strong inactivation of WSSV by some rabbit sera was observed, in a manner independent of anti-vp28 antibodies. These results underscore the problems associated with using heterogeneous reagents (e.g. whole rabbit antiserum) in viral neutralization experiments aimed at defining proteins involved in infection by WSSV. In light of this, the potential of anti-vp28 antibodies to specifically neutralize WSSV should be reconsidered.

siRNA injection induces sequence-independent protection in Penaeus monodon against white spot syndrome virus

White spot syndrome virus (WSSV) is a major disease in crustaceans, particularly shrimp, due to the current intensity of aquaculture practices. Novel strategies including vaccination to control this virus would be highly desirable. However, invertebrates lack a true adaptive immune response system and seem to rely on various innate immune responses. An alternative and more specific approach to counteract WSSV infections in shrimp could be by the exploitation of RNA interference. As long dsRNA molecules induce a general, sequence-independent anti-viral immunity in shrimp [Robalino, J., Browdy, C.L., Prior, S., Metz, A., Parnell, P., Gross, P., Warr, G., 2004. J. Virol. 78, 10442-10448], it was investigated whether shorter 21 nt siRNAs with homology to the WSSV vp15 and vp28 genes would give a sequence-specific interference response in the shrimp Penaeus monodon. Vp28 siRNAs as well as nonspecific control gfp siRNAs were able to specifically and efficiently silence their homologous genes in a heterologous baculovirus insect cell expression system. However, in shrimps no such a specific effect was observed. Shrimp injected with vp15 or vp28 siRNAs before WSSV challenge gave a significantly lower mortality rate, but not significantly different when shrimps were injected with gfp siRNA. Thus, large dsRNA molecules as well as siRNAs induce a sequence-independent anti-viral immunity when injected in shrimp.

Identification and characterization of a prawn white spot syndrome virus gene that encodes an envelope protein VP31

Based on a combination of SDS-PAGE and mass spectrometry, a protein with an apparent molecular mass of 31 kDa (termed as VP31) was identified from purified shrimp white spot syndrome virus (WSSV) envelope fraction. The resulting amino acid (aa) sequence matched an open reading frame (WSV340) of the WSSV genome. This ORF contained 783 nucleotides (nt), encoding 261 aa. A fragment of WSV340 was expressed in Escherichia coli as a glutathione S-transferase (GST) fusion protein with a 6His-tag, and then specific antibody was raised. Western blot analysis and the immunoelectron microscope method (IEM) confirmed that VP31 was present exclusively in the viral envelope fraction. The neutralization experiment suggested that VP31 might play an important role in WSSV infectivity.

Transcription and identification of a novel envelope protein (VP124) gene of shrimp white spot syndrome virus

White spot syndrome virus (WSSV) is one of the most virulent pathogens in shrimp culture worldwide. Combining SDS-PAGE with mass spectrometry, a novel envelope protein from WSSV was identified to match an open reading frame (ORF) of WSSV genome. This ORF contained 3582nt, encoding 1194 aa, and was termed the vp124 gene. One part of the whole gene (named vp124p) was cloned into pET-GST vector and expressed as a fusion protein with glutathione S-transferase (GST) in Escherichia coli strain BL21 (DE3). Specific antibodies were raised using the purified fusion protein (GST-VP124P). Temporal transcription analysis revealed that the vp124 gene was a late gene. Western blot analysis showed that the mouse anti-GST-VP124P antibodies reacted specifically with VP124 present either in the WSSV virions or in the viral envelopes, and did not react with the proteins of the viral nucleocapsids. VP124 was located in the WSSV virions as an envelope protein using immunoelectron microscopy.

Vp28 of shrimp white spot syndrome virus is involved in the attachment and penetration into shrimp cells

White spot disease (WSD) is caused by the white spot syndrome virus (WSSV), which results in devastating losses to the shrimp farming industry around the world. However, the mechanism of virus entry and spread into the shrimp cells is unknown. A binding assay in vitro demonstrated VP28-EGFP (envelope protein VP28 fused with enhanced green fluorescence protein) binding to shrimp cells. This provides direct evidence that VP28-EGFP can bind to shrimp cells at pH 6.0 within 0.5 h. However, the protein was observed to enter the cytoplasm 3 h post-adsorption. Meanwhile, the plaque inhibition test showed that the polyclonal antibody against VP28 (a major envelope protein of WSSV) could neutralize the WSSV and block an infection with the virus. The result of competition ELISA further confirmed that the envelope protein VP28 could compete with WSSV to bind to shrimp cells. Overall, VP28 of the WSSV can bind to shrimp cells as an attachment protein, and can help the virus enter the cytoplasm.

A simple and efficient method for purification of intact white spot syndrome virus (WSSV) viral particles

A new simple and efficient method for isolation of intact WSSV viral particles from infected crayfish tissues with high yield was developed. Abundant viral particles could be obtained with only a few steps of conventional differential centrifugations, while no density gradient centrifugation or ultracentrifugation was required. The concentrated virus preparations were further studied by transmission electron microscopy and polyacrylamide gel electrophoresis. Using negative-staining TEM, we found that purified viral particles were coated with integral envelope. At least 23 major structural proteins from purified WSSV virions could be observed by SDS-PAGE. By this method, about 10(12) viral particles could be recovered from 10 g of infected crayfish tissues. Moreover, purified virus does not lose its biological activity. Using purified virus, the minimal amount of WSSV that could initiate a successful virus proliferation in crayfish was determined.

Gene-expression profiling of White spot syndrome virus in vivo

White spot syndrome virus, type species of the genus Whispovirus in the family Nimaviridae, is a large, double-stranded DNA (dsDNA) virus that infects crustaceans. The genome of the completely sequenced isolate WSSV-TH encodes 184 putative open reading frames (ORFs), the functions of which are largely unknown. To study the transcription of these ORFs, a DNA microarray was constructed, containing probes corresponding to nearly all putative WSSV-TH ORFs. Transcripts of 79 % of these ORFs could be detected in the gills of WSSV-infected shrimp (Penaeus monodon). Clustering of the transcription profiles of the individual genes during infection showed two major classes of genes: the first class reached maximal expression at 20 h post-infection (p.i.) (putative early) and the other class at 2 days p.i. (putative late). Nearly all major and minor structural virion-protein genes clustered in the latter group. These data provide evidence that, similar to other large, dsDNA viruses, the WSSV genes at large are expressed in a coordinated and cascaded fashion. Furthermore, the transcriptomes of the WSSV isolates WSSV-TH and TH-96-II, which have differential virulence, were compared at 2 days p.i. The TH-96-II genome encodes 10 ORFs that are not present in WSSV-TH, of which at least seven were expressed in P. monodon as well as in crayfish (Astacus leptodactylus), suggesting a functional but not essential role for these genes during infection. Expression levels of most other ORFs shared by both isolates were similar. Evaluation of transcription profiles by using a genome-wide approach provides a better understanding of WSSV transcription regulation and a new tool to study WSSV gene function.

Production of recombinant enveloped structural proteins from the Chinese WSSV isolate

The white spot syndrome virus (WSSV) is one of the deadly pathogens of penaeid shrimps and other crustaceans. The WSSV virion consists of an enveloped rod-shaped nucleocapsid enclosing a large circular double stranded DNA genome of 305 Kb with 181 open reading frames. The two major structural genes, VP19 and VP28 were amplified from the genomic DNA of Chinese isolate of WSSV and cloned in pUCm-T vector and sub cloned in pET-30a (+) vector. The expressions of genes inE. coli (BL21) were confirmed by SDS-PAGE analysis. The clones were sequenced, submitted to the gene bank and the Xiang Shan strain of WSSV were compared with the previous reported sequence of WSSV of various regions which revealed that VP19 and VP28 gene sequences had certain differences from the sequences of similar genes of the isolate already reported. The recombinant proteins expressed, purified and characterized.

Interaction of white spot syndrome virus VP26 protein with actin

VP26 protein, the product of the WSV311 gene of white spot syndrome virus (WSSV), is one of major structural proteins of virus. In this study, when purified virions were treated with Triton X-100 detergent, VP26 protein was present in both the envelope and the nucleocapsid fraction. We have rationalized this finding by suggesting that VP26 protein might be located in the space between the envelope and the nucleocapsid. By using a fluorescent probe method, we have investigated the interaction between VP26 protein and some proteins of host cells. Three major VP26-binding proteins were purified from crayfish hemocytes by affinity-chromatography, in which the protein with an apparent molecular mass of 42 kDa was identified as actin by mass spectrometry (MS). Moreover, the association of VP26 protein with actin microfilaments was confirmed by coimmunoprecipitation.

Protein expression in white spot syndrome virus infected Penaeus monodon fabricius

White spot syndrome virus (WSSV) is the causative agent of the white spot disease of shrimp. Penaeus monodon were captured from Muttukadu Estuary in Chennai, India, transported to the laboratory and maintained in an aerated system with continuous water circulation-biofiltration. WSSV-free P. monodon were challenged by feeding them only once with WSSV-infected tissues of P. monodon. Cumulative mortality (100%) of the infected individuals was determined. Tissues from infected and uninfected shrimp such as muscles, hepatopancreas, heart, gills and eye tissues (100mg of each) and haemolymph (50 microl) were subjected to SDS-PAGE. In infected muscle tissue, six newly expressed proteins were detected. In infected haemolymph, four new proteins and three intensely expressed high molecular weight proteins were observed. Three intensely expressed high molecular weight proteins were detected in infected heart tissue and two new proteins in infected hepatopancreatic tissues. In infected gill tissues, two new protein bands and three intensely expressed high molecular weight proteins were detected on comparison with uninfected tissues. Similarly, two intensely expressed protein bands were seen in the infected as compared with the uninfected eye tissues. The protein profiles of the muscle tissue from 50 different naturally infected (WSSV) shrimp were analyzed. Eleven different new protein bands appeared in the infected muscle tissues when compared to the control muscle tissues whereas; in muscle tissue six new proteins were observed both in naturally and experimentally WSSV infected shrimp. The current study has shown that the protein expression patterns of the infected tissues of P. monodon have been drastically altered by WSSV infection. Western blot analysis revealed that one of the newly expressed 53 kDa protein in the infected muscle represents the WSSV envelope protein.

Fitness and virulence of an ancestral White Spot Syndrome Virus isolate from shrimp

White Spot Syndrome Virus, the type species of the virus family Nimaviridae, is a large dsDNA virus infecting shrimp and other crustaceans. Genomic analysis of three completely sequenced WSSV isolates identified two major polymorphic loci, "variable region ORF14/15" and "variable region ORF23/24". Here, we characterize a WSSV isolate originating from shrimp collected in Thailand in 1996 (TH-96-II). This isolate contains the largest WSSV genome ( approximately 312 kb) identified so far, mainly because of its sequences in both major polymorphic loci. Analysis of "variable region ORF14/15" suggests that TH-96-II may be ancestral to the WSSV isolates described to date. A comparison for virulence was made between TH-96-II and WSSV-TH, a well characterized isolate containing the smallest genome ( approximately 293 kb) identified at present. After injection of the isolates into Penaeus monodon the mortality rates showed that the median lethal time (LT50) of TH-96-II was approximately 14 days, compared to 3.5 days for WSSV-TH. When both isolates were mixed in equal amounts and serially passaged in shrimp, WSSV-TH outcompeted TH-96-II within four passages. These data suggest a higher virulence of WSSV-TH compared to TH-96-II. The molecular basis for the difference in virulence remains unclear, but a replication advantage of the 19 kb smaller WSSV-TH genome could play a role.

A novel envelope protein involved in White spot syndrome virus infection

One open reading frame (designated vp76) from the White spot syndrome virus (WSSV) genome has the motif of a cytokine I receptor and has been identified as a structural protein. In this paper, vp76 was expressed in Escherichia coli and used to prepare a specific antibody to determine the location of the corresponding protein in the intact virion, the nucleocapsids and the envelope of WSSV. Western blotting with the VP76 antiserum confirmed that VP76 was an envelope protein of WSSV. To investigate the function of the VP76, WSSV was neutralized with the VP76-specific antiserum at different concentrations and injected intramuscularly into crayfish. The mortality curves showed that the VP76 antiserum could partially attenuate infection with WSSV, suggesting that VP76 is an envelope protein involved in WSSV infection.

Protection of crayfish, Cambarus clarkii, from white spot syndrome virus by polyclonal antibodies against a viral envelope fusion protein

White spot syndrome virus (WSSV) is a large double-stranded DNA virus, causing considerable mortality in penaeid shrimp and other crustaceans. WSSV produces five major structural proteins, including two major envelope proteins, VP28 and VP19. To produce VP28 and VP19 as a single protein for antibody production, DNA sequences encoding both open reading frames were fused together and cloned into pET-22b(+) expression vector. The fusion protein, VP(19+28), was expressed in Escherichia coli, purified using Ni2+ His affinity chromatography and injected into a rabbit. Antiserum collected from the immunized rabbit was tested in vivo for ability to protect crayfish, Cambarus clarkii, from disease caused by WSSV. Fifteen days after challenge with WSSV, treatment with VP(19+28) antiserum gave 100% protection against disease in the ambient temperature range of 15-22 degrees C and 65% protection at a constant temperature of 26 degrees C. These results demonstrated VP(19+28) antiserum is effective in protection of crayfish from WSSV and confirmed that VP19 and VP28 play an important role in WSSV host infection. Targeting both VP19 and VP28 may be effective for the design of both immunotherapeutic medicines and reagents to detect WSSV.

Identification of white spot syndrome virus (WSSV) envelope proteins involved in shrimp infection

White spot syndrome virus (WSSV) is a major shrimp pathogen causing large economic losses. In an attempt to identify the envelope proteins involved in virus infection, antisera against six WSSV envelope proteins were used in neutralization assays conducted in vivo. The results showed that the virus infection could be significantly delayed or neutralized by antibodies against three WSSV envelope proteins (VP68, VP281 and VP466). This neutralization was further confirmed by quantitative PCR. It could be concluded that the viral envelope proteins VP68, VP281 and VP466 played roles in WSSV infection to shrimp.

Genomic and proteomic analysis of thirty-nine structural proteins of shrimp white spot syndrome virus

White spot syndrome virus (WSSV) virions were purified from the hemolymph of experimentally infected crayfish Procambarus clarkii, and their proteins were separated by 8 to 18% gradient sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) to give a protein profile. The visible bands were then excised from the gel, and following trypsin digestion of the reduced and alkylated WSSV proteins in the bands, the peptide sequence of each fragment was determined by liquid chromatography-nano-electrospray ionization tandem mass spectrometry (LC-nanoESI-MS/MS) using a quadrupole/time-of-flight mass spectrometer. Comparison of the resulting peptide sequence data against the nonredundant database at the National Center for Biotechnology Information identified 33 WSSV structural genes, 20 of which are reported here for the first time. Since there were six other known WSSV structural proteins that could not be identified from the SDS-PAGE bands, there must therefore be a total of at least 39 (33 + 6) WSSV structural protein genes. Only 61.5% of the WSSV structural genes have a polyadenylation signal, and preliminary analysis by 3' rapid amplification of cDNA ends suggested that some structural protein genes produced mRNA without a poly(A) tail. Microarray analysis showed that gene expression started at 2, 6, 8, 12, 18, 24, and 36 hpi for 7, 1, 4, 12, 9, 5, and 1 of the genes, respectively. Based on similarities in their time course expression patterns, a clustering algorithm was used to group the WSSV structural genes into four clusters. Genes that putatively had common or similar roles in the viral infection cycle tended to appear in the same cluster.

Protection of Penaeus monodon against white spot syndrome virus using a WSSV subunit vaccine

Although invertebrates lack a true adaptive immune response, the potential to vaccinate Penaeus monodon shrimp against white spot syndrome virus (WSSV) using the WSSV envelope proteins VP19 and VP28 was evaluated. Both structural WSSV proteins were N-terminally fused to the maltose binding protein (MBP) and purified after expression in bacteria. Shrimp were vaccinated by intramuscular injection of the purified WSSV proteins and challenged 2 and 25 days after vaccination to assess the onset and duration of protection. As controls, purified MBP- and mock-vaccinated shrimp were included. VP19-vaccinated shrimp showed a significantly better survival (p<0.05) as compared to the MBP-vaccinated control shrimp with a relative percent survival (RPS) of 33% and 57% at 2 and 25 days after vaccination, respectively. Also, the groups vaccinated with VP28 and a mixture of VP19 and VP28 showed a significantly better survival when challenged two days after vaccination (RPS of 44% and 33%, respectively), but not after 25 days. These results show that protection can be generated in shrimp against WSSV using its structural proteins as a subunit vaccine. This suggests that the shrimp immune system is able to specifically recognize and react to proteins. This study further shows that vaccination of shrimp may be possible despite the absence of a true adaptive immune system, opening the way to new strategies to control viral diseases in shrimp and other crustaceans.

Protection of Penaeus monodon against white spot syndrome virus by oral vaccination

White spot syndrome virus (WSSV) occurs worldwide and causes high mortality and considerable economic damage to the shrimp farming industry. No adequate treatments against this virus are available. It is generally accepted that invertebrates such as shrimp do not have an adaptive immune response system such as that present in vertebrates. As it has been demonstrated that shrimp surviving a WSSV infection have higher survival rates upon subsequent rechallenge, we investigated the potential of oral vaccination of shrimp with subunit vaccines consisting of WSSV virion envelope proteins. Penaeus monodon shrimp were fed food pellets coated with inactivated bacteria overexpressing two WSSV envelope proteins, VP19 and VP28. Vaccination with VP28 showed a significant lower cumulative mortality compared to vaccination with bacteria expressing the empty vectors after challenge via immersion (relative survival, 61%), while vaccination with VP19 provided no protection. To determine the onset and duration of protection, challenges were subsequently performed 3, 7, and 21 days after vaccination. A significantly higher survival was observed both 3 and 7 days postvaccination (relative survival, 64% and 77%, respectively), but the protection was reduced 21 days after the vaccination (relative survival, 29%). This suggests that contrary to current assumptions that invertebrates do not have a true adaptive immune system, a specific immune response and protection can be induced in P. monodon. These experiments open up new ways to benefit the WSSV-hampered shrimp farming industry.

Replication of white spot syndrome virus in ovarian primary cultures from the kuruma shrimp, Marsupenaeus japonicus

Propagation of white spot syndrome virus (WSSV) was investigated in primary ovarian cultures from the kuruma shrimp Marsupenaeus japonicus. A WSSV strain, purified by sucrose density gradient centrifugation, was inoculated into 10-day-old primary ovarian cultures. WSSV infection induced marked cytopathic effect (CPE) on primary ovarian cells. Initially, virus-infected cells began to shrink 72 h post-inoculation, followed by the rounding of most cells which detached finally from flask surface. Electron microscopic observations clearly showed that the replication of WSSV occurred in nuclei of ovarian cells. Immunoblot analysis with antibodies against the WSSV envelope protein VP28 provided the evidence that the levels of WSSV antigens in culture supernatant gradually increased during the period between 24 and 120 h after virus inoculation. The results suggest that the use of primary ovarian cultures of the kuruma shrimp will facilitate characterization of the WSSV infection.

A synthetic antibacterial peptide from Mytilus galloprovincialis reduces mortality due to white spot syndrome virus in palaemonid shrimp

White spot syndrome virus (WSSV) isolated from Penaeus monodon was found to be highly infective for the western Mediterranean shrimp, Palaemon sp. Using polymerase chain reaction (PCR), it was demonstrated that such shrimp are not naturally carriers of WSSV. Following challenge with virus, mortality reached 100% 3.5-4 days after injection at 22 degrees C. Incubation of infected shrimp at 10 degrees C totally suppressed the mortality which rapidly developed when shrimp were returned to 18 or 22 degrees C. Preincubation of WSSV with mature synthetic mytilin significantly reduced shrimp mortality with a 50% efficient dose of about 5 microM. Survival of shrimp was not due to the development of an active mechanism of defence as re-injection of WSSV produced the same mortality pattern. Mortality was probably due to WSSV replication as dot blot failed to detect viral DNA in the injection sample but was positive 1 day post-injection. Protection by mytilin was by interaction at the virus level, preventing replication as no WSSV nucleic acid was detected by PCR even after 7 days in shrimp injected with WSSV preincubated with 10 or 50 microM mytilin.

Transcriptional analysis of the white spot syndrome virus major virion protein genes

White spot syndrome virus (WSSV) is a member of a new virus family (Nimaviridae) infecting crustaceans. The regulation of transcription of WSSV genes is largely unknown. Transcription of the major WSSV structural virion protein genes, vp28, vp26, vp24, vp19 and vp15, was studied to search for common promoter motifs for coordinate expression. The temporal expression of these genes and both 5' and 3' ends of the mRNA were determined, using infected crayfish gill tissue as a RNA source. RT-PCR showed that all five genes are expressed late in infection compared to the early ribonucleotide reductase large subunit gene. 5' RACE studies revealed a consensus late transcription initiation motif for only two of the five major virion protein genes. This motif was only found in one other upstream region of the putative translational start site of a gene with unknown function (ORF 158). No other conserved sequence motifs could be detected in the sequences surrounding the transcriptional start sites of the five major virion protein genes. All 5' ends were located about 25 nt downstream of an A/T rich sequence, including the consensus TATA-box sequence for vp15. The absence of a consensus motif is distinct from gene regulation of other large dsDNA viruses and suggests a unique regulation of WSSV transcription, in line with its unique taxonomic position.

Transcriptional analysis of the DNA polymerase gene of shrimp white spot syndrome virus

The white spot syndrome virus DNA polymerase (DNA pol) gene (WSSV dnapol) has already been tentatively identified based on the presence of highly conserved motifs, but it shows low overall homology with other DNA pols and is also much larger (2351 amino acid residues vs 913-1244 aa). In the present study we perform a transcriptional analysis of the WSSV dnapol gene using the total RNA isolated from WSSV-infected shrimp at different times after infection. Northern blot analysis with a WSSV dnapol-specific riboprobe found a major transcript of 7.5 kb. 5'-RACE revealed that the major transcription start point is located 27 nucleotides downstream of the TATA box, at the nucleotide residue A within a CAGT motif, one of the initiator (Inr) motifs of arthropods. In a temporal expression analysis using differential RT-PCR, WSSV dnapol transcripts were detected at low levels at 2-4 h.p.i., increased at 6 h.p.i., and remained fairly constant thereafter. This is similar to the previously reported transcription patterns for genes encoding the key enzyme of nucleotide metabolism, ribonucleotide reductase. Phylogenetic analysis showed that the DNA pols from three different WSSV isolates form an extremely tight cluster. In addition, similar to an earlier phylogenetic analysis of WSSV protein kinase, the phylogenetic tree of viral DNA pols further supports the suggestion that WSSV is a distinct virus (likely at the family level) that does not belong to any of the virus families that are currently recognized.

Identification of VP19 and VP15 of white spot syndrome virus (WSSV) and glycosylation status of the WSSV major structural proteins

White spot syndrome virus (WSSV) infects penaeid shrimp and other crustaceans. The WSSV virion consists of an enveloped rod-shaped nucleocapsid enclosing a large circular double-stranded DNA genome of 293 kbp. The virion envelope contains two major proteins of 28 (VP28) and 19 kDa (VP19) and the nucleocapsid consists of three major proteins of 26 (VP26), 24 (VP24) and 15 kDa (VP15). Study on the morphogenesis of the WSSV particle requires the genomic identification and chemical characterization of these WSSV virion proteins. An internal amino acid sequence of envelope protein VP19 was obtained by amino acid sequencing and used to locate the VP19 open reading frame of this protein on the genome, as WSSV ORF182. VP19 contained two putative transmembrane domains, which may anchor this protein in the WSSV envelope. Similarly, the gene for VP15 was located on the WSSV genome as ORF109. N-terminal amino acid sequencing on VP15 suggested that this protein was expressed from the second ATG of its ORF and the first methionine is lost by N-terminal protein processing. The 15 kDa protein is very basic and is a candidate DNA-binding protein in the WSSV nucleocapsid. None of the five major structural WSSV proteins appear to be glycosylated, which is an unusual feature among enveloped animal viruses.