Cloning, sequence analysis and expression of the major outer capsid protein gene of an aquareovirus

The nucleotide and deduced amino acid sequences of genome segment 10 of aquareovirus strain SBR, encoding the major outer capsid protein (VP7), have been determined. Genome segment 10 of SBR virus is 986 nucleotides long and encodes a polypeptide of 298 amino acids with a predicted molecular mass of 32,430 Da. There are 26 non-translated nucleotides at the 5' end and 66 non-translated nucleotides at the 3' end. Using a recombinant baculovirus system, the VP7 protein of SBR virus was expressed to a high level. The baculovirus-produced VP7 protein was similar both in its size and antigenic properties to the authentic aquareovirus VP7 protein. Antiserum from a rabbit immunized with the baculovirus-produced VP7 protein failed to neutralize the homologous aquareovirus strain. As determined by Western blotting, this antiserum reacted with aquareovirus strains belonging to the same genogroup as SBR virus, but did not react with aquareovirus strains belonging to the other genogroups.

Identification of a new genogroup of aquareovirus by RNA-RNA hybridization

The relative mobilities of the 11 dsRNA genomic segments of 22 aquareovirus isolates from fish and shellfish obtained from different geographical areas of the world were compared by PAGE. Using reciprocal RNA-RNA dot blot hybridization, a new sixth genetic group of aquareovirus (genogroup F) was identified. Genogroup A was represented by eight and genogroup B by 12 isolates. The remaining two isolates represented the new sixth genogroup (genogroup F). The genetic relationship of these aquareoviruses with mammalian rotavirus group A (SA 11) was also examined by reciprocal RNA-RNA blot hybridization but none was found under any of the stringency conditions used.

The structure of aquareovirus shows how the different geometries of the two layers of the capsid are reconciled to provide symmetrical interactions and stabilization

BACKGROUND

Aquareoviruses are important pathogens of aquatic animals and have severe consequences in aquaculture. These viruses belong to the family Reoviridae. A structural feature common to members of the Reoviridae is a multilayered capsid, formed by several concentric icosahedral shells with different protein compositions. How these proteins, which often are present in unequal stoichiometries, interact between icosahedral layers to stabilize the capsid is not well understood.

RESULTS

We have determined the three-dimensional structure of aquareovirus to 23 A resolution using electron cryomicroscopy and computer image analysis. The protein capsid is composed of two structurally distinct icosahedral layers: an outer layer approximately 100 A thick, with incomplete T=13 left-handed symmetry, surrounds an inner layer 600 A in diameter that has T=1 symmetry and is perforated by channels near the fivefold axes. There are 120 subunits, arranged in dimers, in the inner layer, each of which interacts with two of the 600 subunits in the outer layer. A separate set of closely interacting proteins forms the fivefold axes of the virus structure, forming continuous density throughout both layers of the capsid. Comparison of full and empty (lacking RNA) virus structures reveals an RNA shell that lies directly beneath the inner layer.

CONCLUSIONS

Our aquareovirus structure displays marked similarity to the mammalian reovirus intermediate subviral particles, suggesting a close evolutionary relationship. However, the noticeable distinction is that aquareovirus lacks the hemagglutinin spike observed in reovirus. The T=1 inner layer organization observed in the aquareovirus appears to be common to other members of the Reoviridae. Such organization may be of fundamental significance in the endogenous transcription of the genome in these viruses.

Ehretianone, a novel quinonoid xanthene from Ehretia buxifolia with antisnake venom activity

Ehretianone (1), a new quinonoid xanthene, together with known sterols, was isolated from a MeOH extract of the root bark of Ehretia buxifolia. The structure of ehretianone was elucidated as 7-hydroxy-9a alpha-(3-methylbut-2-enyl)-4a alpha,9 alpha-(2-methylprop-2-enyl)-4a, 9a-dihydro-1,4-dioxoxanthene on the basis of spectroscopic data and X-ray crystallographic analysis. The antisnake venom activity of ehretianone against Echis carinatus venom in mice is also reported.

The Fc segment of IgE influences the kinetics of dissociation of a symmetrical bivalent ligand from cyclic dimeric complexes

As part of a systematic effort to determine the features of immunoglobulin E-receptor (IgE-Fc epsilon RI) aggregation that are critical for cellular activation, we used fluorescence to examine the dissociation of a soluble bivalent ligand, N, N'-bis[[epsilon-[(2,4-dinitrophenyl)amino]caproyl]-L-tyrosyl]-L-cystine ((DCT)2-cys), from soluble bivalent IgE and its bivalent F(ab')2 and monovalent Fab' fragments. Cross-linking of Fab' fragments by (DCT)2-cys is limited to linear dimers, and we find that (DCT)2-cys dissociation from Fab' occurs with a single kinetic coefficient [(4.2 +/- 0.6) x 10-3 s-1] that corresponds to the lower of the two kinetic coefficients observed with the bivalent IgE [(4.7 +/- 0.7) x 10-2 s-1 and (4.4 +/- 0.3) x 10-3 s-1]. Similarly, the lower value is obtained for dissociation of (DCT)2-cys that is monovalently bound to IgE after incubation with a large excess of the ligand. (DCT)2-cys can bind to bivalent F(ab')2 fragments and form a variety of linear and cyclic aggregates, similarly to IgE, but, unlike IgE, we find that dissociation occurs with a single kinetic coefficient similar to that observed for Fab'. We find that IgE and its (Fab')2 fragments form highly stable cyclic dimer rings with two (DCT)2-cys. We demonstrate that the kinetic coefficients are independent of enhanced fluorescence quenching observed for bound sites in cyclic dimers. Together, the results show that the rate constant for breaking a linear cross-link formed by (DCT)2-cys is the same as that for dissociation of the monovalently bound (DCT)2-cys. Further, they show that opening of a bond in a dimer ring for the F(ab')2 fragment occurs with approximately the same dissociation rate constant as opening a bond in a linear cross-link. This rate constant is about three times smaller than that observed with IgE, suggesting that steric strain is caused by apposed Fc segments in cyclic IgE dimers. Such structural interference may affect the functional consequences of IgE-Fc epsilon RI aggregation on the cell surface.