The nucleocapsid protein of bovine rotavirus was shown to exist in trimeric units in both the virus particle and in infected cells, with the subunits linked by non-covalent interactions. These trimeric units complex further by disulphide bridges into larger units which may represent the hexameric structures observed by electron microscopy. Visualization of various nucleocapsid protein complexes was also achieved on polyacrylamide gels by treating virus preparations with urea at 37 °C or boiling in the presence and absence of 2-mercaptoethanol. Since virus particles devoid of nucleic acid were also broken down into trimeric subunits by such treatments, assembly of virus particles appears not to require an RNA-protein interaction. Four nucleocapsid-specific monoclonal antibodies with low neutralizing ability reacted with the monomeric (45 000 mol. wt., 45K), dimeric (90K), trimeric (135K) and trimeric pair (270K) subunits, indicating that a site responsible for neutralization is probably exposed after assembly of these subunits. Analysis of radiolabelled virus revealed that a high proportion (80%) of infectious particles could be immunoprecipitated by these monoclonal antibodies, suggesting that the virus particles are either partially double-shelled or have the nucleocapsid exposed on the surface. The monoclonal antibodies also cross-reacted with the nucleocapsid proteins of simian (SA11), pig (OSU), bovine (NCDV and UK) and human (Wa and ST4) rotaviruses in an immunoblot ELISA reaction. Since these six viruses belong to two different subgroups, it is likely that the antibodies did not recognize the subgroup-specific site, but a shared exposed antigenic determinant. Due to the hexameric configuration of the nucleocapsid in virus particles the neutralizing epitope may be repeatedly presented and, therefore, may contribute to the immunogenicity of this protein.


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