Expression and functional characterization of bluetongue virus VP2 protein: role in cell entry

Segment 2 of bluetongue virus (BTV) serotype 10, which encodes the outer capsid protein VP2, was tagged with the S-peptide fragment of RNase A and expressed by a recombinant baculovirus. The recombinant protein was subsequently purified to homogeneity by virtue of the S tag, and the oligomeric nature of the purified protein was determined. The data obtained indicated that the majority of the protein forms a dimer and, to a lesser extent, some trimer. The recombinant protein was used to determine various biological functions of VP2. The purified VP2 was shown to have virus hemagglutinin activity and was antigenically indistinguishable from the VP2 of the virion. Whether VP2 is responsible for BTV entry into permissive cells was subsequently assessed by cell surface attachment and internalization studies with an immunofluorescence assay system. The results demonstrated that VP2 alone is responsible for virus entry into mammalian cells. By competition assay, it appeared that both VP2 and the BTV virion attached to the same cell surface molecule(s). The purified VP2 also had a strong affinity for binding to glycophorin A, a sialoglycoprotein component of erythrocytes, indicating that VP2 may be responsible for BTV transmission by the Culicoides vector to vertebrate hosts during blood feeding. Further, by various enzymatic treatments of BTV-permissive L929 cells, preliminary data have been obtained which indicated that the BTV receptor molecule(s) is likely to be a glycoprotein and that either the protein moiety of the glycoprotein or a second protein molecule could also serve as a coreceptor for BTV infection.