Characterization of vaccinia surface antigen expressed by recombinant baculovirus

The gene encoding the vaccinia surface antigen (S antigen) was inserted into a baculovirus transfer vector and a recombinant virus was isolated. The S antigen was expressed on the surface of Spodoptera frugiperda cells (Sf cells) infected with the recombinant baculovirus. Recombinant proteins were detected in immunoblotting with anti-vaccinia serum and have the apparent molecular weight of 40 and 50-kDa. The 50-kDa polypeptide was tunicamycin sensitive and was thus glycosylated. The glycosylated 50-kDa polypeptide was also secreted into culture supernatants. Antiserum directed against the expressed S antigen specifically reacted to the authentic S antigen that is located on the mammalian cells infected with vaccinia virus, but did not affect the production of infectious progeny virus. Antisera against both the terminal regions of the S antigen were prepared by immunizing rabbits with the recombinant fusion proteins produced in the bacterial expression vector. The S antigen on the cell surface reacted by immunofluorescence with anti-C-terminus serum but not or very weakly with anti N-terminus serum, indicating that the hydrophobic N-terminus functions both as a signal and a membrane anchor sequence. Interleukin (IL)-1 alpha failed to bind to the S antigen expressed on insect and mammalian cells, although the homology between IL-1 receptor and the S antigen has been reported by computer analysis of amino acid sequence.