Analysis of the S spike (peplomer) glycoprotein of bovine coronavirus synthesized in insect cells

The bovine coronavirus (BCV) spike glycoprotein precursor (S, formerly termed peplomer) and its two subunit polypeptides (S1 and S2) were individually expressed in Spodoptera frugiperda (Sf9) insect cells. Each recombinant baculovirus expressed both glycosylated (S, 170K; S1, 95K; S2, 80K) and unglycosylated (S(0), 140K; S1(0), 75K; and S2(0), 65K) forms of BCV spike polypeptides in Sf9 cells. The mature 95K S1 polypeptide was secreted whereas the S and S2 polypeptides remained cell-associated. The S precursor was partially cleaved in Sf9 cells, and the resulting S1 was also released into the medium. Neutralizing monoclonal antibodies representing two antigenic domains bound to recombinant S and S1 but not the S2 polypeptides, indicating that two major epitopes for BCV neutralization are located on the S1 subunit.