We have identified the binding site of monoclonal antibodies (MAbs) against the S2 subunit of the bovine coronavirus spike (S) glycoprotein. The location of this site was first investigated by using prokaryotic expression of DNA restriction fragments covering the entire S gene. The amino acid sequence containing the antibody binding site was shortened from 70 to 20 amino acids by digestion of plasmid DNA with exonuclease III, followed by sequencing of the smallest digestion product encoding an immunoreactive fusion protein. Finally we synthesized a set of nonapeptides covering the 20 amino acid sequence extending from the N-terminal residue of the S2 subunit (Ala 769 to Tyr 798). MAbs reacted mainly with six consecutive overlapping peptides with the sequence TTGYRFTN-FEPFTV. Polyclonal antibodies from hyperimmunized or convalescent animals reacted only with the recombinant proteins identified by MAbs, and the hyperimmune serum bound to the same set of peptides. This suggests that this highly conserved linear antigenic determinant corresponds to an immunodominant region. This region resembles both in location and immunodominance the linear determinant defined on the infectious bronchitis virus S2 subunit. The presence of similar regions in the N-terminal region of the S2 subunit of other coronaviruses is discussed.


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