Localization of neutralizing epitopes and receptor-binding site in murine coronavirus spike protein.

To identify the localization of the epitopes recognized by monoclonal antibodies (MAbs) against the S1 subunit of the murine coronavirus JHMV spike protein, we have expressed the S1 proteins with different deletions from the C terminus of the S1. All of MAbs in groups A and B recognized the S1N(330) composed of 330 amino acids (aa) from the N terminus of the S1 and the larger S1 deletion mutants, but failed to react with the S1N(220) composed of 220 aa. MAbs in group C reacted only with the S1utt protein without any deletion. These results indicated that the S1N330 comprised the cluster of epitopes recognized by MAbs in groups A and B. These results together with the fact that all the MAbs in group B retained the high neutralizing activity suggested that the N terminus 330 aa are responsible for binding to the MHV-specific receptors. In pursuit of this possibility, we have expressed the receptor protein and examined the binding of each S1 deletion mutants to the receptor. It was demonstrated that the S1N(330) protein as well as other S1 deletion mutants larger than S1N(330) bound to the receptor. These results indicated that a domain composed of 330 aa at the N terminus of the S1 protein is responsible for binding to the MHV-specific receptor.