Epitope mapping and cellular localization of swine acute diarrhea syndrome coronavirus nucleocapsid protein using a novel monoclonal antibody

A swine acute diarrhea syndrome coronavirus (SADS-CoV) that causes severe diarrhea in suckling piglets was identified in Southern China in 2017. To develop an antigen that is specific, sensitive, and easy to prepare for serological diagnosis, antigenic sites in the SADS-CoV nucleocapsid (N) protein were screened. We generated and characterized an N-reactive monoclonal antibody (mAb) 3E9 from mice immunized with recombinant N protein. Through fine epitope mapping of mAb 3E9 using a panel of eukaryotic-expressed polypeptides with GFP-tags, we identified the motif (343)DAPVFTPAP(351) as the minimal unit of the linear B-cell epitope recognized by mAb 3E9. Protein sequence alignment indicated that (343)DAPVFTPAP(351) was highly conserved in different SADS-CoV strains and SADS-related coronaviruses from bat, with one substitution in this motif in HKU2-related bat coronavirus. Using mAb 3E9, we observed that N protein was expressed in the cytoplasm and was in the nucleolus during SADS-CoV replication. N protein was immunoprecipitated from SADS-CoV-infected Vero E6 cells. Taken together, our results indicated that 3E9 mAb could be a useful tool to investigate the structure and function of N protein during viral replication.