Plasmids for expression of full spike protein and fragments
SARS-CoV-2 S-protein-expressing plasmids were codon-optimised and generated by gene synthesis (Bio Basic Asia Pacific, Singapore) according to GenBank accession number: QHD43416.1. One plasmid is for expressing untagged full-length S protein while the other is for expressing a Myc-tagged S-protein fragment consisting of residues 1048–1206 (SARS-CoV-2 numbering). The pXJ40-Myc expression vector was used as an empty vector control and pXJ40-Myc-HBcAg plasmid expressing Myc-tagged hepatitis B virus core antigen (HBcAg) was used as a negative control.