Figure 1. Comparison of RBD expression constructs.

Four different gene designs were utilized in this work. All constructs contain the CoV-2 spike RBD domain, either containing amino acids 319–541 (Sinai, orange) or 319–529 (Ragon, green). All proteins contain a signal peptide for secretion of the RBD from mammalian cells—these leaders are from the CoV-2 spike protein (S, yellow) or human tissue plasminogen activator (TPA, cyan). Carboxyterminal tags are present on all constructs consisting of a 6 or 8 polyhistidine tag (with or without an HRV-3C protease cleavage site, red) and a streptavidin-binding protein tag (SBP, blue).