Fig. 4

Secretion of automatically supercharged SARS-CoV-2 S1 protein NTD and RBD. (a) Top graph: LCD analysis result of wildtype SARS-CoV-2 S1 protein NTD sequence (cationic supercharged regions of LCD > 2 shaded green). Middle graph: Sequence mutated by PySupercharge to make NTD (LCD ≤ 2). Note that all the shaded regions from the top graph have been modified to stay below or on the LCD = 2 reference line. LCD > 1 regions have been shaded in red for comparison with NTD (LCD ≤ 1). Bottom graph: Sequence of NTD (LCD ≤ 1), mutated in a similar manner but with a cutoff value of 1. The LCD values for the SARS-CoV-2 spike protein domains were calculated with histidine charge configuration + 0.1. All of the regions have been mutated to stay below or on the green LCD = 1 reference line. (b) Similar set of graphs for SARS-CoV-2 S1 protein RBD. (c) The western blot analysis results of the NTD (1st -6th lane) and RBD (7th -12th lane). Supercharged NTD and RBD sequences clearly display greater secretion ability. A separate high-exposure western blot of wildtype RBD is in an additional file [see Additional File 4] due to low visibility in this figure. (*, dimer protein; ▶, trimer protein; ■, tetramer protein)