Figure 2. Quantification of the interaction between PB1_1–25 and PA.

(A) Determination of the 50% inhibitory concentration (IC₅₀) of PB1_1–25A by competitive ELISA using the indicated increasing concentrations of peptides and cell extract containing HA-tagged PA of FluA. Error bars represent standard deviations from triplicate experiments. (B) IC50 of PB1_1–25A-derived peptides. S.D. is shown in parenthesis. Asterisks indicate highest concentrations of peptides (3000 nM) used without detectable inhibitory effect. Grey boxes highlight amino acids that are part of the 3₁₀-helix, which was postulated to comprise the core PA-binding region of PB1. Amino acids known to form hydrogen bonds with PA residues are represented in bold.