Schematic diagram of the PspA fragments. The whole PspA molecule, containing the N-terminal α-helical domain (A and B), the proline-rich region (C), and the choline-binding domain, is shown on the top. The recombinant fragments PspA1, PspA2, PspA3, PspA4, PspA4AB, PspA4Pro, PspA5, and Psp3-NonPro are represented with the distinct domains. P indicates the proline blocks, and NP indicates the NonPro block. CDR, clade-defining region.