Shown are the native strain PAO and PAK pilin sequences of the receptor binding domain, residues 128–144, and two mutants. The mutated residues in PAO and PAK pilins (29–144) are boxed. The PAO pilin (29–144) mutant contains three PAK pilin residues, A128K, D143S, and N144K and is denoted PAO (KSK). The PAK pilin (29–144) mutant contains three PAO pilin residues, K128A, S143D, and K144N and is denoted PAK (ADN).