Interaction of a bacterially expressed peptide from the receptor binding domain of Pseudomonas aeruginosa pili strain PAK with a cross-reactive antibody: conformation of the bound peptide.

The C-terminal receptor binding region of Pseudomonas aeruginosa pilin protein strain PAK (residues 128-144) has been the target for the design of a vaccine effective against P. aeruginosa infections. We have recently cloned and expressed a (15)N-labeled PAK pilin peptide spanning residues 128-144 of the PAK pilin protein. The peptide exists as a major (trans) and minor (cis) species in solution, arising from isomerization around a central Ile(138)-Pro(139) peptide bond. The trans isomer adopts two well-defined turns in solution, a type I beta-turn spanning Asp(134)-Glu-Gln-Phe(137) and a type II beta-turn spanning Pro(139)-Lys-Gly-Cys(142). The cis isomer adopts only one well-defined type II beta-turn spanning Pro(139)-Lys-Gly-Cys(142) but displays evidence of a less ordered turn spanning Asp(132)-Gln-Asp-Glu(135). These turns have been implicated in cross-reactive antibody recognition. (15)N-edited NMR spectroscopy was used to study the binding of the (15)N-labeled PAK pilin peptide to an Fab fragment of a cross-reactive monoclonal antibody, PAK-13, raised against the intact PAK pilus. The results of these studies are as follows: the trans and cis isomers bind with similar affinity to the Fab, despite their different topologies; both isomers maintain the conformational integrity of their beta-turns when bound; binding leads to the preferential stabilization of the first turn over the second turn in each isomer; and binding leads to the perturbation of resonances within regions of the trans and cis backbone that undergo microsecond to millisecond motions. These slow motions may play a role in induced fit binding of the first turn to Fab PAK-13, which would allow the same antibody combining site to accommodate either trans or cis topology. More importantly for vaccine design, these motions may also play a role in the development of a broad-spectrum vaccine capable of generating an antibody therapeutic effective against the multiple strains of P. aeruginosa.