NMR structure of a type IVb pilin from Salmonella typhi and its assembly into pilus

The structure of the N-terminal-truncated Type IVb structural pilin (t-PilS) from Salmonella typhi was determined by NMR. Although topologically similar to the recently determined x-ray structure of pilin from Vibrio cholerae toxin-coregulated pilus, the only Type IVb pilin with known structure, t-PilS contains many distinct structural features. The protein contains an extra pair of beta-strands in the N-terminal alphabeta loop that align with the major beta-strands to form a continuous 7-stranded antiparallel beta-sheet. The C-terminal disulfide-bonded region of t-PilS is only half the length of that of toxin-coregulated pilus pilin. A model of S. typhi pilus has been proposed and mutagenesis studies suggested that residues on both the alphabeta loop and the C-terminal disulfide-bonded region of PilS might be involved in binding specificity of the pilus. This model structure reveals an exposed surface between adjacent subunits of PilS that could be a potential binding site for the cystic fibrosis transmembrane conductance regulator.