Structures of T4 pilins and minor pilins. Structures of T4 pilins and minor pilins. Type IV pilins are characterized by conserved N-terminal α-helices (cyan) connected to a β-sheet (gray) by the αβ-loop (magenta). The N-terminal α-helix is divided into two subdomains, α1-N (amino acids ∼1 to 28) and α1-C (amino acids ∼29 to 52), as indicated on the N. gonorrhoeae PilE structure. In most pilins, two cysteine residues form a disulfide bond to form the variable D region (blue). Although the Dichelobacter nodosus FimA protein lacks these characteristic residues, its C terminus retains a similar architecture through alternative interactions (169). (A) T4a pilins, represented by Neisseria gonorrhoeae PilE (Protein Data Bank [PDB] accession no. 1AY2), Dichelobacter nodosus FimA (PDB accession no. 3SOK), Pseudomonas aeruginosa PAK PilA (PDB accession no. 1DZO), and P. aeruginosa Pa110594 PilA (PDB accession no. 3JZZ), share a shallow S-shaped N-terminal helix and a four-stranded continuous antiparallel β-sheet. The architectures of the αβ-loop and the D region of T4a pilins vary, with defined secondary structure in the GC pilin structure (top left) and the Pa110594 PilA structure (top right). Pa110594 PilA has an extended loop region, with an additional α-helix and β-strand between β3 and β4 (green) differentiating it from other T4a pilins. (B) T4b pilins, represented by Salmonella Typhi PilS (PDB accession no. 1Q5F), Vibrio cholerae TcpA (PDB accession no. 1OQV), and enteropathogenic E. coli BfpA (PBD accession no. 1ZWT), have a different protein fold, with discontinuous β-strands making up the β-sheet. The D region is embedded in the protein, with the C terminus forming the last strand of the β-sheet. The αβ-loop contains an α-helix oriented roughly 90° relative to α1. The structure of the noncore minor pilin PilX from N. meningitidis (PDB accession no. 2OPD) is similar to that of the major pilins but has two α-helices in the αβ-loop. The D region contains a short α-helix with a hook implicated in function (176). Figures were prepared using MacPymol (DeLano Scientific). Carmen L. Giltner et al. Microbiol. Mol. Biol. Rev. 2012; doi:10.1128/MMBR.00035-12