Fig. 2.

Three finger-like loops form the hydrophobic tip of the Csu pilus. (A) Stereoview of the CsuC-CsuE complex. CsuE is colored cyan, except for the hydrophobic binding motifs in loops B₁2-C₁2 and D₁1″-α2 (yellow, 1), F₁2-G₁1 (orange, 2), and G₁1-G₁2 (light orange, 3). CsuC is colored magenta, except for the G₁ “donor” strand, which is violet. (B) Topology diagram of CsuE. (C) Cartoon diagram and molecular surface of CsuE. Residues comprising the hydrophobic sequences are shown as ball-and-stick. (D) Cartoon diagram and semitransparent surface of CsuE viewed from the pilus tip. (E) Alignment of TDAs of archaic pili from Acinetobacter baumannii, Burkholderia pseudomallei, Pseudomonas aeruginosa, and Yersinia pestis. Amino acid identities and similar residues are indicated by background shading in cyan and gray, respectively. Hydrophobic binding motifs are framed. Within motifs, hydrophobic residues are shown on a yellow background. Cysteines are colored red. Disulfide bonds are indicated with connecting lines. Limits and nomenclature for secondary structure elements are shown above the sequence. Dashed and wavy lines indicate unstructured and domain-linking sequences, respectively. Alignment of a large set of archaic TDAs is shown in SI Appendix, Fig. S11.