A, structural features of the different groups of colicin pores. The pore-forming domains of colicins S4, E1, and Ia shown in the top and side views; the hydrophobic hairpin is drawn in red. The major differences between the colicin groups are the length of the hydrophobic hairpin (see also B), the angle between Pα1 and Pα2 (colicin S4, 5°; colicin E1, 34°; and colicin Ia, 37 °), and the position of the hairpin comprising Pα5 and the loop connection to Pα6. B, superimposition of hydrophobic hairpins. Left, colicin S4 (group PI, in red) versus colicin E1 (group PIIa, in pink); right, colicin E1 (group PIIa, in pink) versus colicin Ia (group PIIb, in purple). C, alignment of the pore-forming domains of colicins. Colicins are grouped according to sequence similarity. Groups PI and PIIa/b differ by several insertions and deletions, whereas groups PIIa and PIIb are highly similar and differ only in several conserved positions, affecting the overall charge and isoelectric point of the domains. Moreover, the structure of colicin Ia contains four very short β-strands where colicin E1 only has unstructured loops.