Figure 8. Conserved charged residues decorate the MxiH needle pore.

(a) Top and (b) side view of the hybrid needle structure showing the surface electrostatic potential computed using APBS⁵⁸ and contoured linearly in the range ±5 kT/e. The helical arrangement of subunits is shown as cartoons for one-half of the full 29-subunit system as a guide. In (b, d) three 11-start protofilaments have been removed for simplicity. (c, d) Same view as in (a, b), with surface coloring according to the sequence conservation index calculated from the alignment of 102 unique needle sequences using Jalview⁵⁹, measured on a 1–10 scale. Marine color indicates high (7–10), light purple medium (4–6) and white low (1–3) sequence conservation index. The boxed region in (d) is enlarged in the inset, indicating the pattern of highly conserved charged residues on the helical structure, as discussed in the main text.