# Cryo-Electron Microscopy of BfpB Reveals a Type IVb Secretin Multimer Adapted to Accommodate the Exceptionally Wide Bundle-Forming Pilus

## TL;DR

This study reveals the unique structure of BfpB, a secretin protein in bacteria, which helps in forming a special type of pilus and provides insights into bacterial secretion systems.

## Contribution

The study presents a novel structural model of BfpB with 17-fold symmetry, adapted for a wider pilus, and identifies unique structural features.

## Key findings

- BfpB has a 17-fold symmetry, differing from typical T4P systems.
- An extended β-hairpin loop in the N3 domain resembles T3SS secretins.
- Structural adaptations in BfpB suggest evolutionary parallels among secretins.

## Abstract

Type IV pili (T4Ps) are multifunctional surface fibers essential for bacterial motility, adhesion, and virulence, found across Gram-negative and Gram-positive bacteria and archaea. Detailed descriptions of T4P structural biology are allowing progress in understanding T4P biogenesis. Secretins, large outer membrane channels, are crucial for T4P extrusion in Gram-negative bacteria. Using cryo-EM and AlphaFold, we modeled the structure of BfpB, the secretin of the Bundle-Forming Pilus (BFP) of enteropathogenic Escherichia coli. BfpB exhibits a unique 17-fold symmetry, correlating with the thicker BFP filaments, and diverging from the 12–15 subunits typical of T4P, type 2 secretion (T2S), and type 3 secretion (T3S) systems. Additionally, we identified an extended β-hairpin loop in the N3 domain, resembling features of distantly related T3SS secretins, and an N-terminal helix where a C-terminal S-domain is seen in some T2S and T3S secretins. These findings reveal evolutionary parallels and structural adaptations in secretins, highlighting the link between oligomerization and pilus structure. This work advances our understanding of T4P biogenesis, secretin evolution, and bacterial secretion systems, offering insights into pathogenic diversity and future research directions.