Cryo-EM structure of a type IV secretion system

TLDR

Bacterial conjugation is the primary mechanism for unidirectional DNA transfer, especially plasmids, spreading antibiotic resistance among Gram‑negative bacteria via the conjugative type IV secretion system (T4SS) that spans both membranes and produces a long extracellular pilus essential for DNA transfer. The authors reconstructed the full T4SS by adding the two missing components to the cryo‑EM model using co‑evolution analysis of protein interfaces, enabling a complete system including the pilus. The high‑resolution cryo‑EM structure of the 2.8‑megadalton T4SS, comprising 92 polypeptides from eight essential components, reveals an exceptionally large protein‑protein interaction network and provides insights into the unique mechanism of pilus assembly.

Abstract

Bacterial conjugation is the fundamental process of unidirectional transfer of DNAs, often plasmid DNAs, from a donor cell to a recipient cell1. It is the primary means by which antibiotic resistance genes spread among bacterial populations2,3. In Gram-negative bacteria, conjugation is mediated by a large transport apparatus-the conjugative type IV secretion system (T4SS)-produced by the donor cell and embedded in both its outer and inner membranes. The T4SS also elaborates a long extracellular filament-the conjugative pilus-that is essential for DNA transfer4,5. Here we present a high-resolution cryo-electron microscopy (cryo-EM) structure of a 2.8 megadalton T4SS complex composed of 92 polypeptides representing 8 of the 10 essential T4SS components involved in pilus biogenesis. We added the two remaining components to the structural model using co-evolution analysis of protein interfaces, to enable the reconstitution of the entire system including the pilus. This structure describes the exceptionally large protein-protein interaction network required to assemble the many components that constitute a T4SS and provides insights on the unique mechanism by which they elaborate pili.