Vidya Chandran, Remi Fronzes, Sebastien
Duquerroy, Nora Cronin, Jorge Navaza,
and Gabriel Waksman*.
Structure of the outer membrane complex of a type IV
secretion system
Nature.
Article, 463: 1011-1015.
* indicates corresponding
author
Type IV secretion systems are secretion nanomachines spanning the two membranes of Gram-negative
bacteria. Three proteins, VirB7, VirB9, and VirB10 assemble into a 1.05 MDa core spanning the inner and outer membranes. This core
consists of 14 copies of each of the proteins and forms two layers, the I and O layers, inserting in the inner and outer
membrane, respectively. Here we present the crystal structure of a ~0.6 MDa outer membrane complex containing the entire O-layer.
This structure is the largest determined for an outer membrane channel and is
also unprecedented in being composed of three proteins. Unexpectedly, this
structure identifies VirB10 as the outer membrane channel with a unique
hydrophobic double helical trans-membrane region. This structure establishes
VirB10 as the only known protein crossing both membranes of Gram-negative
bacteria. Comparison of the cryo-EM
and crystallographic structures point to conformational changes regulating
channel opening and closing.