The substrate specificity switch FlhB assembles onto the export gate to regulate type three secretion

• Published in: Nature communications Vol. 11; no. 1; pp. 1296 - 10
• Main Authors: Kuhlen, Lucas, Johnson, Steven, Zeitler, Andreas, Bäurle, Sandra, Deme, Justin C., Caesar, Joseph J. E., Debo, Rebecca, Fisher, Joseph, Wagner, Samuel, Lea, Susan M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 10.03.2020; Nature Publishing Group; Nature Portfolio
• Subjects: 101/28; 631/326/421; 631/45/535; 631/535/1258/1259; Bacteria; Bacterial Proteins - chemistry; Bacterial Proteins - metabolism; Bacterial Proteins - ultrastructure; Bacterial Secretion Systems - metabolism; Electron microscopy; Escherichia coli - metabolism; Exports; Flagella; Gating; Helices; Humanities and Social Sciences; Hydrophobic and Hydrophilic Interactions; Models, Molecular; multidisciplinary; Mutagenesis; Periplasm; Protein Domains; Protein Structure, Secondary; Protein Transport; Proteins; Science; Science (multidisciplinary); Secretion; Substrate Specificity; Substrates; Topology; Vibrio - metabolism; Virulence; Waterborne diseases
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/s41467-020-15071-9

Protein secretion through type-three secretion systems (T3SS) is critical for motility and virulence of many bacteria. Proteins are transported through an export gate containing three proteins (FliPQR in flagella, SctRST in virulence systems). A fourth essential T3SS protein (FlhB/SctU) functions to “switch” secretion substrate specificity once the growing hook/needle reach their determined length. Here, we present the cryo-electron microscopy structure of an export gate containing the switch protein from a Vibrio flagellar system at 3.2 Å resolution. The structure reveals that FlhB/SctU extends the helical export gate with its four predicted transmembrane helices wrapped around FliPQR/SctRST. The unusual topology of the FlhB/SctU helices creates a loop wrapped around the bottom of the closed export gate. Structure-informed mutagenesis suggests that this loop is critical in gating secretion and we propose that a series of conformational changes in the T3SS trigger opening of the gate through interactions between FlhB/SctU and FliPQR/SctRST. Export of proteins by type three secretion systems occurs through an export gate that is localized in the periplasm. Here, the authors present the cryo-EM structure of the Vibrio mimicus export gate complex with FlhB, which plays a major role in switching of the specificity of secretion substrates and propose a mechanism for export gate opening.