“Core components of bacterial protein secretion systems revealed at high resolution by cryo-electron microscopy”

The cell envelope of Gram-negative bacteria serves as a shield to protect against hostile environments. Due to the nature of the permeability barrier imposed by the cell envelope, bacteria have evolved highly intricate and efficient protein cargo delivery systems to cross both inner and outer membranes. Protein secretion systems are essential for the growth of many pathogenic bacteria; they are necessary for nutrient acquisition, motility, cell-to-cell communication, host defense, and delivery of virulence factors. Novel high-resolution structures of core membrane protein complexes from various secretion systems, revealed by single particle reconstruction cryo-electron microscopy (cryo-EM), will be presented. Emphasis will be placed on the supra-membrane helical assembly of the export gate of the Type III secretion system (T3SS) flagellar assembly apparatus. Recent advances in structure and stoichiometry elucidation of the twin-arginine translocation (tat) substrate receptor complex by cryo-EM will also be highlighted. Structural understanding of protein secretion systems that are critical for bacterial pathogenesis will aid in the development of novel antibacterial therapies.