The Architecture of EssB, an Integral Membrane Component of the Type VII Secretion System

The membrane-bound EssB is an integral and essential component of the bacterial type VII secretion system that can contribute to pathogenicity. The architecture of Geobacillus thermodenitrificans EssB has been investigated by combining crystallographic and EPR spectroscopic methods. The protein forms a dimer that straddles the cytoplasmic membrane. A helical fold is observed for the C-terminal segment, which is positioned on the exterior of the membrane. This segment contributes most to dimer formation. The N-terminal segment displays a structure related to the pseudokinase fold and may contribute to function by recognizing substrates or secretion system partners. The remaining part of EssB may serve as an anchor point for the secretion apparatus, which is embedded in the cytoplasmic membrane with the C-terminal domain protruding out to interact with partner proteins or components of peptidoglycan.