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Area of Science:

  • Microbiology
  • Structural Biology
  • Biochemistry

Background:

  • Gram-negative bacteria utilize type 2 secretion systems (T2SSs) for secreting proteins crucial for virulence and adaptation.
  • T2SS function relies on the assembly of inner membrane-anchored fibers, termed pseudopili, for transporting folded proteins.

Purpose of the Study:

  • To determine the atomic structure of a T2SS pseudopilus from Klebsiella oxytoca.
  • To elucidate the mechanistic link between pseudopilus assembly and protein secretion through structure-based functional analysis.

Main Methods:

  • Nuclear Magnetic Resonance (NMR) spectroscopy to determine the structure of the calcium-bound PulG subunit.
  • Cryo-electron microscopy (cryo-EM) to obtain a ~5-Å resolution reconstruction of assembled pseudopili.
  • Integration of NMR and cryo-EM data to generate an atomic model of the pseudopilus.
  • Site-directed mutagenesis and functional assays to investigate the role of calcium and specific residues.

Main Results:

  • An atomic model of the T2SS pseudopilus revealed extensive inter-subunit contacts and unique structural features.
  • Identified a disordered central region within the PulG helical stem and flexible C-terminal residues on the fiber surface.
  • Demonstrated the critical role of calcium in PulG folding, stability, and pseudopilus assembly.
  • Observed calcium-dependent fiber disassembly, suggesting a mechanism for pseudopilus length control.

Conclusions:

  • The atomic structure provides unprecedented insight into T2SS pseudopilus architecture and assembly.
  • Calcium is essential for pseudopilus stability and function, influencing fiber length control.
  • The findings support the Archimedes screw model for protein translocation via the T2SS.