Structural similarity of secretins from type II and type III secretion systems

Structure. 2014 Sep 2;22(9):1348-1355. doi: 10.1016/j.str.2014.07.005. Epub 2014 Aug 21.


Secretins, the outer membrane components of several secretion systems in Gram-negative bacteria, assemble into channels that allow exoproteins to traverse the membrane. The membrane-inserted, multimeric regions of PscC, the Pseudomonas aeruginosa type III secretion system secretin, and PulD, the Klebsiella oxytoca type II secretion system secretin, were purified after cell-free synthesis and their structures analyzed by single particle cryoelectron microscopy. Both homomultimeric, barrel-like structures display a "cup and saucer" architecture. The "saucer" region of both secretins is composed of two distinct rings, with that of PulD being less segmented than that of PscC. Both secretins have a central chamber that is occluded by a plug linked to the chamber walls through hairpin-like structures. Comparisons with published structures from other bacterial systems reveal that secretins have regions of local structural flexibility, probably reflecting their evolved functions in protein secretion and needle assembly.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Outer Membrane Proteins / chemistry*
  • Bacterial Outer Membrane Proteins / ultrastructure
  • Bacterial Secretion Systems
  • Cryoelectron Microscopy
  • Klebsiella oxytoca / chemistry
  • Models, Molecular
  • Protein Structure, Quaternary
  • Pseudomonas aeruginosa / chemistry
  • Secretin / chemistry*
  • Structural Homology, Protein


  • Bacterial Outer Membrane Proteins
  • Bacterial Secretion Systems
  • Secretin