Structural insights into the secretin PulD and its trypsin-resistant core.
Limited proteolysis, secondary structure and biochemical analyses, mass spectrometry, and mass measurements by scanning transmission electron microscopy were combined with cryo-electron microscopy to generate a three-dimensional model of the homomultimeric complex formed by the outer membrane secretin PulD, an essential channel-forming component of the type II secretion system from Klebsiella oxytoca. The complex is a dodecameric structure composed of two rings that sandwich a closed disc. The two rings form chambers on either side of a central plug that is part of the middle disc. The PulD polypeptide comprises two major, structurally quite distinct domains; an N domain, which forms the walls of one of the chambers, and a trypsin-resistant C domain, which contributes to the outer chamber, the central disc, and the plug. The C domain contains a lower proportion of potentially transmembrane beta-structure than classical outer membrane proteins, suggesting that only a small part of it is embedded within the outer membrane. Indeed, the C domain probably extends well beyond the confines of the outer membrane bilayer, forming a centrally plugged channel that penetrates both the peptidoglycan on the periplasmic side and the lipopolysaccharide and capsule layers on the cell surface. The inner chamber is proposed to constitute a docking site for the secreted exoprotein pullulanase, whereas the outer chamber could allow displacement of the plug to open the channel and permit the exoprotein to escape.
Chami
Mohamed
M
ME Müller Institute, Biozentrum, University of Basel, Switzerland.
Guilvout
Ingrid
I
Gregorini
Marco
M
Rémigy
Hervé W
HW
Müller
Shirley A
SA
Valerio
Marielle
M
Engel
Andreas
A
Pugsley
Anthony P
AP
Bayan
Nicolas
N
D001425:Bacterial Outer Membrane Proteins / Q000737:chemistry* / Q000378:metabolism* / Q000648:ultrastructure; D004926:Escherichia coli / Q000235:genetics; D041121:Klebsiella oxytoca / Q000737:chemistry*; D011487:Protein Conformation; D014357:Trypsin / Q000378:metabolism*
D001425:Bacterial Outer Membrane Proteins; D014357:Trypsin