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Secretins form large pores in the outer membrane, participating in protein secretion during the type II and type III terminal branch of the General Secretion Pathway (GSP) of Gram-negative bacteria. The type II secretion pathway is dependent on the Sec system, since the secreted proteins must carry a signal peptide sufficient for the translocation through the inner membrane and is responsible for the secretion of toxins and exo-enzymes. Type III secretion pathway is Sec-independent and allows the translocation of effector proteins from bacteria to the eukaryotic target cells. Members of the family include PilQ of Neisseria meningitidis, PulD of Klebsiella oxytoca, GspD of Escherichia coli, the pIV protein (which plays a role in the assembly of the filamentous bacteriophage) and other proteins of Gram-negative bacteria. Electron microscopy suggests that secretins form large channels in the outer membrane with an internal diameter of approximately 7nm. The structure of PilQ has been determined, suggesting a 12-meric structure. It is believed that the C-terminal part of secretins forms the transmembrane ß-barrel domain, however there is a lack of a low-resolution three-dimensional model and thus the true folding state of the channel is ambiguous. There is also evidence that secretins play an active role in the assembly of the pili besides acting solely as channels.
Ushers and secretins: channels for the secretion of folded proteins across the bacterial outer membrane
J Mol Microbiol Biotechnol. 2002 Jan;4(1):11-20.
Secretins of Pseudomonas aeruginosa: large holes in the outer membrane
Arch Microbiol. 2003 May;179(5):307-14. doi: 10.1007/s00203-003-0541-8. Epub 2003 Mar 28.
Protein secretion mechanisms in Gram-negative bacteria
Int J Med Microbiol. 2000 Oct;290(4-5):325-31. doi: 10.1016/S1438-4221(00)80033-8.
Interaction with type IV pili induces structural changes in the bacterial outer membrane secretin PilQ
J Biol Chem. 2005 May 13;280(19):18923-30. doi: 10.1074/jbc.M411603200. Epub 2005 Mar 7.
Structure of the Neisseria meningitidis outer membrane PilQ secretin complex at 12 A resolution
J Biol Chem. 2004 Sep 17;279(38):39750-6. doi: 10.1074/jbc.M405971200. Epub 2004 Jul 14.