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The Bacterial type II and III secretion system protein (Secretin) Family [Function: Biogenesis/Secretion] Seed alignment | Full alignment | Pfam page | TC-DB page

The members of this family 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. The structures of the periplasmic domains from N. meningitidis PilQ: the N-terminus is shown to consist of two ß-domains, which are unique to the type IV pilus-dependent secretins. The structure of the second ß-domain revealed an eight-stranded ß-sandwich structure which is a novel variant of the HSP20-like fold. The central part of PilQ consists of two ?/ß fold domains: the structure of the first of these is similar to domains from other secretins, but with an additional ?-helix which links it to the second ?/ß domain. We also determined the structure of the entire PilQ dodecamer by cryoelectron microscopy: it forms a cage-like structure, enclosing a cavity which is approximately 55 Å in internal diameter at its largest extent. The Vibrio cholerae T4aP secretin PilQ is a member of the bacterial secretin superfamily. In each VcPilQ monomer, four ß strands come together to form a ß sheet. Once assembled, VcPilQ forms a 56-strand ß barrel.

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Literature references

Yeast can express and assemble bacterial secretins in the mitochondrial outer membrane
Microb Cell. 2019 Nov 19;7(1):15-27. doi: 10.15698/mic2020.01.703.
PMID: 31921930

Structural insights into the secretin translocation channel in the type II secretion system
Nat Struct Mol Biol. 2017 Feb;24(2):177-183. doi: 10.1038/nsmb.3350. Epub 2017 Jan 9.
PMID: 28067918

Prepore Stability Controls Productive Folding of the BAM-independent Multimeric Outer Membrane Secretin PulD
J Biol Chem. 2017 Jan 6;292(1):328-338. doi: 10.1074/jbc.M116.759498. Epub 2016 Nov 30.
PMID: 27903652

Lipids assist the membrane insertion of a BAM-independent outer membrane protein
Sci Rep. 2015 Oct 14;5:15068. doi: 10.1038/srep15068.
PMID: 26463896

Structure and assembly of a trans-periplasmic channel for type IV pili in Neisseria meningitidis
PLoS Pathog. 2012 Sep;8(9):e1002923. doi: 10.1371/journal.ppat.1002923. Epub 2012 Sep 13.
PMID: 23028322

Structure and function of PilQ, a secretin of the DNA transporter from the thermophilic bacterium Thermus thermophilus HB27
J Biol Chem. 2011 Mar 25;286(12):9977-84. doi: 10.1074/jbc.M110.212688. Epub 2011 Feb 1.
PMID: 21285351

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.
PMID: 15753075

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.
PMID: 15254043

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.
PMID: 12664194

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.
PMID: 11763968

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.
PMID: 11111906

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