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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.
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.
Structural insights into the secretin translocation channel in the type II secretion system
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Prepore Stability Controls Productive Folding of the BAM-independent Multimeric Outer Membrane Secretin PulD
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Lipids assist the membrane insertion of a BAM-independent outer membrane protein
Sci Rep. 2015 Oct 14;5:15068. doi: 10.1038/srep15068.
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.
Structure and function of PilQ, a secretin of the DNA transporter from the thermophilic bacterium Thermus thermophilus HB27
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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.
Secretins of Pseudomonas aeruginosa: large holes in the outer membrane
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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.
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.