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 α/ß fold domains: the structure of the first of these is similar to domains from other secretins, but with an additional α-helix which links it to the second α/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.