Involved in a type II secretion system (T2SS, formerly general secretion pathway, GSP) for the export of folded proteins across the outer membrane. This subunit would form the outer membrane channel. Forms a cylindrical channel with 15 subunits; approximately 25% of the particles have 16-subunit channels. Closed pentadeacameric channels are 180 Angstroms long and 145 Angstroms in diameter. Each subunit turns in a clock-wise manner around the channel. Most of the protein is in the periplasm which it traverses to contact proteins of the cell inner membrane. Silenced by the DNA-binding protein H-NS under standard growth conditions. The N0, N1, N2 and N3 domains are periplasmic, while the secretin and S domains form a channel that is partially inserted in the outer membrane. The N1, N2 and N3 domains each form a periplasmic ring; the N0 domain was present but not resolved by electron microscopy. The secretin domain forms a double beta-barrel structure; the outer barrel has a diameter of about 110 Angstroms while the inner barrel forms the central gate with a small pore in the closed state. Part of a cryptic operon that encodes proteins involved in type II secretion machinery in other organisms, but is not expressed in strain K12. Belongs to the bacterial secretin family. GSP D subfamily. Extended N-terminus.