Involved in a type II secretion system (T2SS, formerly general secretion pathway, GSP) for the export of proteins. Required for secretion of cholera toxin through the outer membrane. This subunit forms the outer membrane channel. Forms a cylindrical channel with 15 subunits; unlike E.coli no 16-subunit channels are seen. The closed pentadeacameric channels are 195 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. 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 cap gate extends out on the extracellular side of the channel partially closing the channel. The secretin domain forms a double beta-barrel structure; the outer barrel has an outer diameter of about 110 Angstroms while the inner barrel forms the central gate with a small pore in the closed state. Belongs to the bacterial secretin family. GSP D subfamily. This may be a natural frameshift.