Involved in a type II secretion system (T2SS, formerly general secretion pathway, GSP) for the export of proteins (By similarity). This subunit forms the outer membrane channel (By similarity). Forms a cylindrical channel with 15 subunits. 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 N1, N2 and N3 domains each form a periplasmic ring. 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. Belongs to the bacterial secretin family. GSP D subfamily.