Involved in a type II secretion system (T2SS, formerly general secretion pathway, GSP) for the export of proteins (Probable). This subunit forms the outer membrane channel (By similarity). Among its substrates are PrpL, elastase LasB, chitin binding protein D (CbpD), aminopeptidase PaAP, and metalloprotease ImpA (PubMed:29042496, PubMed:29042493). Forms a cylindrical channel with 15 subunits. The closed pentadecameric channel is 170 Angstroms long and 140 Angstroms in diameter. 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 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 140 Angstroms while the inner barrel forms the central gate with a small pore in the closed state (PubMed:29042496). Isolated N-terminus (domains N0, N1 and N2) assembles as a hexamer of dimers (i.e. dodecamers). The N0 domain probably moves during export (PubMed:29042493). It has been suggested that the C15 symmetry of the C-terminus may transit to a C6 symmetry by displacement of 3 of the N-terminii (Probable). Loss of export of PrpL, elastase LasB, chitin binding protein D (CbpD), aminopeptidase PaAP, and metalloprotease ImpA, still exports LapA and AprA. Belongs to the bacterial secretin family. GSP D subfamily.