Figure 1.

Model of general type II secretion as proposed by Douzi et al. (2012). Exoproteins (gray circles) are initially translocated over the inner membrane (IM) into the periplasmic space via the Sec or Tat pathway (not shown). The prepilin peptidase GspO processes the pseudopilins (GspG–GspK) by cleavage of the leader peptide. The cleaved pseudopilins are then assembled by ATP hydrolysis of the cytosolic NTPase (GspE) to form the pseudopilus. GspE is anchored to the inner membrane by GspL and thereby interacts with the IM-embedded GspF. In active state, the selection of secretion substrates is presumably performed by an interaction of GspC and GspD components. Then the appropriate exoproteins can enter the T2SS and are pushed into the extracellular milieu by the assembling pseudopilus. The exoproteins cross the outer membrane (OM) through the ring-formed channel (secretin) of GspD-subunits. In some species, GspS stabilizes GspD-subunits and prevents degradation of the secretin.