This pathway represents a third branch of the GSP system. The type II pathway is employed for the secretion of a wide variety of pro- teins in Gram-negative bacteria including pullulanase by Klebsiella oxytoca. More than 12 different proteins are involved in this path- way. They are collectively referred to as the secreton. Their location
extends from the cytoplasmic membrane to the outer membrane. An outer membrane secreton protein called secretin (GspD) forms highly stable ring-shaped complexes of 12-14 subunits with central Channels ranging from 5-10 nm in diameter, large enough to accommodate folded substrates. The Cytoplasmic secreton proteln (GspE) has a conserved ATP-binding motif, and interacts with cyto- plasmic membrane secreton proteins. GspE hydrolyzes ATP to pro- vide energy for the secretion through confonnational changes in the cytoplasmic membrane secreton proteins. Peptides exported to the periplasm fold before being secreted through the secretin com- plex (Figure 3.13). In Pseudomonas aeruginosa, the type 4 pilin is transported in a similar mechanism.