Type II secretion system

The first step involves the Sec and Tat secretory pathways, which are responsible for transporting proteins across the inner membrane into the periplasm.

Once these secretory proteins are located in the periplasm, the second step can then take place, whereby they are secreted across the outer membrane into the extracellular milieu.

GspS is also transported into the periplasm using the Sec translocation machinery, at which point it is inserted into the inner layer of the outer membrane where it remains closely associated with GspD.

It is thought that GspS plays an important role in the stabilization of the secretin GspD and helps prevent it from breaking down in the presence of highly degradative periplasmic enzymes.

GspL is however known to form tight interactions with the secretion ATPase and these are needed to hold it in close association with the rest of the inner membrane complex.

The N2D domain is not fully understood but observations show that it is the N1D which is responsible for forming the tight interactions seen with the inner membrane complex subunit GspL.

Its name it derived from the fact that it is made up of a number of pilin like proteins or pseudopilins, known as GspG, GspH, GspI, GspJ and GspK.

[3] They are known as pseudopilins due to their similarity to the pilins (like PilA) that make up the type IV pili found in gram negative bacteria.

Once the Sec machinery has transported the pre-pseudopilin across the inner membrane, but before the protein itself is released into the periplasm, the N-terminal signal sequence is cleaved at a conserved stretch of positively charged amino acid residues.

Mature pilins and pseudopilins have a lollipop-shaped structure, made up of a long hydrophobic tail and a globular hydrophilic head domain.

It is thought that this constant extension and retraction of the pseudopilus causes it to act like a piston and push secretory proteins out through the outer membrane secretin.

This mechanism can be broken down into several steps: It is believed that quorum sensing plays a key role in controlling the activation of the type II secretion system and the initiation of exoprotein release.

[6] Specifically quorum sensing helps regulate the transcription of the genes encoding these exoproteins and ensures that they are only produced when other like bacteria are nearby and environmental conditions are conducive to survival and infection.