microbial physiology
1) Type III protein secretion systems are related to the flagellar assembly systems in their inner membrane proteins. Type III systems are independent of the general secretory pathway and are used for the secretion of virulence proteins in various gram negative bacteria. There is a high level of conservation in the proteins that make up the apparatus, although many different proteins are secreted through this pathway. (Figure 1) Type II protein secretion systems are the main route for secretion in gram negative bacteria. This system utilizes a two step process for secretion. The first step involves using the general secretory pathway to move the pre-protein to the periplasm, upon which the leader sequence of the polypeptide is removed with proteolytic enzymes. The second step of the secretion is not fully understood, however it has been suggested that the polypeptide may be transported via vesicles. There is a high degree of specificity in the interaction between the polypeptide and apparatus for secretion. This suggests that the system can differentiate foreign from self proteins. Type II secretion systems resemble pillus formation systems and there is homology between the two systems. (Figure 2) Type I protein secretion systems secrete proteins directly from the cytoplasm to outside of the cell without the use of leader sequences or the sec genes. The system contains three secretory proteins, two inner membrane proteins and one outer membrane protein. One of the inner membrane proteins is an ABC transporter which hydrolyses ATP to drive export. The other two membrane proteins are an auxiliary proteins, one on the inner membrane the other on the outer. The inner protein spans the periplasm to the outer protein. The structure leads researchers to believe that the proteins form a channel to the outside of the cell. (Figure 3) Differences between Type III and Type I and II include the type of proteins secreted, dependence on the GSP, and target of the proteins which are secreted.