3.8 Export of cell surface structural components
Large numbers of protein and polysaccharides constitute the cytoplasmic membrane, periplasm( including the cell wall), and outer membrane. Many prokaryotes also secrete extracellular enzymes and toxins. These must be translocated through the cytoplasmic membrane after their synthesis in the cytoplasmic. Translocation into and through the cytoplasmic membrane is referred to as protein transport. The term ‘secretion’ is used to refer to protein translocation further away from the cytoplasmic membrane to the cell surface and to the extracellular medium. Proteins are transported through one of three mechanisms. These are the general secretory pathway (GSP), the ABC pathway and the twin-arginine translocation (TAT) pathway.
The outer membrane is another barrier for protein secretion in Gram-negative bacteria and several different mechanisms have been identified as protein secretion pathways.