Secretion - Secretion in Gram Positive Bacteria

Secretion in Gram Positive Bacteria

Proteins with appropriate N-terminal targeting signals are synthesized in the cytoplasm and then directed to a specific protein transport pathway. During, or shortly after its translocation across the cytoplasmic membrane, the protein is processed and folded into its active form. Then the translocated protein is either retained at the extracytoplasmic side of the cell or released into the environment. Since the signal peptides that target proteins to the membrane are key determinants for transport pathway specificity, these signal peptides are classified according to the transport pathway to which they direct proteins. Signal peptide classification is based on the type of signal peptidase (SPase) that is responsible for the removal of the signal peptide. The majority of exported proteins are exported from the cytoplasm via the general Secretory (Sec) pathway. Most well known virulence factors (e.g. exotoxins of Staphylococcus aureus, protective antigen of Bacillus anthracis, listeriolysin O of Listeria monocytogenes) that are secreted by Gram-positive pathogens have a typical N-terminal signal peptide that would lead them to the Sec-pathway. Proteins that are secreted via this pathway are translocated across the cytoplasmic membrane in an unfolded state. Subsequent processing and folding of these proteins takes place in the cell wall environment on the trans-side of the membrane. In some Staphylococcus and Streptococcus species, the accessory secretory system handles the export of highly repetitive adhesion glycoproteins. In addition to the Sec and accessory-Sec systems, some Gram-positive bacteria contain the Tat-system that is able to translocate folded proteins across the membrane. This is especially appropriate for proteins that need co-factors, such as iron-sulfur clusters and molybdopterin, which are incorporated in the cytoplasm. Pathogenic bacteria may contain certain special purpose export systems that are specifically involved in the transport of only a few proteins. For example, several gene clusters have been identified in mycobacteria that encode proteins that are secreted into the environment via specific pathways (ESAT-6) and are important for mycobacterial pathogenesis. Specific ATP-binding cassette (ABC) transporters direct the export and processing of small antibacterial peptides called bacteriocins. Genes for endolysins that are responsible for the onset of bacterial lysis are often located near genes that encode for holin-like proteins, suggesting that these holins are responsible for endolysin export to the cell wall.