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Bacterial secretion system
Bacterial secretion systems are protein complexes present on the cell membranes of bacteria for secretion of substances. Specifically, they are the cellular devices used by pathogenic bacteria to secrete their virulence factors (mainly of proteins) to invade the host cells. They can be classified into different types based on their specific structure, composition and activity. Generally, proteins can be secreted through two different processes. One process is a one-step mechanism in which proteins from the cytoplasm of bacteria are transported and delivered directly through the cell membrane into the host cell. Another involves a two-step activity in which the proteins are first transported out of the inner cell membrane, then deposited in the periplasm, and finally through the outer cell membrane into the host cell.
These major differences can be distinguished between Gram-negative diderm bacteria and Gram-positive monoderm bacteria. But the classification is by no means clear and complete. There are at least eight types specific to Gram-negative bacteria, four to Gram-positive bacteria, while two are common to both. In addition, there is appreciable difference between diderm bacteria with lipopolysaccharide on the outer membrane (diderm-LPS) and those with mycolic acid (diderm-mycolate).
The export pathway is responsible for crossing the inner cell membrane in diderms, and the only cell membrane in monoderms.
The general secretion (Sec) involves secretion of unfolded proteins that first remain inside the cells. In Gram-negative bacteria, the secreted protein is sent to either the inner membrane or the periplasm. But in Gram-positive bacteria, the protein can stay in the cell or is mostly transported out of the bacteria using other secretion systems. Among Gram-negative bacteria, Escherichia coli, Vibrio cholerae, Klebsiella pneumoniae, and Yersinia enterocolitica use the Sec system. Staphylococcus aureus and Listeria monocytogenes are Gram-positive bacteria that use the Sec system.
The Sec system utilises two different pathways for secretion: the SecA and signal recognition particle (SRP) pathways. SecA is an ATPase motor protein and has many related proteins including SecD, SecE, SecF, SegG, SecM, and SecY. SRP is a ribonucleoprotein (protein-RNA complex) that recognizes and targets specific proteins to the endoplasmic reticulum in eukaryotes and to the cell membrane in prokaryotes. The two pathways require different molecular chaperones and ultimately use a protein-transporting channel SecYEG for transporting the proteins across the inner cell membrane. In the SecA-dependent pathway, SecB acts as a chaperone, keeping the newly synthesized protein in an unfolded state and delivers it to translocon-bound SecA. Then the pre-protein is secreted to the periplasm through the SecYEG translocon. Whereas in the SRP pathway, SRP recruits the ribosome-nascent chain complex (RNC) to the cell membrane during protein synthesis. In Escherichia coli, inner membrane proteins are mainly targeted by the SRP pathway and outer membrane proteins are targeted by the SecA pathway. However, a recent selective ribosome profiling study suggest that inner membrane proteins with large periplasmic loops are targeted by the SecA pathway.
Proteins are synthesised in ribosomes by a process of serially adding amino acids, called translation. In SecA pathway, a chaperone trigger factor (TF) first bind to the exposed N-terminal signal sequence of the peptide chain. As elongation of peptide chain continues, TF is replaced by SecB. SecB specifically maintains the peptide in an unfolded state, and aids in the binding of SecA. The complex can then bind to SecYEG, by which SecA is activated by binding with ATP. Driven by ATP energy, SecA pushes the protein through the secYEG channel. SecD/F complex also helps in the pulling of the protein from the other side of the cell membrane.
The SecA pathway has also been suggested to have a co-translational targeting mechanism, meaning that the polypeptide would be targeted directly by SecA during its synthesis.
In this pathway, SRP competes with TF and binds to the N-terminal signal sequence. Proteins from inner membrane stops the process of chain elongation. The SRP then binds to a membrane receptor, FtsY. The peptide chain-SRP-FtsY complex is then transported to SecY, where peptide elongation resumes.
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Bacterial secretion system
Bacterial secretion systems are protein complexes present on the cell membranes of bacteria for secretion of substances. Specifically, they are the cellular devices used by pathogenic bacteria to secrete their virulence factors (mainly of proteins) to invade the host cells. They can be classified into different types based on their specific structure, composition and activity. Generally, proteins can be secreted through two different processes. One process is a one-step mechanism in which proteins from the cytoplasm of bacteria are transported and delivered directly through the cell membrane into the host cell. Another involves a two-step activity in which the proteins are first transported out of the inner cell membrane, then deposited in the periplasm, and finally through the outer cell membrane into the host cell.
These major differences can be distinguished between Gram-negative diderm bacteria and Gram-positive monoderm bacteria. But the classification is by no means clear and complete. There are at least eight types specific to Gram-negative bacteria, four to Gram-positive bacteria, while two are common to both. In addition, there is appreciable difference between diderm bacteria with lipopolysaccharide on the outer membrane (diderm-LPS) and those with mycolic acid (diderm-mycolate).
The export pathway is responsible for crossing the inner cell membrane in diderms, and the only cell membrane in monoderms.
The general secretion (Sec) involves secretion of unfolded proteins that first remain inside the cells. In Gram-negative bacteria, the secreted protein is sent to either the inner membrane or the periplasm. But in Gram-positive bacteria, the protein can stay in the cell or is mostly transported out of the bacteria using other secretion systems. Among Gram-negative bacteria, Escherichia coli, Vibrio cholerae, Klebsiella pneumoniae, and Yersinia enterocolitica use the Sec system. Staphylococcus aureus and Listeria monocytogenes are Gram-positive bacteria that use the Sec system.
The Sec system utilises two different pathways for secretion: the SecA and signal recognition particle (SRP) pathways. SecA is an ATPase motor protein and has many related proteins including SecD, SecE, SecF, SegG, SecM, and SecY. SRP is a ribonucleoprotein (protein-RNA complex) that recognizes and targets specific proteins to the endoplasmic reticulum in eukaryotes and to the cell membrane in prokaryotes. The two pathways require different molecular chaperones and ultimately use a protein-transporting channel SecYEG for transporting the proteins across the inner cell membrane. In the SecA-dependent pathway, SecB acts as a chaperone, keeping the newly synthesized protein in an unfolded state and delivers it to translocon-bound SecA. Then the pre-protein is secreted to the periplasm through the SecYEG translocon. Whereas in the SRP pathway, SRP recruits the ribosome-nascent chain complex (RNC) to the cell membrane during protein synthesis. In Escherichia coli, inner membrane proteins are mainly targeted by the SRP pathway and outer membrane proteins are targeted by the SecA pathway. However, a recent selective ribosome profiling study suggest that inner membrane proteins with large periplasmic loops are targeted by the SecA pathway.
Proteins are synthesised in ribosomes by a process of serially adding amino acids, called translation. In SecA pathway, a chaperone trigger factor (TF) first bind to the exposed N-terminal signal sequence of the peptide chain. As elongation of peptide chain continues, TF is replaced by SecB. SecB specifically maintains the peptide in an unfolded state, and aids in the binding of SecA. The complex can then bind to SecYEG, by which SecA is activated by binding with ATP. Driven by ATP energy, SecA pushes the protein through the secYEG channel. SecD/F complex also helps in the pulling of the protein from the other side of the cell membrane.
The SecA pathway has also been suggested to have a co-translational targeting mechanism, meaning that the polypeptide would be targeted directly by SecA during its synthesis.
In this pathway, SRP competes with TF and binds to the N-terminal signal sequence. Proteins from inner membrane stops the process of chain elongation. The SRP then binds to a membrane receptor, FtsY. The peptide chain-SRP-FtsY complex is then transported to SecY, where peptide elongation resumes.