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Bacterial secretion system
Chhandama: added Category:Membrane biology using HotCat
'''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. These major differences can be distinguished between Gram-negative and Gram-negative 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.<ref name="green16">Liquid error: wrong number of arguments (1 for 2)</ref> 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.<ref name="bocian17">Liquid error: wrong number of arguments (1 for 2)</ref>
==Type I secretion system (T1SS or TOSS)==
Type I secretion is a chaperone dependent secretion system employing the Hly and Tol gene clusters. The process begins as a leader sequence HlyA is recognized and binds HlyB on the membrane. This signal sequence is extremely specific for the ABC transporter. The HlyAB complex stimulates HlyD which begins to uncoil and reaches the outer membrane where TolC recognizes a terminal molecule or signal on HlyD. HlyD recruits TolC to the inner membrane and HlyA is excreted outside of the outer membrane via a long-tunnel protein channel.
Type I secretion system transports various molecules, from ions, drugs, to proteins of various sizes (20 – 900 kDa). The molecules secreted vary in size from the small ''[[Escherichia coli]]'' peptide colicin V, (10 kDa) to the ''[[Pseudomonas fluorescens]]'' cell adhesion protein LapA of 520 kDa.<ref>Liquid error: wrong number of arguments (1 for 2)</ref> The best characterized are the [[RTX toxin]]s and the lipases. Type I secretion is also involved in export of non-proteinaceous substrates like cyclic β-glucans and polysaccharides.
==Type II secretion system (T2SS)==
Proteins secreted through the type II system, or main terminal branch of the general secretory pathway, depend on the Sec or Tat system for initial transport into the [[periplasm]]. Once there, they pass through the outer membrane via a multimeric (12–14 subunits) complex of pore forming secretin proteins. In addition to the secretin protein, 10–15 other inner and outer membrane proteins compose the full secretion apparatus, many with as yet unknown function. Gram-negative [[Pilus#Type IV pili|type IV pili]] use a modified version of the type II system for their biogenesis, and in some cases certain proteins are shared between a pilus complex and type II system within a single bacterial species.
===Type III secretion system (T3SS or TTSS)===
It is homologous to the basal body in bacterial flagella. It is like a molecular syringe through which a bacterium (e.g. certain types of ''[[Salmonella]]'', ''[[Shigella]]'', ''[[Yersinia]]'', ''[[Vibrio]]'') can inject proteins into eukaryotic cells. The low Ca<sup>2+</sup> concentration in the cytosol opens the gate that regulates T3SS. One such mechanism to detect low calcium concentration has been illustrated by the lcrV (Low Calcium Response) antigen utilized by ''[[Yersinia pestis]]'', which is used to detect low calcium concentrations and elicits T3SS attachment. The Hrp system in plant pathogens inject harpins and pathogen effector proteins through similar mechanisms into plants. This secretion system was first discovered in ''[[Yersinia pestis]]'' and showed that toxins could be injected directly from the bacterial cytoplasm into the cytoplasm of its host's cells rather than simply be secreted into the extracellular medium.<ref>Salyers, A. A. & Whitt, D. D. (2002). ''Bacterial Pathogenesis: A Molecular Approach'', 2nd ed., Washington, D.C.: ASM Press. Liquid error: wrong number of arguments (1 for 2)</ref>
==Type IV secretion system (T4SS or TFSS)==
It is homologous to [[Bacterial conjugation|conjugation]] machinery of bacteria. It is capable of transporting both DNA and proteins. It was discovered in ''Agrobacterium tumefaciens'', which uses this system to introduce the T-DNA portion of the Ti plasmid into the plant host, which in turn causes the affected area to develop into a crown gall (tumor). ''[[Helicobacter pylori]]'' uses a type IV secretion system to deliver [[CagA]] into gastric epithelial cells, which is associated with gastric carcinogenesis.<ref name="pmid16367902">Liquid error: wrong number of arguments (1 for 2)</ref> ''[[Bordetella pertussis]]'', the causative agent of whooping cough, secretes the [[pertussis toxin]] partly through the type IV system. ''[[Legionella pneumophila]]'', the causing agent of legionellosis (Legionnaires' disease) utilizes a [[type IVB secretion system]], known as the icm/dot ('''i'''ntra'''c'''ellular '''m'''ultiplication / '''d'''efect in '''o'''rganelle '''t'''rafficking genes) system, to translocate numerous [[Bacterial effector protein|effector proteins]] into its eukaryotic host.<ref name="pmid15035043">Liquid error: wrong number of arguments (1 for 2)</ref> The prototypic Type IVA secretion system is the VirB complex of ''[[Agrobacterium tumefaciens]]''.<ref name="pmid16153176"></ref>
Protein members of this family are components of the type IV secretion system. They mediate [[intracellular]] transfer of [[macromolecule]]s via a [[Nuclear receptor#Mechanism of action|mechanism]] ancestrally related to that of [[bacterial conjugation]] machineries.<ref name="pmid15546668">Liquid error: wrong number of arguments (1 for 2)</ref><ref name="pmid14673074">Liquid error: wrong number of arguments (1 for 2)</ref>
=== Function ===
In short, Type IV secretion system (T4SS), is the general mechanism by which bacterial cells secrete or take up macromolecules. Their precise mechanism remains unknown. T4SS is encoded on [[Gram negative bacteria|Gram-negative]] conjugative elements in [[bacteria]].T4SS are cell envelope-spanning complexes or in other words 11–13 core proteins that form a channel through which DNA and proteins can travel from the cytoplasm of the donor cell to the cytoplasm of the recipient cell. Additionally, T4SS also secrete [[virulence]] factor proteins directly into host cells as well as taking up DNA from the medium during natural [[Transformation (bacteria)|transformation]], which shows the versatility of this macromolecular secretion apparatus.<ref name="pmid12855161">Liquid error: wrong number of arguments (1 for 2)</ref>
=== Interactions ===
T4SS has two effector proteins: firstly, ATS-1, which stands for Anaplasma translocated substrate 1, and secondly [[Ankyrin repeat|AnkA]], which stands for ankyrin repeat domain-containing protein A. Additionally, T4SS coupling proteins are VirD4, which bind to VirE2.<ref name="pmid20670295">Liquid error: wrong number of arguments (1 for 2)</ref>
===Type V secretion system (T5SS)==
Also called the autotransporter system,<ref name=Thanassi2005></ref> type V secretion involves use of the ''Sec'' system for crossing the inner membrane. Proteins which use this pathway have the capability to form a beta-barrel with their C-terminus which inserts into the outer membrane, allowing the rest of the peptide (the passenger domain) to reach the outside of the cell. Often, autotransporters are cleaved, leaving the beta-barrel domain in the outer membrane and freeing the passenger domain. Some researchers believe remnants of the autotransporters gave rise to the porins which form similar beta-barrel structures.Liquid error: wrong number of arguments (1 for 2) A common example of an autotransporter that uses this secretion system is the [[Trimeric Autotransporter Adhesins (TAA)|Trimeric Autotransporter Adhesins]].<ref name="pmid17482513">Liquid error: wrong number of arguments (1 for 2)</ref>
==Type VI secretion system (T6SS)==
Type VI secretion systems were originally identified in 2006 by the group of John Mekalanos at the Harvard Medical School (Boston, USA) in two bacterial pathogens, ''[[Vibrio cholerae]]'' and ''[[Pseudomonas aeruginosa]]''.<ref name="pmid16432199">Liquid error: wrong number of arguments (1 for 2)</ref><ref name="pmid16763151">Liquid error: wrong number of arguments (1 for 2)</ref> These were identified when mutations in the Hcp and VrgG genes in ''Vibrio Cholerae'' led to decreased virulence and pathogenicity. Since then, Type VI secretion systems have been found in a quarter of all proteobacterial genomes, including animal, plant, human pathogens, as well as soil, environmental or marine bacteria.<ref name="pmid18289922">Liquid error: wrong number of arguments (1 for 2)</ref><ref name="pmid18617888">Liquid error: wrong number of arguments (1 for 2)</ref> While most of the early studies of Type VI secretion focused on its role in the pathogenesis of higher organisms, more recent studies suggested a broader physiological role in defense against simple eukaryotic predators and its role in inter-bacteria interactions.<ref>Liquid error: wrong number of arguments (1 for 2)</ref><ref name="Coulthurst 2013 S0923-2508"></ref> The Type VI secretion system gene clusters contain from 15 to more than 20 genes, two of which, Hcp and VgrG, have been shown to be nearly universally secreted substrates of the system. Structural analysis of these and other proteins in this system bear a striking resemblance to the tail spike of the T4 phage, and the activity of the system is thought to functionally resemble phage infection.<ref name="Silverman 2012 453-472"></ref>
==References==
[[Category:Secretion]]
[[Category:Cellular processes]]
[[Category:Membrane biology]]
==Type I secretion system (T1SS or TOSS)==
Type I secretion is a chaperone dependent secretion system employing the Hly and Tol gene clusters. The process begins as a leader sequence HlyA is recognized and binds HlyB on the membrane. This signal sequence is extremely specific for the ABC transporter. The HlyAB complex stimulates HlyD which begins to uncoil and reaches the outer membrane where TolC recognizes a terminal molecule or signal on HlyD. HlyD recruits TolC to the inner membrane and HlyA is excreted outside of the outer membrane via a long-tunnel protein channel.
Type I secretion system transports various molecules, from ions, drugs, to proteins of various sizes (20 – 900 kDa). The molecules secreted vary in size from the small ''[[Escherichia coli]]'' peptide colicin V, (10 kDa) to the ''[[Pseudomonas fluorescens]]'' cell adhesion protein LapA of 520 kDa.<ref>Liquid error: wrong number of arguments (1 for 2)</ref> The best characterized are the [[RTX toxin]]s and the lipases. Type I secretion is also involved in export of non-proteinaceous substrates like cyclic β-glucans and polysaccharides.
==Type II secretion system (T2SS)==
Proteins secreted through the type II system, or main terminal branch of the general secretory pathway, depend on the Sec or Tat system for initial transport into the [[periplasm]]. Once there, they pass through the outer membrane via a multimeric (12–14 subunits) complex of pore forming secretin proteins. In addition to the secretin protein, 10–15 other inner and outer membrane proteins compose the full secretion apparatus, many with as yet unknown function. Gram-negative [[Pilus#Type IV pili|type IV pili]] use a modified version of the type II system for their biogenesis, and in some cases certain proteins are shared between a pilus complex and type II system within a single bacterial species.
===Type III secretion system (T3SS or TTSS)===
It is homologous to the basal body in bacterial flagella. It is like a molecular syringe through which a bacterium (e.g. certain types of ''[[Salmonella]]'', ''[[Shigella]]'', ''[[Yersinia]]'', ''[[Vibrio]]'') can inject proteins into eukaryotic cells. The low Ca<sup>2+</sup> concentration in the cytosol opens the gate that regulates T3SS. One such mechanism to detect low calcium concentration has been illustrated by the lcrV (Low Calcium Response) antigen utilized by ''[[Yersinia pestis]]'', which is used to detect low calcium concentrations and elicits T3SS attachment. The Hrp system in plant pathogens inject harpins and pathogen effector proteins through similar mechanisms into plants. This secretion system was first discovered in ''[[Yersinia pestis]]'' and showed that toxins could be injected directly from the bacterial cytoplasm into the cytoplasm of its host's cells rather than simply be secreted into the extracellular medium.<ref>Salyers, A. A. & Whitt, D. D. (2002). ''Bacterial Pathogenesis: A Molecular Approach'', 2nd ed., Washington, D.C.: ASM Press. Liquid error: wrong number of arguments (1 for 2)</ref>
==Type IV secretion system (T4SS or TFSS)==
It is homologous to [[Bacterial conjugation|conjugation]] machinery of bacteria. It is capable of transporting both DNA and proteins. It was discovered in ''Agrobacterium tumefaciens'', which uses this system to introduce the T-DNA portion of the Ti plasmid into the plant host, which in turn causes the affected area to develop into a crown gall (tumor). ''[[Helicobacter pylori]]'' uses a type IV secretion system to deliver [[CagA]] into gastric epithelial cells, which is associated with gastric carcinogenesis.<ref name="pmid16367902">Liquid error: wrong number of arguments (1 for 2)</ref> ''[[Bordetella pertussis]]'', the causative agent of whooping cough, secretes the [[pertussis toxin]] partly through the type IV system. ''[[Legionella pneumophila]]'', the causing agent of legionellosis (Legionnaires' disease) utilizes a [[type IVB secretion system]], known as the icm/dot ('''i'''ntra'''c'''ellular '''m'''ultiplication / '''d'''efect in '''o'''rganelle '''t'''rafficking genes) system, to translocate numerous [[Bacterial effector protein|effector proteins]] into its eukaryotic host.<ref name="pmid15035043">Liquid error: wrong number of arguments (1 for 2)</ref> The prototypic Type IVA secretion system is the VirB complex of ''[[Agrobacterium tumefaciens]]''.<ref name="pmid16153176"></ref>
Protein members of this family are components of the type IV secretion system. They mediate [[intracellular]] transfer of [[macromolecule]]s via a [[Nuclear receptor#Mechanism of action|mechanism]] ancestrally related to that of [[bacterial conjugation]] machineries.<ref name="pmid15546668">Liquid error: wrong number of arguments (1 for 2)</ref><ref name="pmid14673074">Liquid error: wrong number of arguments (1 for 2)</ref>
=== Function ===
In short, Type IV secretion system (T4SS), is the general mechanism by which bacterial cells secrete or take up macromolecules. Their precise mechanism remains unknown. T4SS is encoded on [[Gram negative bacteria|Gram-negative]] conjugative elements in [[bacteria]].T4SS are cell envelope-spanning complexes or in other words 11–13 core proteins that form a channel through which DNA and proteins can travel from the cytoplasm of the donor cell to the cytoplasm of the recipient cell. Additionally, T4SS also secrete [[virulence]] factor proteins directly into host cells as well as taking up DNA from the medium during natural [[Transformation (bacteria)|transformation]], which shows the versatility of this macromolecular secretion apparatus.<ref name="pmid12855161">Liquid error: wrong number of arguments (1 for 2)</ref>
=== Interactions ===
T4SS has two effector proteins: firstly, ATS-1, which stands for Anaplasma translocated substrate 1, and secondly [[Ankyrin repeat|AnkA]], which stands for ankyrin repeat domain-containing protein A. Additionally, T4SS coupling proteins are VirD4, which bind to VirE2.<ref name="pmid20670295">Liquid error: wrong number of arguments (1 for 2)</ref>
===Type V secretion system (T5SS)==
Also called the autotransporter system,<ref name=Thanassi2005></ref> type V secretion involves use of the ''Sec'' system for crossing the inner membrane. Proteins which use this pathway have the capability to form a beta-barrel with their C-terminus which inserts into the outer membrane, allowing the rest of the peptide (the passenger domain) to reach the outside of the cell. Often, autotransporters are cleaved, leaving the beta-barrel domain in the outer membrane and freeing the passenger domain. Some researchers believe remnants of the autotransporters gave rise to the porins which form similar beta-barrel structures.Liquid error: wrong number of arguments (1 for 2) A common example of an autotransporter that uses this secretion system is the [[Trimeric Autotransporter Adhesins (TAA)|Trimeric Autotransporter Adhesins]].<ref name="pmid17482513">Liquid error: wrong number of arguments (1 for 2)</ref>
==Type VI secretion system (T6SS)==
Type VI secretion systems were originally identified in 2006 by the group of John Mekalanos at the Harvard Medical School (Boston, USA) in two bacterial pathogens, ''[[Vibrio cholerae]]'' and ''[[Pseudomonas aeruginosa]]''.<ref name="pmid16432199">Liquid error: wrong number of arguments (1 for 2)</ref><ref name="pmid16763151">Liquid error: wrong number of arguments (1 for 2)</ref> These were identified when mutations in the Hcp and VrgG genes in ''Vibrio Cholerae'' led to decreased virulence and pathogenicity. Since then, Type VI secretion systems have been found in a quarter of all proteobacterial genomes, including animal, plant, human pathogens, as well as soil, environmental or marine bacteria.<ref name="pmid18289922">Liquid error: wrong number of arguments (1 for 2)</ref><ref name="pmid18617888">Liquid error: wrong number of arguments (1 for 2)</ref> While most of the early studies of Type VI secretion focused on its role in the pathogenesis of higher organisms, more recent studies suggested a broader physiological role in defense against simple eukaryotic predators and its role in inter-bacteria interactions.<ref>Liquid error: wrong number of arguments (1 for 2)</ref><ref name="Coulthurst 2013 S0923-2508"></ref> The Type VI secretion system gene clusters contain from 15 to more than 20 genes, two of which, Hcp and VgrG, have been shown to be nearly universally secreted substrates of the system. Structural analysis of these and other proteins in this system bear a striking resemblance to the tail spike of the T4 phage, and the activity of the system is thought to functionally resemble phage infection.<ref name="Silverman 2012 453-472"></ref>
==References==
[[Category:Secretion]]
[[Category:Cellular processes]]
[[Category:Membrane biology]]
September 17, 2018 at 12:39AM
