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Bacterial Secretion Systems: An Overview
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- Authors: Erin R. Green1, Joan Mecsas2,3
- Editor: Indira T. Kudva4
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VIEW AFFILIATIONS HIDE AFFILIATIONSAffiliations: 1: Program in Molecular Microbiology, Sackler School of Graduate Biomedical Sciences, Tufts University, Boston, MA 02111; 2: Program in Molecular Microbiology, Sackler School of Graduate Biomedical Sciences, Tufts University, Boston, MA 02111; 3: Department of Molecular Biology and Microbiology, Tufts University School of Medicine, Boston, MA 02111; 4: National Animal Disease Center, Agricultural Research Service, U.S. Department of Agriculture, Ames, IA
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Received 11 March 2015 Accepted 06 July 2015 Published 26 February 2016
- Correspondence: Joan Mecsas, [email protected]
Source:
microbiolspec
February 2016
vol. 4 no. 1 doi:10.1128/microbiolspec.VMBF-0012-2015
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Abstract:
Bacterial pathogens utilize a multitude of methods to invade mammalian hosts, damage tissue sites, and thwart the immune system from responding. One essential component of these strategies for many bacterial pathogens is the secretion of proteins across phospholipid membranes. Secreted proteins can play many roles in promoting bacterial virulence, from enhancing attachment to eukaryotic cells, to scavenging resources in an environmental niche, to directly intoxicating target cells and disrupting their functions. Many pathogens use dedicated protein secretion systems to secrete virulence factors from the cytosol of the bacteria into host cells or the host environment. In general, bacterial protein secretion apparatuses can be divided into classes, based on their structures, functions, and specificity. Some systems are conserved in all classes of bacteria and secrete a broad array of substrates, while others are only found in a small number of bacterial species and/or are specific to only one or a few proteins. In this chapter, we review the canonical features of several common bacterial protein secretion systems, as well as their roles in promoting the virulence of bacterial pathogens. Additionally, we address recent findings that indicate that the innate immune system of the host can detect and respond to the presence of protein secretion systems during mammalian infection.
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Citation: Green E, Mecsas J. 2016. Bacterial Secretion Systems: An Overview. Microbiol Spectrum 4(1):VMBF-0012-2015. doi:10.1128/microbiolspec.VMBF-0012-2015.
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Abstract:
Bacterial pathogens utilize a multitude of methods to invade mammalian hosts, damage tissue sites, and thwart the immune system from responding. One essential component of these strategies for many bacterial pathogens is the secretion of proteins across phospholipid membranes. Secreted proteins can play many roles in promoting bacterial virulence, from enhancing attachment to eukaryotic cells, to scavenging resources in an environmental niche, to directly intoxicating target cells and disrupting their functions. Many pathogens use dedicated protein secretion systems to secrete virulence factors from the cytosol of the bacteria into host cells or the host environment. In general, bacterial protein secretion apparatuses can be divided into classes, based on their structures, functions, and specificity. Some systems are conserved in all classes of bacteria and secrete a broad array of substrates, while others are only found in a small number of bacterial species and/or are specific to only one or a few proteins. In this chapter, we review the canonical features of several common bacterial protein secretion systems, as well as their roles in promoting the virulence of bacterial pathogens. Additionally, we address recent findings that indicate that the innate immune system of the host can detect and respond to the presence of protein secretion systems during mammalian infection.
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Bacterial Secretion Systems: An Overview
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Citation:
Green E, Mecsas J. 2016. Bacterial secretion systems: an overview. microbiolspec 4(1): doi:10.1128/microbiolspec.VMBF-0012-2015
/content/microbiolspec/10.1128/microbiolspec.VMBF-0012-2015.fig1
FIGURE 1a
Export through the Sec pathway. In bacteria, the Sec pathway transports unfolded proteins across the cytoplasmic membrane. Proteins secreted by this pathway may either become embedded in the inner membrane or will be released into the periplasm. In Gram-negative organisms, these periplasmic proteins may be released extracellularly with the help of an additional secretion system. (A) Proteins destined for the periplasm (or extracellular release) are translocated by a posttranslational mechanism and contain a removable signal sequence recognized by the SecB protein. SecB binds presecretory proteins and prevents them from folding while also delivering its substrates to SecA. SecA both guides proteins to the SecYEG channel and serves as the ATPase that provides the energy for protein translocation. Following transport through the SecYEG channel, proteins are folded in the periplasm. (B) The Sec pathway utilizes a cotranslational mechanism of export to secrete proteins destined for the inner membrane. These proteins contain a signal sequence recognized by the signal recognition particle (SRP). During translation, the SRP binds target proteins as they emerge from the ribosome and recruits the docking protein FtsY. FtsY delivers the ribosome-protein complex to the SecYEG channel, which translocates the nascent protein across the cytoplasmic membrane. During translocation across the channel, the transmembrane domain is able to escape through the side of the channel into the membrane, where the protein remains attached.
microbiolspec/4/1/VMBF-0012-2015-fig1a_thmb.gif
microbiolspec/4/1/VMBF-0012-2015-fig1a.gif
Click to view
FIGURE 1a
Export through the Sec pathway. In bacteria, the Sec pathway transports unfolded proteins across the cytoplasmic membrane. Proteins secreted by this pathway may either become embedded in the inner membrane or will be released into the periplasm. In Gram-negative organisms, these periplasmic proteins may be released extracellularly with the help of an additional secretion system. (A) Proteins destined for the periplasm (or extracellular release) are translocated by a posttranslational mechanism and contain a removable signal sequence recognized by the SecB protein. SecB binds presecretory proteins and prevents them from folding while also delivering its substrates to SecA. SecA both guides proteins to the SecYEG channel and serves as the ATPase that provides the energy for protein translocation. Following transport through the SecYEG channel, proteins are folded in the periplasm. (B) The Sec pathway utilizes a cotranslational mechanism of export to secrete proteins destined for the inner membrane. These proteins contain a signal sequence recognized by the signal recognition particle (SRP). During translation, the SRP binds target proteins as they emerge from the ribosome and recruits the docking protein FtsY. FtsY delivers the ribosome-protein complex to the SecYEG channel, which translocates the nascent protein across the cytoplasmic membrane. During translocation across the channel, the transmembrane domain is able to escape through the side of the channel into the membrane, where the protein remains attached.
Source:
microbiolspec
February 2016
vol. 4 no. 1 doi:10.1128/microbiolspec.VMBF-0012-2015
/content/microbiolspec/10.1128/microbiolspec.VMBF-0012-2015.fig1b
FIGURE 1b
Export through the Sec pathway. In bacteria, the Sec pathway transports unfolded proteins across the cytoplasmic membrane. Proteins secreted by this pathway may either become embedded in the inner membrane or will be released into the periplasm. In Gram-negative organisms, these periplasmic proteins may be released extracellularly with the help of an additional secretion system. (A) Proteins destined for the periplasm (or extracellular release) are translocated by a posttranslational mechanism and contain a removable signal sequence recognized by the SecB protein. SecB binds presecretory proteins and prevents them from folding while also delivering its substrates to SecA. SecA both guides proteins to the SecYEG channel and serves as the ATPase that provides the energy for protein translocation. Following transport through the SecYEG channel, proteins are folded in the periplasm. (B) The Sec pathway utilizes a cotranslational mechanism of export to secrete proteins destined for the inner membrane. These proteins contain a signal sequence recognized by the signal recognition particle (SRP). During translation, the SRP binds target proteins as they emerge from the ribosome and recruits the docking protein FtsY. FtsY delivers the ribosome-protein complex to the SecYEG channel, which translocates the nascent protein across the cytoplasmic membrane. During translocation across the channel, the transmembrane domain is able to escape through the side of the channel into the membrane, where the protein remains attached.
microbiolspec/4/1/VMBF-0012-2015-fig1b_thmb.gif
microbiolspec/4/1/VMBF-0012-2015-fig1b.gif
Click to view
FIGURE 1b
Export through the Sec pathway. In bacteria, the Sec pathway transports unfolded proteins across the cytoplasmic membrane. Proteins secreted by this pathway may either become embedded in the inner membrane or will be released into the periplasm. In Gram-negative organisms, these periplasmic proteins may be released extracellularly with the help of an additional secretion system. (A) Proteins destined for the periplasm (or extracellular release) are translocated by a posttranslational mechanism and contain a removable signal sequence recognized by the SecB protein. SecB binds presecretory proteins and prevents them from folding while also delivering its substrates to SecA. SecA both guides proteins to the SecYEG channel and serves as the ATPase that provides the energy for protein translocation. Following transport through the SecYEG channel, proteins are folded in the periplasm. (B) The Sec pathway utilizes a cotranslational mechanism of export to secrete proteins destined for the inner membrane. These proteins contain a signal sequence recognized by the signal recognition particle (SRP). During translation, the SRP binds target proteins as they emerge from the ribosome and recruits the docking protein FtsY. FtsY delivers the ribosome-protein complex to the SecYEG channel, which translocates the nascent protein across the cytoplasmic membrane. During translocation across the channel, the transmembrane domain is able to escape through the side of the channel into the membrane, where the protein remains attached.
Source:
microbiolspec
February 2016
vol. 4 no. 1 doi:10.1128/microbiolspec.VMBF-0012-2015
/content/microbiolspec/10.1128/microbiolspec.VMBF-0012-2015.fig2
FIGURE 2
Secretion through the Tat pathway. Bacteria secrete folded proteins across the cytoplasmic membrane using the Tat secretion pathway. This pathway consists of two or three components (TatA, TatB, and TatC). In Gram-negative bacteria, TatB and TatC bind a specific N-terminal signal peptide containing a “twin” arginine motif on folded Tat secretion substrates. TatB and TatC then recruit TatA to the cytoplasmic membrane, where it forms a channel. Folded proteins are then translocated across the channel and into the periplasm. In Gram-negative bacteria, these proteins may remain in the periplasm or can be exported out of the cell by the T2SS.
microbiolspec/4/1/VMBF-0012-2015-fig2_thmb.gif
microbiolspec/4/1/VMBF-0012-2015-fig2.gif
Click to view
FIGURE 2
Secretion through the Tat pathway. Bacteria secrete folded proteins across the cytoplasmic membrane using the Tat secretion pathway. This pathway consists of two or three components (TatA, TatB, and TatC). In Gram-negative bacteria, TatB and TatC bind a specific N-terminal signal peptide containing a “twin” arginine motif on folded Tat secretion substrates. TatB and TatC then recruit TatA to the cytoplasmic membrane, where it forms a channel. Folded proteins are then translocated across the channel and into the periplasm. In Gram-negative bacteria, these proteins may remain in the periplasm or can be exported out of the cell by the T2SS.
Source:
microbiolspec
February 2016
vol. 4 no. 1 doi:10.1128/microbiolspec.VMBF-0012-2015
/content/microbiolspec/10.1128/microbiolspec.VMBF-0012-2015.fig3
FIGURE 3
Secretion systems in Gram-negative bacteria. Gram-negative bacteria utilize a number of dedicated protein secretion systems to transport proteins across one, two, or three phospholipid membranes. Some proteins are secreted in a two-step, Sec- or Tat-dependent mechanism. These proteins cross the inner membrane with the help of either the Sec or Tat secretion pathways and are then transported across the outer membrane using a second secretion system. The T2SS and T5SS secrete proteins in this manner. Because it secretes folded substrates, the T2SS translocates proteins initially transported by either the Tat or Sec pathway (where Sec substrates are folded in the periplasm). In contrast, autotransporters of the T5SS must be unfolded prior to outer membrane transport and thus must be secreted across the inner membrane by the Sec pathway. Additionally, several Gram-negative protein secretion systems transport their substrates across both bacterial membranes in a one-step, Sec- or Tat-independent process. These include the T1SS, T3SS, T4SS, and T6SS. All of these pathways contain periplasm-spanning channels and secrete proteins from the cytoplasm outside the cell, but their mechanisms of protein secretion are quite different. Three of these secretion systems, the T3SS, T4SS, and T6SS, can also transport proteins across an additional host cell membrane, delivering secreted proteins directly to the cytosol of a target cell.
microbiolspec/4/1/VMBF-0012-2015-fig3_thmb.gif
microbiolspec/4/1/VMBF-0012-2015-fig3.gif
Click to view
FIGURE 3
Secretion systems in Gram-negative bacteria. Gram-negative bacteria utilize a number of dedicated protein secretion systems to transport proteins across one, two, or three phospholipid membranes. Some proteins are secreted in a two-step, Sec- or Tat-dependent mechanism. These proteins cross the inner membrane with the help of either the Sec or Tat secretion pathways and are then transported across the outer membrane using a second secretion system. The T2SS and T5SS secrete proteins in this manner. Because it secretes folded substrates, the T2SS translocates proteins initially transported by either the Tat or Sec pathway (where Sec substrates are folded in the periplasm). In contrast, autotransporters of the T5SS must be unfolded prior to outer membrane transport and thus must be secreted across the inner membrane by the Sec pathway. Additionally, several Gram-negative protein secretion systems transport their substrates across both bacterial membranes in a one-step, Sec- or Tat-independent process. These include the T1SS, T3SS, T4SS, and T6SS. All of these pathways contain periplasm-spanning channels and secrete proteins from the cytoplasm outside the cell, but their mechanisms of protein secretion are quite different. Three of these secretion systems, the T3SS, T4SS, and T6SS, can also transport proteins across an additional host cell membrane, delivering secreted proteins directly to the cytosol of a target cell.
Source:
microbiolspec
February 2016
vol. 4 no. 1 doi:10.1128/microbiolspec.VMBF-0012-2015
/content/microbiolspec/10.1128/microbiolspec.VMBF-0012-2015.fig4
FIGURE 4
Four secretion systems in Gram-positive bacteria. Gram-positive bacteria contain a single cytoplasmic membrane surrounded by a very thick cell wall. These organisms can secrete proteins across the membrane using the Tat and Sec secretion systems. In contrast to Gram-negative organisms, many Gram-positive bacteria use an additional factor for Sec secretion of a smaller subset of proteins, called SecA2. Additionally, there is evidence that some Gram-positive bacteria may use dedicated secretion apparatuses, called “injectosomes” to transport proteins from the bacterial cytoplasm into the cytoplasm of a host cell in a two-step process. The specific mechanism of this process has not been determined, though it has been proposed that the injectosome may utilize a protected channel to transport proteins across the cell wall during export.
microbiolspec/4/1/VMBF-0012-2015-fig4_thmb.gif
microbiolspec/4/1/VMBF-0012-2015-fig4.gif
Click to view
FIGURE 4
Four secretion systems in Gram-positive bacteria. Gram-positive bacteria contain a single cytoplasmic membrane surrounded by a very thick cell wall. These organisms can secrete proteins across the membrane using the Tat and Sec secretion systems. In contrast to Gram-negative organisms, many Gram-positive bacteria use an additional factor for Sec secretion of a smaller subset of proteins, called SecA2. Additionally, there is evidence that some Gram-positive bacteria may use dedicated secretion apparatuses, called “injectosomes” to transport proteins from the bacterial cytoplasm into the cytoplasm of a host cell in a two-step process. The specific mechanism of this process has not been determined, though it has been proposed that the injectosome may utilize a protected channel to transport proteins across the cell wall during export.
Source:
microbiolspec
February 2016
vol. 4 no. 1 doi:10.1128/microbiolspec.VMBF-0012-2015
/content/microbiolspec/10.1128/microbiolspec.VMBF-0012-2015.fig5
FIGURE 5
The T7SS. Certain Gram-positive organisms, including members of the genus Mycobacteria, contain a cell wall layer that is heavily modified by lipids, called a mycomembrane. These organisms contain a distinct protein secretion apparatus called a T7SS. T7SSs contain several core inner membrane proteins that interact with cytosolic chaperones and form a channel through which proteins are secreted. Additionally, it has been proposed that T7SSs may contain an additional, mycomembrane-spanning channel that aids in extracellular secretion of substrates, though this model has not been experimentally proven.
microbiolspec/4/1/VMBF-0012-2015-fig5_thmb.gif
microbiolspec/4/1/VMBF-0012-2015-fig5.gif
Click to view
FIGURE 5
The T7SS. Certain Gram-positive organisms, including members of the genus Mycobacteria, contain a cell wall layer that is heavily modified by lipids, called a mycomembrane. These organisms contain a distinct protein secretion apparatus called a T7SS. T7SSs contain several core inner membrane proteins that interact with cytosolic chaperones and form a channel through which proteins are secreted. Additionally, it has been proposed that T7SSs may contain an additional, mycomembrane-spanning channel that aids in extracellular secretion of substrates, though this model has not been experimentally proven.
Source:
microbiolspec
February 2016
vol. 4 no. 1 doi:10.1128/microbiolspec.VMBF-0012-2015
/content/microbiolspec/10.1128/microbiolspec.VMBF-0012-2015.fig6
FIGURE 6
Mechanisms of innate immune recognition of bacterial secretion systems. To distinguish between pathogenic and commensal bacteria, the mammalian innate immune system has developed methods to directly recognize patterns unique to bacterial pathogens, such as the use of protein secretion apparatuses. The immune system can sense several facets of bacterial protein secretion. These include the pore formation by secretion systems or secreted proteins, aberrant translocation of bacterial molecules into the cytosol, the presence of effector proteins and/or their activities, as well as the components of the secretion systems themselves.
microbiolspec/4/1/VMBF-0012-2015-fig6_thmb.gif
microbiolspec/4/1/VMBF-0012-2015-fig6.gif
Click to view
FIGURE 6
Mechanisms of innate immune recognition of bacterial secretion systems. To distinguish between pathogenic and commensal bacteria, the mammalian innate immune system has developed methods to directly recognize patterns unique to bacterial pathogens, such as the use of protein secretion apparatuses. The immune system can sense several facets of bacterial protein secretion. These include the pore formation by secretion systems or secreted proteins, aberrant translocation of bacterial molecules into the cytosol, the presence of effector proteins and/or their activities, as well as the components of the secretion systems themselves.
Source:
microbiolspec
February 2016
vol. 4 no. 1 doi:10.1128/microbiolspec.VMBF-0012-2015
Tables
Bacterial Secretion Systems: An Overview
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Citation:
Green E, Mecsas J. 2016. Bacterial secretion systems: an overview. microbiolspec 4(1): doi:10.1128/microbiolspec.VMBF-0012-2015
/content/microbiolspec/10.1128/microbiolspec.VMBF-0012-2015.T1
TABLE 1
Classes of bacterial protein secretion systems
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TABLE 1
Classes of bacterial protein secretion systems
Source:
microbiolspec
February 2016
vol. 4 no. 1 doi:10.1128/microbiolspec.VMBF-0012-2015
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