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Secretion is the movement of material from one point to another, such as a secreted
chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by having volume. All everyday objects that can be touched are ultimately composed of atoms, which ...
from a cell or
gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλά ...

gland
. In contrast,
excretion Excretion is a process in which metabolic waste Metabolic wastes or excrements are Chemical substance, substances left over from metabolism, metabolic processes (such as cellular respiration) which cannot be used by the organism (they are surplus ...
, is the removal of certain substances or waste products from a cell or organism. The classical mechanism of cell secretion is via secretory portals at the cell plasma membrane called
porosomes 280px Porosomes are cup-shaped supramolecular structures in the cell membranes of eukaryotic Eukaryotes () are organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual conti ...
. Porosomes are permanent cup-shaped lipoprotein structure at the cell plasma membrane, where secretory vesicles transiently dock and fuse to release intra-vesicular contents from the cell. Secretion in bacterial species means the transport or translocation of effector molecules for example:
proteins Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

proteins
,
enzymes Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates int ...
or
toxin A toxin is a harmful substance produced within living cells or organisms; synthetic toxicants created by artificial processes are thus excluded. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived from the word toxic ...
s (such as
cholera toxin Cholera toxin (also known as choleragen and sometimes abbreviated to CTX, Ctx or CT) is AB5 multimeric protein complex is a protein complex functioning as a molecular biological machine. It uses protein domain dynamics on nanoscales A protei ...

cholera toxin
in
pathogenic bacteria Pathogenic bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of biological cell The cell (from Latin ''cella'', meaning "small room") is the basic structural, functional, and biological unit of a ...
for example ''
Vibrio cholerae ''Vibrio cholerae'' is a species of Gram-negative Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain o ...

Vibrio cholerae
'') from across the interior (
cytoplasm In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes ...
or
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a s ...
) of a bacterial cell to its exterior. Secretion is a very important mechanism in bacterial functioning and operation in their natural surrounding environment for adaptation and survival.


In eukaryotic cells


Mechanism

Eukaryotic Eukaryotes () are organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interact ...

Eukaryotic
cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...
, including
human cells There are many different types of cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse liv ...

human cells
, have a highly
evolved Evolution is change in the heritable Heredity, also called inheritance or biological inheritance, is the passing on of Phenotypic trait, traits from parents to their offspring; either through asexual reproduction or sexual reproduction, ...

evolved
process of secretion. Proteins targeted for the outside are
synthesized Synthesis or synthesize may also refer to: Science Chemistry and biochemistry *Chemical synthesis, the execution of chemical reactions to form a more complex molecule from chemical precursors **Organic synthesis, the chemical synthesis of or ...

synthesized
by
ribosome Ribosomes ( ), also called Palade granules, are molecular machine, macromolecular machines, found within all cell (biology), cells, that perform Translation (biology), biological protein synthesis (mRNA translation). Ribosomes link amino acids ...

ribosome
s docked to the rough
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
(ER). As they are synthesized, these proteins translocate into the ER lumen, where they are
glycosylated Glycosylation (see also chemical glycosylationA chemical glycosylation reaction involves the coupling of a glycosyl donor, to a glycosyl acceptor forming a glycoside. If both the donor and acceptor are sugars, then the product is an oligosacchar ...

glycosylated
and where molecular chaperones aid
protein folding Protein folding is the physical process Physical changes are changes affecting the form of a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass ...

protein folding
.
Misfolded proteins Protein folding is the physical process by which a protein chain is Translation (biology), translated to its Native state, native protein tertiary structure, three-dimensional structure, typically a "folded" Protein structure, conformation by w ...
are usually identified here and retrotranslocated by ER-associated degradation to the
cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a s ...
, where they are degraded by a
proteasome Proteasomes are protein complex A protein complex or multiprotein complex is a group of two or more associated polypeptide chain Peptides (from Greek language Greek (modern , romanized: ''Elliniká'', Ancient Greek, ancient , ''Hellēn ...

proteasome
. The
vesicle Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry) In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and liv ...
s containing the properly folded proteins then enter the
Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that stu ...

Golgi apparatus
. In the Golgi apparatus, the glycosylation of the proteins is modified and further posttranslational modifications, including cleavage and functionalization, may occur. The
proteins Proteins are large biomolecule , showing alpha helices, represented by ribbons. This poten was the first to have its suckture solved by X-ray crystallography by Max Perutz and Sir John Cowdery Kendrew in 1958, for which they received a No ...

proteins
are then moved into secretory vesicles which travel along the
cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filament In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proces ...

cytoskeleton
to the edge of the cell. More modification can occur in the secretory vesicles (for example
insulin Insulin (, from Latin ''insula'', 'island') is a peptide hormone produced by beta cells of the pancreatic islets; it is considered to be the main Anabolism, anabolic hormone of the body. It regulates the metabolism of carbohydrates, fats and p ...

insulin
is cleaved from
proinsulin
proinsulin
in the secretory vesicles). Eventually, there is
vesicle fusionVesicle fusion is the merging of a vesicle with other vesicles Vesicle may refer to: ; In cellular biology or chemistry * Vesicle (biology and chemistry), a supramolecular assembly of lipid molecules, like a cell membrane * Synaptic vesicle ; In h ...
with the
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane A biological membrane, biomembrane or cell membrane is a selectively permeable membra ...

cell membrane
at a structure called the porosome, in a process called
exocytosis Exocytosis () is a form of active transport In cellular biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, phys ...
, dumping its contents out of the cell's environment. Strict
biochemical Biochemistry or biological chemistry, is the study of chemical process In a scientific Science () is a systematic enterprise that Scientific method, builds and organizes knowledge in the form of Testability, testable explanations and pr ...
control is maintained over this sequence by usage of a
pH
pH
gradient: the pH of the cytosol is 7.4, the ER's pH is 7.0, and the cis-golgi has a pH of 6.5. Secretory vesicles have pHs ranging between 5.0 and 6.0; some secretory vesicles evolve into
lysosome A lysosome () is a membrane-bound organelle found in many animal Cell (biology), cells. They are spherical Vesicle (biology and chemistry), vesicles that contain Hydrolysis, hydrolytic enzymes that can break down many kinds of biomolecules. A ly ...

lysosome
s, which have a pH of 4.8.


Nonclassical secretion

There are many proteins like
FGF1 FGF1, also known as acidic fibroblast growth factor (aFGF), is a growth factor A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Usually it i ...
(aFGF),
FGF2 FGF2, also known as basic fibroblast growth factor (bFGF) and FGF-β, is a growth factor A growth factor is a naturally occurring substance capable of stimulating cell proliferation, wound healing, and occasionally cellular differentiation. Us ...
(bFGF),
interleukin-1 The Interleukin-1 family (IL-1 family) is a group of 11 cytokines Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Cytokines are peptides and cannot cross the lipid bilayer of cells to ente ...
(IL1) etc. which do not have a signal sequence. They do not use the classical ER-Golgi pathway. These are secreted through various nonclassical pathways. At least four nonclassical (unconventional) protein secretion pathways have been described. They include 1) direct translocation of proteins across the plasma membrane likely through membrane transporters, 2)
blebbing Image:Apoptotic cell disassembly.png, 500px, During apoptosis, blebbing is the first phase (left) of cell disassembly. In cell biology, a bleb is a bulge of the plasma membrane of a cell, human bioparticulate or abscess with an internal environment ...
, 3) lysosomal secretion, and 4) release via exosomes derived from multivesicular bodies. In addition, proteins can be released from cells by mechanical or physiological wounding and through nonlethal, transient oncotic pores in the plasma membrane induced by washing cells with serum-free media or buffers.


In human tissues

Many human cell types have the ability to be secretory cells. They have a well-developed
endoplasmic reticulum The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
, and
Golgi apparatus The Golgi apparatus (), also known as the Golgi complex, Golgi body, or simply the Golgi, is an organelle In cell biology Cell biology (also cellular biology or cytology) is a branch of biology Biology is the natural science that stu ...

Golgi apparatus
to fulfill this function. Tissues that produce secretions include the
gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system The human digestive system consists of the gastrointestinal tract The gastrointestinal tract, (GI tract, GIT, d ...
which secretes
digestive enzyme Digestive may refer to: Biology *Digestion Digestion is the breakdown of large insoluble food Food is any substance consumed to provide nutritional Nutrition is the biochemical Biochemistry or biological chemistry, is the study of c ...
s and
gastric acid Gastric acid, gastric juice, or stomach acid, is a digestive fluid formed within the stomach lining. With a pH between 1 and 3, gastric acid An acid is a or capable of donating a (hydrogen ion H+) (a ), or, alternatively, capable of for ...
, the
lung The lungs are the primary organs An organ is a group of tissues with similar functions. Plant life and animal life rely on many organs that co-exist in organ systems. A given organ's tissues can be broadly categorized as parenchyma ...

lung
s which secrete
surfactant Surfactants are compounds that lower the surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension is what allows objects with a higher density than wate ...

surfactant
s, and
sebaceous gland A sebaceous gland is a microscopic exocrine Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in motion") is a ...
s which secrete
sebum A sebaceous gland is a microscopic exocrine Exocrine glands are gland In animals, a gland is a group of cells in an animal's body that synthesizes substances (such as hormone A hormone (from the Greek participle , "setting in motion") is a ...

sebum
to lubricate the skin and hair.
Meibomian gland Meibomian glands (also called tarsal glands, palpebral glands, and tarsoconjunctival glands) are sebaceous gland A sebaceous gland is a microscopic exocrine Exocrine glands are glands that secrete substances onto an Epithelium, epithelial surfa ...
s in the
eyelid An eyelid is a thin fold of skin that covers and protects an eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo respon ...

eyelid
secrete
meibum Meibomian glands (also called tarsal glands, palpebral glands, and tarsoconjunctival glands) are sebaceous glands along the rims of the eyelid inside the tarsal plate. They produce meibum, an oily substance that prevents evaporation of the human ...
to lubricate and protect the eye.


In gram-negative bacteria

Secretion is not unique to eukaryotes - it is also present in bacteria and archaea as well. ATP binding cassette (ABC) type transporters are common to the three domains of life. Some secreted proteins are translocated across the cytoplasmic membrane by the SecYEG
translocon The translocon (also known as a translocator or translocation channel) is a complex of protein Proteins are large s and s that comprise one or more long chains of . Proteins perform a vast array of functions within organisms, including , , , ...
, one of two translocation systems, which requires the presence of an N-terminal signal peptide on the secreted protein. Others are translocated across the cytoplasmic membrane by the twin-arginine translocation pathway (Tat).
Gram-negative bacteria Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), do ...
have two membranes, thus making secretion topologically more complex. There are at least six specialized secretion systems in gram-negative bacteria. Many secreted proteins are particularly important in bacterial pathogenesis.


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 on the protein to be secreted is recognized by HlyA 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 ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative bacteria, Gram-negative, Facultative anaerobic organism, facultative anaer ...

Escherichia coli
'' peptide colicin V, (10 kDa) to the ''
Pseudomonas fluorescens ''Pseudomonas fluorescens'' is a common Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram stain, gram-staining method of bacterial differentiation. They are characterized by their cell ...

Pseudomonas fluorescens
'' cell adhesion protein LapA of 520 kDa. The best characterized are the RTX toxins 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 400px, cell_wall.html"_;"title="Gram-negative_cell_wall">Gram-negative_cell_wall_ The_periplasm_is_a_concentrated_gel-like_matrix_(biology).html" ;"title="cell_wall_.html" ;"title="cell_wall.html" ;"title="Gram-negative cell wall">Gram-negative ce ...
. 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
type IV pili A pilus (Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it bec ...
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 ''Salmonella'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circ ...
'', ''
Shigella ''Shigella'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to ...
'', ''
Yersinia ''Yersinia'' is a genus of bacteria in the family Yersiniaceae. ''Yersinia'' species are Gram-negative, coccobacilli bacteria, a few micrometers long and fractions of a micrometer in diameter, and are facultative anaerobes. Some members of ''Yers ...

Yersinia
'', ''
Vibrio ''Vibrio'' is a genus Genus /ˈdʒiː.nəs/ (plural genera /ˈdʒen.ər.ə/) is a taxonomic rank In biological classification In biology, taxonomy () is the scientific study of naming, defining (Circumscription (taxonomy), circumscr ...
'') can inject proteins into eukaryotic cells. The low Ca2+ 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 ''Yersinia pestis'' (''Y. pestis'') (formerly ''Pasteurella __NOTOC__ ''Pasteurella'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the ...

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 ''Yersinia pestis'' (''Y. pestis'') (formerly ''Pasteurella __NOTOC__ ''Pasteurella'' is a genus Genus (plural genera) is a taxonomic rank Taxonomy (general) is the practice and science of classification of things or concepts, including the ...

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.


Type IV secretion system (T4SS or TFSS)

It is homologous to
conjugation Conjugation or conjugate may refer to: Linguistics * Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics * Complex conjugation, the change ...
machinery of bacteria, the conjugative pili. 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 ''Helicobacter pylori'', previously known as ''Campylobacter pylori'', is a gram-negative, microaerophile, microaerophilic, spiral bacteria, spiral (helical) bacterium usually found in the stomach. Its helical shape (from which the genus name, ...

Helicobacter pylori
'' uses a type IV secretion system to deliver
CagA ''Helicobacter pylori ''Helicobacter pylori'', previously known as ''Campylobacter pylori'', is a gram-negative, microaerophile, microaerophilic, spiral bacteria, spiral (helical) bacterium usually found in the stomach. Its helical shape (from ...
into gastric epithelial cells, which is associated with gastric carcinogenesis. ''
Bordetella pertussis ''Bordetella pertussis'' is a Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram stain, gram-staining method of bacterial differentiation. They are characterized by their cell envelope ...

Bordetella pertussis
'', the causative agent of whooping cough, secretes the
pertussis toxin Pertussis toxin (PT) is a protein-based AB5-type exotoxin produced by the bacterium Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domai ...
partly through the type IV system. ''
Legionella pneumophila ''Legionella pneumophila'' is a thin, aerobic, pleomorphic, flagellated, non-spore-forming, Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram stain, gram-staining method of bacteri ...
'', the causing agent of legionellosis (Legionnaires' disease) utilizes a type IVB secretion system, known as the icm/dot (intracellular multiplication / defect in organelle trafficking genes) system, to translocate numerous effector proteins into its eukaryotic host. The prototypic Type IVA secretion system is the VirB complex of ''
Agrobacterium tumefaciens ''Agrobacterium tumefaciens'' (updated scientific name ''Rhizobium radiobacter'', synonym ''Agrobacterium radiobacter'') is the causal agent of crown gall disease (the formation of tumours) in over 140 species of eudicots. It is a rod-shaped, Gram ...

Agrobacterium tumefaciens
''. Protein members of this family are components of the type IV secretion system. They mediate
intracellular This glossary of biology terms is a list of definitions of fundamental terms and concepts used in biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemic ...
transfer of
macromolecule A macromolecule is a very large molecule A molecule is an electrically Electricity is the set of physical phenomena associated with the presence and motion Image:Leaving Yongsan Station.jpg, 300px, Motion involves a change in ...
s via a
mechanism Mechanism may refer to: *Mechanism (engineering) In engineering, a mechanism is a Machine, device that transforms input forces and movement into a desired set of output forces and movement. Mechanisms generally consist of moving components which m ...

mechanism
ancestrally related to that of
bacterial conjugation Bacterial conjugation is the transfer of genetic material between Bacteria, bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells. This takes place through a pilus. It is a parasexual mode of reproduction in ...
machineries.


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 Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few microm ...
conjugative elements in
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

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 Virulence is a pathogen's or microorganism's ability to cause damage to a host. In most contexts, especially in animal systems, virulence refers to the degree of damage caused by a microbe to its host (biology), host. The Pathogen#Pathogenicity, ...
factor proteins directly into host cells as well as taking up DNA from the medium during natural
transformation Transformation may refer to: Science and mathematics In biology and medicine * Metamorphosis, the biological process of changing physical form after birth or hatching * Malignant transformation, the process of cells becoming cancerous * Transf ...
, which shows the versatility of this macromolecular secretion apparatus.


Structure

As shown in the above figure, TraC, in particular consists of a three helix bundle and a loose globular appendage.


Interactions

T4SS has two effector proteins: firstly, ATS-1, which stands for Anaplasma translocated substrate 1, and secondly AnkA, which stands for ankyrin repeat domain-containing protein A. Additionally, T4SS coupling proteins are VirD4, which bind to VirE2.


Type V secretion system (T5SS)

Also called the autotransporter system, 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. A common example of an autotransporter that uses this secretion system is the Trimeric Autotransporter Adhesins.


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 ''Vibrio cholerae'' is a species of Gram-negative Gram-negative bacteria are bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain o ...

Vibrio cholerae
'' and ''
Pseudomonas aeruginosa ''Pseudomonas aeruginosa'' is a common encapsulated, Gram-negative Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram stain, gram-staining method of bacterial differentiation. They are characterized ...
''. 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. 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. 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.


Release of outer membrane vesicles

In addition to the use of the multiprotein complexes listed above, Gram-negative bacteria possess another method for release of material: the formation of
bacterial outer membrane vesicles Bacterial outer membrane vesicles (OMVs) are vesicles of lipids In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...
. Portions of the outer membrane pinch off, forming nano-scale spherical structures made of a lipopolysaccharide-rich lipid bilayer enclosing periplasmic materials, and are deployed for
membrane vesicle traffickingMembrane vesicle trafficking in ''eukaryotic'' animal cells involves movement of important biochemical signal molecules from synthesis-and-packaging locations in the Golgi body to specific 'release' locations on the inside of the plasma membrane ...
to manipulate environment or invade at host-pathogen interface. Vesicles from a number of bacterial species have been found to contain virulence factors, some have immunomodulatory effects, and some can directly adhere to and intoxicate host cells. release of vesicles has been demonstrated as a general response to stress conditions, the process of loading cargo proteins seems to be selective.


Secretion in gram-positive bacteria

In some ''Staphylococcus'' and ''Streptococcus'' species, the accessory secretory system handles the export of highly repetitive adhesion glycoproteins.


See also

* Bacterial effector protein *
Bacterial outer membrane vesicles Bacterial outer membrane vesicles (OMVs) are vesicles of lipids In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...
* Host-pathogen interface *
Membrane vesicle traffickingMembrane vesicle trafficking in ''eukaryotic'' animal cells involves movement of important biochemical signal molecules from synthesis-and-packaging locations in the Golgi body to specific 'release' locations on the inside of the plasma membrane ...
* Secretomics *
Secretory protein A secretory protein is any protein, whether it be endocrine or exocrine, which is secreted by a cell. Secretory proteins include many hormones, enzymes, toxins, and antimicrobial peptides. Secretory proteins are synthesized in the endoplasmic ...
s * Secretor status


References

Z. Esna Ashari, N. Dasgupta, K. Brayton & S. Broschat,
An optimal set of features for predicting type IV secretion system effector proteins for a subset of species based on a multi-level feature selection approach
, PLOS ONE Journal, 2018, 13, e0197041. (doi.org/10.1371/journal.pone.0197041.)


Further reading

* * *


External links

* * T5SS / Autotransporter illustration at

{{Authority control Biochemistry Cell biology Physiology pl:Wydzielanie