Secretion is the process of segregating, elaborating and releasing chemicals from a cell, or a secreted
chemical substanceor amount of substance. In contrast to excretion, the substance may have a certain function, rather than being a waste product.
Secretion in humans include e.g.:
In humans, just as in all eukaryotic cells, there is a highly evolved process of secretion.
Proteins targeted for the outside are synthesized by ribosomes docked to the rough endoplasmic reticulum. As they are synthesized, these proteins translocate into the ER lumen, where they are glycosylated and where molecular chaperones aid protein folding. Misfolded proteins are usually identified here and retrotranslocated by ER-associated degradation to the cytosol, where they are degraded by a proteasome. The vesicles containing the properly-folded proteins then enter the Golgi apparatus.
Golgi apparatus, the glycosylation of the proteins is modified and further posttranslational modifications, including cleavage and functionalization, may occur. The proteins are then moved into secretory vesicles which travel along the cytoskeletonto the edge of the cell. More modification can occur in the secretory vesicles (for example insulinis cleaved from proinsulin in the secretory vesicles).
Eventually, the vesicle fuses with the
cell membraneat a structure called the porosome, in a process called exocytosis, dumping its contents out of the cell's environment.cite journal |author=Anderson LL |title=Discovery of the 'porosome'; the universal secretory machinery in cells |journal=J. Cell. Mol. Med. |volume=10 |issue=1 |pages=126–31 |year=2006 |pmid=16563225 |url=http://www.blackwell-synergy.com/openurl?genre=article&sid=nlm:pubmed&issn=1582-1838&date=2006&volume=10&issue=1&spage=126 |doi=10.1111/j.1582-4934.2006.tb00294.x]
Strict biochemical control is maintained over this sequence by usage of a
pHgradient: 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 lysosomes, which have a pH of 4.8.
There are many proteins like FGF1 (aFGF), FGF2 (bFGF), interleukin1 (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.lll
human cell typeshave the ability to be secretory cells. They have a well developed endoplasmic reticulumand Golgi apparatusto fulfill their function.
Secretion in Gram negative bacteria
Secretion is not unique to eukaryotes alone, it is present in bacteria and archaea as well.
ATP binding cassette(ABC) type transporters are common to all the three domains of life. The Sec systemis also another conserved secretion system which is homologous to the translocon in the eukaryotic endoplasmic reticulum consisting of Sec 61 translocon complex in yeast and Sec Y-E-G complex in bacteria. Secretion via the Sec pathway generally requires the presence of an N-terminal signal peptide on the secreted protein. Gram negative bacteria have two membranes, thus making secretion topologically more complex. So there are at least six specialized secretion system in Gram negative bacteria:
Type I secretion system (T1SS)
It is similar to the ABC transporter, however it has additional proteins that, together with the ABC protein, form a contiguous channel traversing the inner and outer membranes of Gram-negative bacteria. It is a simple system, which consists of only three protein subunits: the ABC protein, membrane fusion protein (MFP), and outer membrane protein (OMP). Type I secretion system transports various molecules, from ions, drugs, to proteins of various sizes (20 - 100 kDa).
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 system for initial transport into the periplasm. Once there, they pass through the outer membrane via a multimeric complex of 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 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)
It is homologous to bacterial flagellar basal body. It is like a molecular syringe through which a bacterium (e.g. certain types of "Salmonella", "Shigella", "Yersinia") 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 Y. pestis, which is used to detect low calcium concentrations and elicits T3SS attachment. The Hrp system in plant pathogens inject harpins through similar mechanisms into plants. This secretion system was first discovered in "Y. pestis" and showed that toxins could be injected directly from the bacterial cytoplasm into the cytoplasm of its host's cells rather than simply into the extracellular medium. [Salyers, A. A. & Whitt, D. D. (2002). "Bacterial Pathogenesis: A Molecular Approach", 2nd ed., Washington, D.C.: ASM Press. ISBN 1-55581-171-X]
Type IV secretion system (T4SS)
It is homologous to conjugation machinery of bacteria (and archaeal flagella). It is capable of transporting both DNA and proteins. It was discovered in "Agrobacterium tumefaciens", which uses this system to introduce the Ti plasmid and proteins into the host which develops the crown gall (tumor). "Helicobactor pylori" uses a type IV secretion system to deliver CagA into gastric epithelial cells. "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 type IV secretion system, known as the icm/dot (intracellular multiplication / defect in organelle trafficking genes) system, to translocate numerous effector proteins into its eukaryotic host.
Type V secretion system (T5SS)
Also called the autotransporter system,cite journal
author = Thanassi, D.G.
coauthors = Stathopoulos, C.; Karkal, A.; Li, H.
year = 2005
title = Protein secretion in the absence of ATP: the autotransporter, two-partner secretion and chaperone/usher pathways of Gram-negative bacteria (Review)
journal = Molecular Membrane Biology
volume = 22
issue = 1
pages = 63–72
doi = 10.1080/09687860500063290] 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 people believe remnants of the autotransporters gave rise to the porins which form similar beta-barrel structures.
Type VI secretion system (T6SS)
Proteins secreted by the type VI system lack N-terminal signal sequences and therefore presumably do not enter the Sec pathway. This system was first characterised in "
Vibrio cholerae" and " Pseudomonas aeruginosa".cite journal |author=Pukatzki S, Ma AT, Sturtevant D, Krastins B, Sarracino D, Nelson WC, Heidelberg JF, Mekalanos JJ |title=Identification of a conserved bacterial protein secretion system in Vibrio cholerae using the Dictyostelium host model system |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=103 |issue=5 |pages=1528–33 |year=2006 |pmid=16432199 |doi=10.1073/pnas.0510322103] cite journal |author=Mougous JD, Cuff ME, Raunser S, Shen A, Zhou M, Gifford CA, Goodman AL, Joachimiak G, Ordoñez CL, Lory S, Walz T, Joachimiak A, Mekalanos JJ |title=A virulence locus of Pseudomonas aeruginosa encodes a protein secretion apparatus |journal=Science |volume=312 |issue=5779 |pages=1526–30 |year=2006 |pmid=16763151 |doi=10.1126/science.1128393] Type VI secretion systems are now known to be widespread in Gram-negative bacteria. cite journal |author=Bingle LEH, Bailey CM, Pallen MJ |title=Type VI secretion: a beginner's guide |journal=Curr. Opin. Microbiol. |volume=11 |issue=1 |pages=3–8 |year=2008 |pmid=18289922 |doi=10.1016/j.mib.2008.01.006]
Bacteria as well as mitochondria and chloroplasts also use many other special transport systems such as the
twin-arginine translocation (Tat) pathwaywhich, in contrast to Sec-depedendent export, transports fully folded proteins across the membrane. The name of the system comes from the requirement for two consecutive arginines in the signal sequence required for targeting to this system.
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 outer membrane vesicles. [ Kuehn, MJ and NC Kesty. "Bacterial outer membrane vesicles and the host-pathogen interaction." "Genes Dev". 19(22):2645-55 (2005)] Portions of the outer membrane pinch off, forming spherical structures made of a lipid bilayer enclosing periplasmic materials. 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. While release of vesicles has been demonstrated as a general response to stress conditions, the process of loading cargo proteins seems to be selective. [ McBroom, AJ and MJ Kuehn "Release of outer membrane vesicles by Gram-negative bacteria is a novel envelope stress response." "Mol. Microbiol." 63(2):545-58 (2007) ]
* Molecular Biology of the Cell 4th edition - Alberts et al
* The Physiology and Biochemistry of Prokaryotes 2nd edition – David White
* Cellsalive.com-David Avon
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sécrétion — [ sekresjɔ̃ ] n. f. • 1711; « séparation » 1495; lat. secretio « séparation, dissolution » 1 ♦ Phénomène physiologique par lequel un tissu produit une substance spécifique, qui peut soit s introduire dans le sang par osmose, soit s écouler à la… … Encyclopédie Universelle
Secretion — Sécrétion Voir « sécrétion » sur le Wiktionnaire … Wikipédia en Français
Secretion — Se*cre tion, n. [L. secretio: cf. F. s[ e]cr[ e]tion.] 1. The act of secreting or concealing; as, the secretion of dutiable goods. [1913 Webster] 2. (Physiol.) The act of secreting; the process by which material is separated from the blood… … The Collaborative International Dictionary of English
secretion — (n.) 1640s, from Fr. sécrétion, from L. secretionem (nom. secretio) separation, noun of action from pp. stem of secernere to separate, set apart (see SECRET (Cf. secret)) … Etymology dictionary
secretion — secretion. См. секреция. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) … Молекулярная биология и генетика. Толковый словарь.
Secretion — (v. lat.), Absonderung. Secretionsgebilde (Secretorĭa orgăna), so v.w. Absonderungsorgane. Secretorisch, absondernd. Secretiv, absonderungsfähig. Secretivität, Absonderungsfähigkeit … Pierer's Universal-Lexikon
Secretion — (vom lat. secernere, absondern, aus scheiden), in der Physiologie die Thätigkeit, wodurch aus dem Blute fortwährend Stoffe zur Ernährung und Erneuerung des Leibes abgesondert werden; vgl. Blut … Herders Conversations-Lexikon
secretion — index concealment Burton s Legal Thesaurus. William C. Burton. 2006 … Law dictionary
secretion — ► NOUN 1) a process by which substances are produced and discharged from a cell, gland, or organ for a particular function in the organism or for excretion. 2) a substance discharged in such a way. ORIGIN Latin, separation , from secernere move… … English terms dictionary
secretion — [si krē′shən] n. [MFr < L secretio, separation < secretus: see SECRET] 1. the act of hiding or concealing something 2. a) a process in which a gland, tissue, etc. produces a biochemical and releases it into the organism for special use by… … English World dictionary