Secretion is the process of segregating, elaborating, and releasing chemicals from a cell, or a secreted chemical substance or amount of substance. A chemical compound is a substance consisting of two or more different elements chemically bonded together in a fixed proportion by Mass. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called A chemical substance is a Material with a definite chemical composition. In contrast to excretion, the substance may have a certain function, rather than being a waste product. Excretion is the process of eliminating waste products of Metabolism and other non-useful materials
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In humans
Secretion in humans include e. g. :
Mechanism
In humans, just as in all eukaryotic cells, there is a highly evolved process of secretion. Digestion enzymes are Enzymes that break down Polymeric Macromolecules into their smaller building blocks Gastric acid is one of the main Secretions of the Stomach, together with several Enzymes and Intrinsic factor. lung is the essential Respiration organ in air-breathing Animals including most Tetrapods a few Fish and a few Snails The most primitive Surfactants are wetting agents that lower the Surface tension of a liquid allowing easier spreading and lower the Interfacial tension between two liquids Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called eVolution is the third Album by eLDee, it was due to be released in 2008 Proteins targeted for the outside are synthesized by ribosomes docked to the rough endoplasmic reticulum. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl This article deals with protein targeting in Eukaryotes except where noted Protein synthesis is the creation of proteins using DNA and' RNA'. Ribosomes ( from ribo nucleic acid and "Greek soma ( meaning body") are complexes of RNA and Protein that The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle As they are synthesized, these proteins translocate into the ER lumen, where they are glycosylated and where molecular chaperones aid protein folding. Protein biosynthesis (synthesis is the process in which cells build Proteins The term is sometimes used to refer only to protein translation but more The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle Glycosylation is the enzymatic process that links Saccharides to produce glycans, either free or attached to Proteins and Lipids This enzymatic This article is about the protein For other uses see Chaperone, a disambiguation page Protein folding is the physical process by which a Polypeptide folds into its characteristic and functional three-dimensional structure. Misfolded proteins are usually identified here and retrotranslocated by ER-associated degradation to the cytosol, where they are degraded by a proteasome. The cytosol or intracellular fluid (or cytoplasmic matrix) is the liquid found inside cells. Proteasomes are large Protein complexes inside all Eukaryotes and Archaea, as well as in some Bacteria. The vesicles containing the properly-folded proteins then enter the Golgi apparatus. The Golgi apparatus (also called the
In the Golgi apparatus, the glycosylation of the proteins is modified and further posttranslational modifications, including cleavage and functionalization, may occur. The Golgi apparatus (also called the The proteins are then moved into secretory vesicles which travel along the cytoskeleton to the edge of the cell. A vesicle is a small bubble of liquid within a cell A more formal definition in Cell biology, would be that a vesicle is a relatively small intracellular membrane-enclosed cytoskeleton (also CSK is a cellular " Scaffolding " or " Skeleton " contained within the Cytoplasm. More modification can occur in the secretory vesicles (for example insulin is cleaved from proinsulin in the secretory vesicles). Insulin is a Hormone with intensive effects on both metabolism and several other body systems (eg vascular compliance
Eventually, the vesicle fuses with the cell membrane at a structure called the porosome, in a process called exocytosis, dumping its contents out of the cell's environment. The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Porosomes or fusion pores are cup-shaped structures in the Cell membranes of eukaryotic cells where Vesicles dock in the process of Secretion Exocytosis (ek-soh-sy-TOH-sis Greek: Έξω - external and κύτος - cell is the durable process by which a cell directs secretory vesicles out of the Cell [1]
Strict biochemical control is maintained over this sequence by usage of a pH gradient: the pH of the cytosol is 7. pH is the measure of the acidity or alkalinity of a Solution. 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. Lysosomes are Organelles that contain Digestive enzymes (acid Hydrolases. 8.
Nonclassical secretion
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.
Secretory cells
Many human cell types have the ability to be secretory cells. There are about 210 known distinct human Cell types. Keratinizing epithelial cells Epidermal Keratinocyte (differentiating They have a well developed endoplasmic reticulum and Golgi apparatus to fulfill their function. The endoplasmic reticulum (Greek endo = "within" (prefix plásma = "formed entity" Latin reticulum = "little net" or ER, is an Organelle The Golgi apparatus (also called the
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. ATP-binding cassette transporters ( ABC-transporter) are members of a superfamily that is one of the largest and most ancient families with representatives in all extant The Sec system is 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 ulitized 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. [2]
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 system, to translocate numerous effectors proteins into its eukaryotic host.
Type V secretion system (T5SS)
Also called the autotransporter system,[3] 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. Vibrio cholerae (also Kommabacillus) is a Gram negative curved-rod shaped Bacterium with a polar Flagella that causes Cholera Pseudomonas aeruginosa is a Gram-negative, aerobic, rod-shaped Bacterium with unipolar motility. [4][5] Type VI secretion systems are now known to be widespread in Gram-negative bacteria. [6]
Twin-arginine translocation
Bacteria as well as mitochondria and chloroplasts also use many other special transport systems such as the twin-arginine translocation (Tat) pathway which, in contrast to Sec-depedendent export, transports fully folded proteins across the membrane. The twin-arginine translocation or Tat pathway is a protein export or Secretion pathway found in Plants, Bacteria, and Archaea. 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. [7] 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. [8]
References
- ^ Anderson LL (2006). "Discovery of the 'porosome'; the universal secretory machinery in cells". J. Cell. Mol. Med. 10 (1): 126–31. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 16563225.
- ^ Salyers, A. A. & Whitt, D. D. (2002). Bacterial Pathogenesis: A Molecular Approach, 2nd ed. , Washington, D. C. : ASM Press. ISBN 1-55581-171-X
- ^ Thanassi, D. G. ; Stathopoulos, C. ; Karkal, A. ; Li, H. (2005). "Protein secretion in the absence of ATP: the autotransporter, two-partner secretion and chaperone/usher pathways of Gram-negative bacteria (Review)". Molecular Membrane Biology 22 (1): 63-72. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
- ^ Pukatzki S, Ma AT, Sturtevant D, Krastins B, Sarracino D, Nelson WC, Heidelberg JF, Mekalanos JJ (2006). "Identification of a conserved bacterial protein secretion system in Vibrio cholerae using the Dictyostelium host model system". Proc. Natl. Acad. Sci. U. S. A. 103 (5): 1528-33. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 16432199.
- ^ 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 (2006). "A virulence locus of Pseudomonas aeruginosa encodes a protein secretion apparatus". Science 312 (5779): 1526-30. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 16763151.
- ^ Bingle LEH, Bailey CM, Pallen MJ (2008). "Type VI secretion: a beginner's guide". Curr. Opin. Microbiol. 11 (1): 3-8. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 18289922.
- ^ Kuehn, MJ and NC Kesty. "Bacterial outer membrane vesicles and the host-pathogen interaction. " Genes Dev. 19(22):2645-55 (2005)
- ^ 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)
Bibliography
- Molecular Biology of the Cell 4th edition - Alberts et al
- The Physiology and Biochemistry of Prokaryotes 2nd edition – David White
- Cellsalive. com-David Avon
See also
A secretory protein is any protein whether it be Endocrine or Exocrine, which is secreted by a cellDapyx Software network: MP3 Explorer | Ebook Manager | Zenithic