| [image] | This article may contain original research or unverified claims. Please help Wikipedia by adding references. <font class |
}}
Secretion is the process of segregating, elaborating, and releasing chemicals from a cell, or a secreted Chemical substance or amount of substance.
Eukaryotic Cells have 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.
In the 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 Cytoskeleton to the edge of the cell. More modification can occur in the secretory vesicles (for example Insulin is cleaved from proinsulin in the secretory vesicles).
Eventually, the vesicle fuses with the Cell membrane in a process called Exocytosis, dumping its contents out of the cell's environment.
Strict biochemical control is maintained over this sequence by usage of a 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 evolove into lysosomes, which have a pH of 4.8.
METHODS OF SECRETION FROM CELLS
The first method I would like to discuss is the opposite of endocytosis: exocytosis. The proteins destined for export must first be labelled correctly so that they are sent to the Trans Golgi network. There are three major destinations for the proteins: lysosomes, secretary vesicles and direct export through the default secretory pathway. The labelling takes the form of either a signal sequences at the end of a polypeptide chain or signal patches, which are dispersed throughout the polypeptide chain but when in a 3D conformation are brought together. All cells must export proteins to the exterior of the plasma membrane, but specialised secretory cells must also control what domain of the plasma membrane the proteins are released on; it would be no good having mucus secreted into the extracellular spaces behind the epithelial cells of the gut, for example. Thus, all cells have the constitutive secretory pathway, or default pathway, but specialised secretory cells also have a regulated secretory pathway. This regulation needs the proteins to be labelled, so that the trans-Golgi network can dispatch the proteins to their correct destination. For example, proteins with a mannose – 6- phosphate group bind to receptor proteins in the Trans Golgi network and are then sorted into lysosomes for later use. In the secretory pathway proteins are packaged into secretory vesicles by a process believed to rely on selective aggregation, though what causes said aggregation is unknown. in neurotransmitter protein aggregation either GPi anchored proteins are used, or lipid rafts are used, where the unusually long lipid tails cause a swelling in the lipid bilayer which selectively aggregates neurotransmitter proteins, as they have a longer then usual transmembrane segment. Immediately after being pinched off from the trans Golgi network, immature secretory vesicles are loose and dilated, but as they mature the contents are concentrated as membrane lipids are recycled back into the trans Golgi network and because of ATP driven pumps pumping H+ into the vesicles. The recycling of membrane components is achieved by clathrin coated pits budding off, just as in pinocytosis, a process that is began even before the secretory vesicle has pinched off. The secretory vesicles then aggregate next to the plasma membrane where they await an extracellular signal to fuse with the plasma membrane and release their contents. An example of this sort of signal is raising Ca+ levels triggered by electrical excitation in the form of an action potential in a nerve cell that causes gated Ca+ ion channels to open. This allows for quick responses to stimuli as the vesicles are pre-prepared and in position. This is especially important in nerve cells that may have to move their secretory vessels up to a meter down an axon to the synaptic cleft. Secretion can also be localised on one area of the cell, by responding specifically to chemical triggers binding to receptor proteins in the plasma membrane. This allows killer lymphocytes to degrade apoplysed cells without damaging their neighbours. Proteins can also be secreted without the use of vesicles; in type I - V secretion, which occurs in gram negative bacteria. Type II + IV secretion rely on the Sec system and chaperone proteins, and the mechanism of secretion follows, and is known as GSP, or general secretory pathway: 1. The preprotein binds to SecB, a chaperone protein, and is delivered to the cell membrane, specifically to SecA/SecY/E membrane complex. 2. The SecB/preprotein complex binds to SecA. 3. the preprotein is bound to SecA and SecB is released 4. Sec A binds to ATP 5. the signal sequence on the preprotein leaves SecA and inserts itself as a loop into the lipid bilayer 6. The carboxyl end of the signal sequence flips to the periplasmic side, causing the preprotein to enter the translocase channel created by SecY/E. 7. ATP is hydrolysed, releasing SecA 8. Translocation is now driven solely by electrochemical potential. The preprotein arrives in the periplasmic space 9. The signal sequence is cleaved, releasing the preprotein. In type IV secretion the protein then automatically passes through the outer membrane; it is speculated that it does this by forming a pore in the membrane, and then is released out of the cell by autoproteolytic cleavage. In type II secretion specialist structures are necessary, but their nature is unknown. Type I and type III secretion are Sec independent. In Type I secretion the protein (Which does not have a signal sequence) the protein is passed out of the cell through and ATP binding Cassette transporter (ABC transporter), which is bound to the membrane by membrane fusion proteins. There is also an outer membrane protein associated with this mechanism, which is thought to provide a direct channel between the ABC transporter and the extracellular space, bypassing the outer membrane. Transport involves the hydrolysing of ATP, though the exact mechanism is unknown. Type III secretion is used by pathogenic gram negative bacteria to insert toxins into their target cell. In Type III secretion needle-like structures evolutionarily related to flagella is used to puncture a hole in the target cells plasma membrane. The contacting of the tube tip to the plasma membrane triggers secretion of proteins into the target cell that cause reorganisation of the cytoskeleton, cytokine production, or in macrophages, the triggering of apoptosis. In salmonella the effect on the cytoskeleton produces ruffles and filipodia in the target cell membrane, which leads to an increased uptake of salmonella. Much like flagella these needle complexes have two large rings (40 nm in diameter, 20 nm wide) that interact and anchor to the plasma membrane and two smaller rings (20nm in diameter, 18 nm wide) that interact with the outer membrane and peptidoglyen layer. These rings anchor the needle complex in place. “The needle structure itself is a stiff, straight tube, 80nm long and 13nm wide” (Kubori et al Science vol 280).
Type V secretion needs no auxiliary structures; the proteins themselves are autotransporters. The proteins are modular, having 3 domains: a signal sequence present at the N terminus and targets the protein to the inner membrane. The middle domain, or passenger domain, is the active part of the protein that does whatever it is the specific protein is designed to do. The last domain is on the C terminus and is responsible for translocation. It has a short linking region with an α helix conformation and a b domain which forms into a β barrel structure when imbedded into the outer membrane, allowing the passenger domain through.
Bibliography
• The Molecular Biology of the Cell 4th edition - Alberts et al • The Physiology and Biochemistry of Prokaryotes 2nd edition – David White • /classes/mimg/spring_04/mimgc106/b_4_22_27_SALMONELLA_3.pdf (I could not find the author name) • The Magic is in the Simplicity – Biotechnology news issue 47 – Ian Henderson – www.biotech.bham.ac.uk/BTnews47/Magic.htm • Kubori et al - Supramolecular Structure of the Salmonella typhimurium Type III secretion system – Science Vol 280 • Quiaoling Jin & Sheng-Yang He - Role of the Hrp Pilus in Type III protein secretion is Pseudomonas syringae – Science vol 294