translation initiation elongation termination
Introduction to protein synthesis, including amino acid activation and transfer RNA and specificity (tRNA) connection; peptide synthesis (including initiation, elongation and termination) and the nascent chain processing into a mature protein 3 big steps. The central part of the peptide chain synthesis. To be ribosomal protein synthesis, mRNA, tRNA, aminoacyl transfer RNA (aminoacyl tRNA) synthetase, soluble protein factor of about 200 kinds of biological macromolecules in synergy to complete. Translation process is: the nucleus of a section of DNA out of the mRNA transcription to wear out from the nuclear pore into the cytoplasm, together with the ribosome. Ribosomal protein synthesis in the. The beginning of protein synthesis, the first combination of ribosomes and mRNA, ribosomes attached to one end of the mRNA (starting position), and then along the mRNA from the 5'3 'direction (when the ribosome to move forward soon, the other ribosomal also incorporates up, so a number of ribosomal mRNA can go straight.) At the same time, free in the cytoplasm of the tRNA to carry specific amino acids that the ribosome on the mRNA of the corresponding position, and then tRNA leaving the ribosome, go to the corresponding amino acid transport, so that in accordance with the mRNA's genetic code, one by one by the tRNA to the amino acid transport connection to each other and become a polypeptide chain, in the synthesis of the beginning, always carrying the tRNA Met into the ribosome first, and then with a second amino acid of the tRNA before entering, this time with a methionyl tRNA to remove the acid methionine, the start codon on the mRNA position, and then he left the ribosome, the-COOH terminal methionine and the second-NH2 amino acids to form peptide bond. Then carried into the third ribosomal tRNA amino acids, the second amino acid and with the third-COOH-NH2 amino acids to form peptide bond. The second to leave the ribosome and tRNA, again carrying the corresponding amino acid, fourth amino acid tRNA enters the ribosome. the order of tRNA into the ribosome, mRNA's genetic code by the decision. In this way, repeated incessantly until he came to the stop codon on the mRNA, the peptide chain synthesis until the end. mRNA's genetic code will be translated into a polypeptide chain, when a polypeptide chain synthesis is completed, the ribosome will polypeptide chain release down the polypeptide chain through the twisting, folding to form a certain spatial structure of protein molecules, while ribosomes from mRNA on the off down, then re-combined with the mRNA, protein synthesis participate in the next, one can have multiple ribosomal mRNA in the above move, a ribosome can synthesize a polypeptide chain, therefore, a synthesis of multiple mRNA can polypeptide chain. Vivo activation of amino acid amino acid peptide chain reaction can not be directly, and the first by a specific tRNA synthetase ammonia phthalocyanine catalytic activation of the carboxyl groups of amino acids and their corresponding tRNA 3 'end hydroxyl reaction of ammonia with high-energy acyl ester bond tRNA. Aminoacyl can be connected to tRNA3 'side of adenosine 3'-hydroxyl (Figure 1) or 2'-hydroxyl, and can move quickly between the two to achieve a balance.
Aminoacyl tRNA structural amino acids and tRNA reaction the whole process in two steps (see transfer RNA), the total reaction expressed as follows: amino acids ATP + tRNA aminoacyl-tRNA + AMP + Ppi these reactions are in-tRNA synthase catalyzed carried out. This enzyme is highly specific, each of at least one amino acid-tRNA synthetase. Aminoacyl tRNA synthetase in different in size, subunit structure and amino acid composition of different, mostly in the molecular weight of 85 000 ~ 110 000, which contained some crystalline enzymes have been obtained. Peptide synthesis of peptides synthesized in 3 steps: initiation, elongation, termination. Synthesis of N-terminal direction from the (N side) to the carboxy-terminal (C terminal) were. mRNA translation direction is from the 5 'â†’ 3' end. Start first by either prokaryotic or eukaryotic initiation factor is, the birds, the three phosphate (GTP), ribosomes, mRNA and aminoacyl-tRNA initiation complex formation. Start codon is AUG (or GUG). Prokaryotes, the initiation factor protein synthesis There are 3 - IF-1, IF-2 and IF-3, in initial aminoacyl tRNA (also known as the starting tRNA) is the formyl methionyl aminoacyl-tRNA (fMet -tRN
Initiation of protein synthesis in prokaryotes AfMet), where A is the acyl enzyme catalyzed formyl methionyl-tRNA added to the. Initiation process comprises the following three steps: â‘ 70S ribosome at the initiation factor IF-3 and IF-1 under the action of dissociation, resulting in 30S and 50S subunits. â‘¡ 30S subunit with the mRNA start codon binding site, fMet-tRNAfMet role of the IF-2, and GTP to participate into the 30S subunit, the release of IF-3, the formation of 30S initiation complex. In the complexes, fMet-tRNAfMet with the anti-codon on the mRNA start codon (translation start signal) formed between the complementary base pairs. â‘¢ 30S initiation complex with 50S subunits, IF-2 (with the ribosome depends on the activity of GTP hydrolysis) hydrolysis of GTP, GDP and inorganic phosphate production, and release energy, so that IF-2, IF-1 and GDP and so is released from the complex to form a 70S initiation complex (including 70S ribosomes, mRNA and fMet-tRNAfMet). At this time, fMet-tRNAfMet occupy the ribosome peptidyl-tRNA on the location (P bit). Pat-70S initiation complex with the peptide chain extension conditions (Figure 2). Peptide synthesis in eukaryotic initiation factor more than the original nucleus (such as rabbit reticulocyte cell with at least 9), the starting methionyl aminoacyl tRNA is a tRNA (Met-tRNAMet), different from the prokaryotic fMet-tRNAfMet. Start the same basic steps and prokaryotes, including the dissociation of ribosomes, the small subunit (40S) initiation complex formation and peptide chain initiation complex (80S) formation. The main difference is that eukaryotic small subunit ribosomal start first with tRNA with aminoacylation, and then combined with the mRNA; and prokaryotic ribosomal small subunit in the formation of initiation complex and mRNA during the first combined then combined with the initial tRNA. Extension
Peptide synthesis cycle by the number of peptide chain extension cycles can extend the process. Each loop so that the ribosome moves along the mRNA one codon (3 nucleotides) the distance, and to add a nascent chain amino acids. Apart from some details, the extension of prokaryotes and eukaryotes about the same cycle, but the former has the extension factor EF-Tu, EF-Ts and EF-G, which is the EF-1 and EF-2. Each cycle consists of the following three steps: â‘ aminoacyl tRNA and the ribosome binding. EF-Tu and GTP to form the first complex, with the addition of the complex outside the fMet-tRNAfMet any combination of aminoacyl tRNA, and then by the ribosomal initiation complex in the A position on the mRNA codon with its choice the corresponding anti-codon tRNA into the A bit of ammonia phthalate, anti-codon and codon hydrogen bonds formed by base pairs. â‘¡ peptide bond formation. Occupy the fMet-tRNAfMet ribosomal P-bit, aminoacyl tRNA occupies the ribosomal A place in the ribosome peptidyl transferase catalyzed, fMet-tRNAfMet on the Î±-carboxy formyl methionine tRNA on the amino acids and ammonia phthalate, Î±-amino peptide bond formed between. At this point, P-bit on the starting tRNA (tRNAfMet) do not carry amino acids, while the A bit on the tRNA 3 'end of the peptide with a second, called the peptidyl-tRNA. Evidence indicates that peptidyl transferase is ribosomal large subunit (for a region on the ribosome, composed of many molecules synergy results. Does not require participation of soluble protein factors and GTP), peptide bond formation and eukaryotic the same basic steps in prokaryotes. However, because of the different inhibitors have different sensitivities, and thus two types of biological activity of peptidyl transferase center of the structure may differ. â‘¢ displacement. In the EF-G (also known as displacement activity), and the role of GTP under. Including 3 associated with the movement, namely, the loss of aminoacyl tRNA (or the starting tRNA) to leave the P-bit; peptidyl-tRNA from the P-bit A bit moved; ribosome along the mRNA towards the 3 'end of the moving distance of a codon, mRNA The next codon in the A position on the ribosome
Ribosomes along the mRNA chain synthetic peptide chain diagram. EF-Tu-tRNA into the A bit later, that fell off from the ribosome, on the other elongation factor EF-Ts with the help of the formation of a new GTP (EF-Tu Â· GTP) complex, in the first two extended loops (Figure 3). In the peptide chain elongation process, when the first one ribosome along the mRNA to move to far away from the initiation codon (about 40 nucleotides), the No. 2 and the start codon and ribosomal binding and start another the synthesis of new peptide chains, the same 3, 4 have the same mRNA ribosomal binding, to form polyribosomes. Multi-body protein synthesis is actually a form of the ribosome (Figure 4).
Termination of peptide synthesis with the extension of the termination of the ongoing cycle of peptide chain gradually extended, and finally, mRNA stop codon on the (UAA, UAG and UGA) in the A position on the ribosome, the cell did not identify these passwords Son aminoacyl tRNA, and therefore this peptide synthesis to stop. At this point, release factor RF-1 or RF-2 and RF-3's participation in the GTP binding to identify and stop codons, followed by activation of peptidyl transferase and make specific changes in the catalytic sites on P peptidyl-tRNA ester bond hydrolysis, and finally off the nascent peptide chain and the tRNA aminoacyl released from the ribosome. Releasing factor is also dependent on the ribosome with guanosine triphosphate hydrolysis activity, which hydrolyze GTP, the ribosome releasing factor from the available energy. Free to enter the next round of ribosome ribosome cycle (Figure 5). Processing of nascent chain polypeptide translated from the mRNA chain is often no biological activity, known as protein precursors. Transformation of precursor processed to become a functional protein. Precursor processing methods generally have the following: remove the general functional proteins that are not N-formyl methionine (prokaryotic) or methionine (eukaryotic); removal of unwanted proteins function exists in the former peptides in the body; on the nascent chain through the oxidation of two cysteine thiol generates a number of functional proteins (especially enzymes) required for disulfide bond; and certain protein modification of amino acid residues, such as phosphorus acylation, glycosylation, methylation. Acetylation and hydroxylation and so on. Regulation of protein synthesis rate of protein synthesis in vivo, mainly at the transcriptional level, then adjust in the process of translation control. It is influenced by gender, hormones, cell cycle, growth and development, health and living environment and other factors involved in protein synthesis and the many changes of biochemical substances. Since the translation and transcription in prokaryotes are usually coupled together, and the short life of mRNA, and thus the rate of protein synthesis by the transcription rate of the major decisions. Weakening the role of translation products by the first impact of excess and lack of transcription, and thus a way to adjust the translation speed. The structure and properties of mRNA can also regulate the rate of protein synthesis.
Eukaryotic initiation factors of protein biosynthesis regulation of eukaryotic transcription and translation are not coupled, the rate of protein synthesis is usually slower than in prokaryotes. Mainly through the addition of eukaryotic transcription and mRNA post-transcriptional processing and the structure and properties (such as the cap structure and poly-A tail, etc.) (see messenger RNA) to control, but by globin biosynthesis studies have shown that eukaryotic from the beginning of translation factor eIF-2 is a limiting factor for speed, so the factors that affect the eIF-2 can regulate the speed of translation. With mammalian cell-free reticulocyte in vitro preparation for studies that when the lack of hemoglobin, because unable to form hemoglobin, the protein synthesis is not necessary. Experiments show that the regulation of heme is regulated by a repressor called hemoglobin (HCR) to achieve. HCR has both active and inactive state. Heme by influencing eIF-2 protein regulation. When hemoglobin is present, inhibited protein synthesis, but also promote the cells do not normally synthesize protein synthesis of hemoglobin, such as the promotion of liver cancer cells, HeLa cells and ascites tumor cells by cell-free preparations of protein synthesis. Protein synthesis inhibitor
Many inhibitors of protein synthesis inhibitors of protein synthesis is highly specific, it is important for the study of synthetic mechanisms. Many clinically effective antibiotics is through specific inhibition of protein synthesis in prokaryotes play a role, they inhibit the growth of bacteria without damaging human cells. Use of different types of biological protein synthesis, can find the treatment of bacterial infections diseases. Table lists some of the more important the role of protein synthesis inhibitors and their location and specificity.
Source Medical Network 2004-6-22 20:52:00 37C Section XV: Translation: Protein biosynthesis - Protein biosynthesis in eukaryotes Anonymous pharmaceutical competitive mergers and acquisitions?
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Eukaryotic protein synthesis
Eukaryotic cells, the basic process of protein biosynthesis in prokaryotic cells, similar to protein biosynthesis process. In addition to the difference in ribosomal protein synthesis in the structure, size and composition and structure of the different mRNA, the main difference in eukaryotic cells, the initial step of protein synthesis. This step involves the initiation factor of at least a dozen, so start the process is more complicated.
Eukaryotic protein synthesis initiation factor (Eucaryotic initiation factor, abbreviated as eIF)
After washing ribosomes with 0.5mmol/LKCl, ribosomes can not complete the synthesis of globin chains, indicating that the protein component is cleared for globin synthesis is required. In 1970, Anderson, who first isolated from eukaryotic cells, three kinds of factors for protein biosynthesis, called M1, M3 and M2, the corresponding prokaryotic cells, IF1, IF2 and IF3. Smith and others made the same year, Eukaryotic protein synthesis and prokaryotic cells in the way of essentially similar. Until 1977, Schreier and Staehelin PAR using the above method of preparation from rabbit reticulocytes isolated from 9 elF, but only in eukaryotic cells elF and in the role of protein synthesis have more understanding. Now elF old and new terminology, molecular weight and correspondence with E. ColiIF list is as follows:
elF molecular weight of old and new terminology and
And E.coli IF
â‰¥ 5 Ã— 106
Note: NIH: National Institutes of Health (Natoional Institutes of Health)
Basel: Basel, Switzerland (Basel) Laboratory
The beginning of eukaryotic protein synthesis
Since the eighties, there has been on the eukaryotic cell protein synthesis and the initiation factor has been further in-depth research. elF role or protein synthesis in eukaryotic cells and prokaryotic cells start the process of starting the process of broadly similar, differences are mainly the following points:
(1) initial type-specific tRNA is Met-tRNAMet, not N terminal formylation.
(2) Met-tRNAMet and GTP with eIF2 to form a separable complex, does not depend on the small subunit. IF1 prokaryotic cells in combination with tRNAMet on the 30S small subunit.
(3) tRNA Met with the combination of 40S small subunit and mRNA prior to binding. In contrast, in prokaryotic cells tRNAMet combination with the small subunit 30S and the 30S small subunit mRNA after the combination.
(4) ATP hydrolysis to ADP to provide energy for the mRNA binding is required.
(5) in eukaryotic cells, only elF the variety, and many elF itself is a multi-subunit protein. The most obvious is elF3 (involved in mRNA binding, similar to the role of prokaryotic IF3), contains about 10 subunits, molecular weight of 106KD, its weight is equivalent to 40S subunit weight of 1 / 3.
(6) mRNA5 'the existence of the end cap is needed for the start (except for certain viruses mRNA, some virus mRNA 5' end of a covalent binding of the protein.)
(7) The process of eukaryotic cells in the initiation of protein synthesis in the presence of a way to adjust the key difference is the last. elF, each cycle through a GTP-GDP exchange, participating in the exchange of another protein called elF2B (also known as GEF, the GTP-GDP exchange factor). Speaking from the enzyme catalysis, a process similar to prokaryotic cells in the peptide chain elongation EF-Tu/Ts cycle (see section II). When the Met-tRNAMet elF2 located in the ribosome after it is transformed into an inactive form of elF2-GDP. Only when elF2 form complexes with elF2B (elF2-elF2B) after, elF2 of GDP to be replaced. In elF2-elF2B complexes. elF2 with a high affinity for GTP, thus re-entered active state and then combined with the Met-tRNAMet. In this loop process, elF2 one of the three subunits on the phosphorylation elF2Î± sensitive. ElF2 phosphorylation complexes formed with the elF2B very stable, easy to dissociate, thus reducing the availability of elF2.
Eukaryotic cells the initial step of protein synthesis can be summarized as:
(1) the formation of 43S ribosomal complex; by the 40S small subunit and elF3 and elF4c composition.
(2) before the formation of 43S initiation complex: the complex in the 43S ribosome, the connection elF2-GTP-Met-tRNAMet complex.
(3) before the formation of 43S initiation complex: the cap-binding protein mRNA and 1 (CBP1), elF4A, elF4B and elF4F constitute an mRNA complexes. mRNA and the 43S complex before the initiation complex, the formation of 48S preinitiation complexes.
(4) the formation of 80S initiation complex: the role of the elF5, 48S initiation complex before the release of all elF, and with the 60S large subunits, forming 80S initiation complex, the 40S subunit - mRNA-Met-tRNAMet-60S subunit.
Eukaryotic protein synthesis in the initial stage, it is worth mentioning that the 40S ribosomal subunit is almost always select the first initiation codon AUG, start of mRNA translation, such a choice than the original cells of the initiation codon The selection method is more simple. First of all, AUG start codon is the only, and in prokaryotic cells (E.coli) can be used as the start codon AUG triplet in addition, there GUG, UUG, or AUU can also be used. Secondly, the positioning in the mRNA5 'end of AUG (usually from the 5' end nucleotides 10-100) Total is selected. In its simplest mode is: 40S subunit and the cap of the mRNA5 'end of the contact and along the mRNA "scan" all the way to arrive at the first AUG and then start translating. The process need to consume ATP. This was mainly due to the anti-Met-tRNAMet codon AUG initiation codon and complementary results. Therefore, as mentioned above, eukaryotic protein synthesis is one of the characteristics: tRNAMet first with 40S subunits, and then combined with the mRNA. From this, "scanning" process is actually based on 43S initiation complex before the form of. TRNAMet in eukaryotic cells, the next it's anti-codon 3 'end of the nucleotide is modified by a large group of A (t6A: N-[9-Î²-D-ribofuranosyl) purine -6- carbamoyl] Thr), t6A response through strengthened into a pile of (Stacking interaction), stable anticodon 3 'terminal nucleotide, codon and does not allow the reaction anticodon first base in the codon wobble. TRNAMet prokaryotic cells lack the modified base, so the reaction process can be allowed to have more swing codon of. This structure also helps to explain the presence of 43S initiation complex before the start codon AUG on the choice of a simple way. Therefore, in eukaryotic mRNA, the additional upstream signal is necessary, such as the SD as the original order as the nuclear mRNA signal. However, through in-depth study on the 40S subunit initiation site selection mechanism. Found that there are certain exceptions, that a number of mRNA translation is not starting from the initiation AUG, but from the downstream AUG at the beginning of open reading frames. The results show that the facts in order for the 40S subunit A 1NNAUGG is ideal for the start sequence. If the order of -3 bit A or G and G 1 G-bit words to be replaced, the initiation codon may be read to be missed, or 40S subunit through AUG continue to "scan" down.
Peptide chain extension and termination
Eukaryotic cells with the initiation factors of protein biosynthesis compared to the steps in the extension and termination factors involved will be very simple.
Eukaryotic peptide chain elongation and termination factors involved in the process of the corresponding prokaryotic factor function of cytokines EF1Î± make aa-tRNA and the ribosome EF-TuEF1Î²Î³ the EF1Î± recycling EF-TsEF2 translocation EF-GRF peptide chain release RF1RF2
Elongation factor (EF) Î±, the molecular weight of about 50KD. EF1Î± with GTP and aa-tRNA complex formation, and the supply of ribosomal aa-tRNA. Research on the EF1Î²Î³ cells, unlike the original EF-Ts so clear, but has proved EF1Î²Î³ can catalyze GDP-GTP exchange, contribute to EF1Î± recycling, EF2 molecular weight 100KD, the equivalent of prokaryotic cells EF-G, GTP catalyzed hydrolysis and the aa-tRNA from the A-bit transfer to the P-bit. Synthetic peptide chain termination release factor relates only to a (RF), the molecular weight of approximately 115KD. It can recognize all three stop codons UAA, UAG and UGA. RF in the activation of the peptide chain acyl-transferase, after the release of the nascent peptide chain, the dissociation from the ribosome, the dissociation process involves GTP hydrolysis, it is a synthetic peptide chain termination of energy consumption.
Eukaryotic specific inhibitor of protein synthesis
Eukaryotic protein synthesis inhibitor, the role of the specific link.
Not surprising, a number of prokaryotic cells to inhibit protein synthesis inhibitors also inhibit eukaryotic protein biosynthesis. Such as the aa-tRNA analogue puromycin peptides may be terminated translation. Also flies spore acid (fusidicacid) feature not only prevent the completion of the original EF-G cells released from the ribosome, but also can prevent the release of eukaryotic EF2. Some inhibitors on the translation of eukaryotic cells is a specific process, which was mainly due to eukaryotic translation system device specificity. Such as 7 - methyl guanosine monophosphate (m7Gp) in vitro inhibition of eukaryotic translation initiation, because m7Gp competitive with the cap binding protein with mRNA5 'end cap combination. Similarly, most other eukaryotic cells, a specific inhibitor of protein biosynthesis can be translated with different devices (components) combined.
Most of these small molecule inhibitor, some peptides, such as ricin (ricin). Ricin is a specific nuclease, can be hydrolyzed by the 60S subunit to extend suspension of inactivation step. By the diphtheria bacteria (Coryne bacterium diptheriae) produced by lethal toxin is more clearly studied eukaryotic protein synthesis inhibitors. Diphtheria toxin is a 65KD protein, it is not encoded by the bacterial genome, but by a parasitic body dissolved in diphtheria phage Î² (phageÎ²) encoding. The toxin is secreted by the diphtheria bacteria transfer, and then into the tissue cells. Once inside the eukaryotic, the diphtheria toxin catalyzed ADP-ribose to connect with EF2. ADP-ribose provided by the NAD, it is modified with the EF2 molecule binding histidine residues. In vitro, this combination can easily be reversed by adding nicotinamide. Once EF2 ADP-ribose is of, EF2 is completely inactivated. Since diphtheria toxin is a catalytic effect, so just trace (perhaps as few as one molecule) can effectively inhibit the overall protein synthesis, which led to cell death. EF2 in the modification of histidine residues is also known as diphtheria amide (diphthamide). If that does not exist in the EF2 amide diphtheria, diphtheria toxin does not kill mammalian cells.
Secretion after protein synthesis
Either prokaryotic or eukaryotic nucleus, the cytoplasm of the cell protein synthesis to be located in the cell-specific region, or secreted out of cells. Cells secrete the protein in the nucleus of eukaryotic cells is more difficult than the original, because the cell bodies of large eukaryotic cells, but also a lot of film of the interval.
Some proteins are secreted into the cell after synthesis, these proteins collectively referred to as secreted proteins. It was found that certain secretory proteins in vitro translation of mRNA, its translation of the molecular weight larger than expected and more. Such as insulin from the 51 acid residues, however, insulin mRNA translation products in rabbit reticulocyte cell-free translation system for the 86 amino acid residues, known as proinsulin (proinsulin). In the wheat germ cell-free translation system for the 110 amino acid residues of preproinsulin. Proved, in the former M-proinsulin end there is a hydrophobic amino acid-rich peptides as a signal peptide (signalpeptide), the former has become proinsulin proinsulin. And insulin are transported to the Golgi complex, cut out the mature insulin peptide C, the final extracellular discharge. Later found that almost all secreted proteins (such as eukaryotic cells prealbumin, immunoglobulin light chain, prolactin, etc., prokaryotic cells, lipoproteins, penicillin enzymes) contain a signal peptide of about 15-30 hydrophobic amino acid residues. In recent years, people have proposed a hypothesis to explain the secretion signal peptide of protein secretion. The mechanism is: When is nascent chain across the membrane, the signal sequence, and then folded into two short segments of spiral segments, and curved into a spiral of antiparallel hairpin, the hairpin structure can be inserted into the hydrophobic lipid double the quality. Once secreted proteins in the membrane anchor N-terminal, follow-up synthetic peptide segments of other parts of the well through the membrane. Hydrophobic signal peptide for the nascent chain across the membrane and fasten it to the film played the role of a walking stick. After the completion signal peptide, the woman was a specific signal peptidase hydrolysis. Mammalian signal recognition particle (SRP) can be seen with the folding of protein domains the two RNA association, Alu sequences that 7SLRNA (7SLRNA gene sequence with the vertebrate family of Alu repeat sequences.) S on behalf of special centers. Eukaryotic protein synthesis inhibitor, the role of the specific link. The length of rod-shaped particles of about the equivalent of ribosomal diameter.
In the mid-80s, Walter, who has made a positive evidence that: control of secretion and secretion in the start area, a small molecule RNA-protein complexes play a key role again. Because of the complex with the nascent signal peptide-specific responses, it was named as the signal recognition particle (Signal Recognition Particle, abbreviated as SRP). SRP by the 7SLRNA (about 300 bp) and 6 close together to form different proteins. Molecular weight proteins ranging from 9-72KD. The role of SRP with ribosome binding and protein binding to stop and prevent translation occurs in the peptide chain extended to about 70 amino acids long. This is exactly the length of the signal peptide from the ribosome completely out of the length of time (after the signal peptide with 30-40 amino acid residues buried in the ribosome at this time.) When the SRP) and endoplasmic reticulum proteins dock (docking protein) after exposure, translation block reversed, SRP proteins spread to go start another transfer.
The SRP plays a negative regulatory role of translation has a very important biological significance. High levels of SRP in the cytoplasm, about 106 copies (about the number of mammalian cell nuclear ribosomal 1 / 10). Many secreted proteins in mammalian species are degradation enzymes (such as nucleic acids, protease, etc.), even though they occasionally appear in the cytoplasm of cells can also cause a big disaster. SRP suspension by using the translation of these proteins to ensure that they did not reach the endoplasmic reticulum membrane proteins can not be completed before the translation. Thus, in the signal peptide and the SRP's together, so that these secreted proteins into the membrane of the chamber in time to complete the transport and secretion.
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