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Ribosomes are the sites where proteins are synthesised
The mechanism for transporting newly synthesized proteins is highly conserved from bacteria to mammals. A key difference, however, is that bacteria translocate the proteins directly across the plasma membrane to the outside world, whereas eukaryotic cells translocate them into a specialized intracellular organelle, the endoplasmic reticulum. Newly synthesized proteins are conveyed from the endoplasmic reticulum to the cell surface via a series of carrier vesicles. It is therefore useful to consider prokaryotic and eukaryotic protein secretion separately.
Translocation of newly synthesized proteins across the ER membrane shows many similarities to translocation across the plasma membrane protein of bacteria (1, 15, 16). Proteins are prevented from folding in the cytoplasm. They are fed across the plasma membrane through a translocon, a proteinaceous pore, which has three subunits very similar to the bacterial proteins made by the secY, E, and G genes. By electron microscopy, these pores are rings about 8 to 10 nm in diameter, with a central pore of 2 nm, sufficient to allow the passage of an extended, hydrated peptide of 1.1 nm in diameter. These pores can now be recognized (17). In yeast, proteins traverse pores in the ER by two different types of translocation mechanisms. One is an ATP-driven process that translocates proteins whose synthesis is complete. The other couples translation to the translocation process. In this transport mode, the ribosome is attached to the proteinaceous transport pore, the translocon, and feeds the nascent train through the pore as it is being synthesized. Mammalian cells only have the co-translation made of translocation. When translocation is co-translational, the nascent chain is recognized in the cytoplasm by a signal recognition particle, which stops further protein synthesis until the complex of ribosome, nascent chain, and signal recognition particle reaches the endoplasmic reticulum (Fig. 3).
Where are proteins synthesised inside the cell? | Yahoo …
Most of the proteins made by a cell are retained intracellularly; only a specialized subset is secreted outside the cell. Proteins secreted by cells have several major functions, including signaling to other cells, formation of an insoluble extracellular matrix surrounding the cell, and degradation of extracellular material. Molecular biology has helped explain how newly synthesized proteins are selected for secretion and transportation across the hydrophobic barrier of the plasma membrane (see Protein biosynthesis).
The majority of protein is digested, and the amino acids not used for gut fuel are metabolized in the intestinal mucosal cells and transported by the portal vein to the liver for protein synthesis or gluconeogenesis.12 In the liver, nonessential amino acids are largely deaminated, and the amino group (nitrogen) removed is converted into urea for excretion in the urine.13 It has been shown that in subjects without and with mild type 2 diabetes, ~5070% of a 50-g protein meal is accounted for over an 8-hour period by deamination in the liver and intestine and synthesis to urea.14 It has been assumed that the remaining carbon skeletons from the nonessential amino acids are available for glucose synthesis, which would then enter into the general circulation.
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This pathway of protein export or secretion is the one followed in most organisms, including yeast, protozoa, and mammalian cells such as muscle cells and fibroblasts. It is commonly referred to as the constitutive pathway, to distinguish it from a specialized pathway found in specialized cells, such as endocrine and exocrine cells, and in cells of the hematopoietic system, such as neutrophils or cytotoxic T lymphocytes . In such cells, newly synthesized proteins are diverted out of the normal biosynthetic pathway to be stored in secretory granules (14). Fusion of the secretory granule with the plasma membrane is usually triggered by an external stimulus. Because export of this class of secreted proteins is controlled by a stimulus, the triggered release of protein from a storage pathway has been called regulated release.
Unlike bacteria, eukaryotic cells are packed with intracellular membranes. A considerable fraction of those membranes is involved in protein export. Thus protein export in eukaryotes involves translocation across membranes but also transport of the newly synthesized protein through intracellular compartments (9). Protein export in eukaryotes is thus a much more complex process than in bacteria. It becomes necessary to know the intracellular pathway taken by a newly synthesized protein before it reaches the eukaryotic cell surface (see also Protein targeting).
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To take correctly folded and oligomerized proteins from the ER, a vesicle forms in the transitional elements and includes proteins to be exported, but excludes resident proteins of the ER lumen, such as BiP (20). The coat that causes the vesicle to form is now known as COPII. Yeast COPII contains four subunits, sec31p, sec13p, sec23p, and sec24p (see sec mutants). Assembly of a COPII coat requires a small GTPase, Sar1, and a guanine nucleotide exchange factor, Sec12p, in the ER membrane (12) (Fig. 4). The coated vesicle leaving the Golgi carries with it a complement of v-SNARE molecules (see Exocytosis) to allow it to fuse with the cis-Golgi network. In yeast, these are Sec22p, Bos1p, and Bet1p. Resident proteins such as BiP may be excluded from the lumen of the coated vesicle because they are oligomerized into complexes that are too big to enter the small vesicle. To some extent, exported proteins are those that lack a retention signal and so are not retained in the ER. Export of secreted proteins would then be by default, because they lack information to go anywhere else. There is evidence, however, that positive sorting occurs (21) (Fig. 5). In yeast, the secreted protein invertase is recognized by a membrane-bound ER protein (Emp24p) that is required for its transport to the Golgi (22). Furthermore, cargo proteins are concentrated as they leave the ER (23, 24). Since most soluble resident proteins in the ER lumen are not glycosylated, an attractive hypothesis is that exported proteins are recognized by a lectin, which concentrates them in budding vesicles. A protein, ERGIC-53, recycles between the ER and the Golgi and is a lectin with the capacity to bind the mannose residues found on newly synthesized secretory proteins (25). Proteins such as ERGIC-53 might bind secreted proteins and actively carry them to the Golgi complex, in the same way that the mannose phosphate receptor carries newly formed lysosomal enzymes to the prelysosomal compartment.
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The third type of glycosylation results in the formation of proteoglycans. Proteoglycans are formed by the addition of xylose to serine residues as the protein moves from the ER to the Golgi (27), followed by the addition of a long, highly charged, unbranched oligosaccharide called a glycosaminoglycan. Glycosaminoglycans are polymers of a disaccharide unit, one of which is usually uronic acid. The other member of the oligosaccharide pair varies, depending on the type of glycosaminoglycan. Part of the strong negative charge on the proteoglycans is due to the addition of sulfate groups to the glycosaminoglycans. Sulfate can also be added directly to tyrosine residues on secreted proteins. Sulfation of proteins only takes place in the late Golgi. Radioactive sulfate is thus commonly used to label exported proteins selectively and to identify a set of proteins at a late Golgi step of transport. Glycosaminoglycan chain synthesis can be initiated in the late Golgi by growing cells in the presence of a membrane-permeable derivation of xylose (28). This technique has also been used to identify membrane traffic from the late Golgi to the surface of the cell.
The extracellular matrix of normal skin
Examples of protein synthesis by the rough endoplasmic reticulum are the proteins produced in secretory cells. These include the digestive produced in the stomach and the protein hormones like insulin produced in the pancreas. Organ systems which produce many proteins have cells with a large amount of rough endoplasmic reticulum.
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