17/01/2018 · Insulin Synthesis and Secretion
Synthesis, Storage, and Secretion of Adrenal Medullary Hormones: Physiology and Pathophysiology.
Insulin Biosynthesis, Secretion, and Action | …
Sodium dodecyl sulfate-acrylamide gel electrophoresis and molecular-sieve chromatography on 8% agarose demonstrate the existence of a very high molecular weight (500,000-600,000), proline-rich protein in cultured 3T6 fibroblasts that appears to be the precursor molecule (procollagen) of collagen. The kinetics of [3H]-proline uptake indicate that this precursor is synthesized at a different rate than are other cell proteins and is secreted apparently unchanged into the medium, where it undergoes modification; it then precipitates around the cells as collagen fibrils that contain the characteristic tropocollagen polypeptide chains. The solubility of this precursor in hot 5% Cl3CCOOH, its hydroxyproline to proline ratio, and its sensitivity to highly-purified bacterial collagenase all indicate that this molecule is of collagenous nature, but that it has considerable regions of noncollagen peptide (about half of the molecule is collagenase sensitive, and it has half of the normal amount of hydroxylated proline residues).
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.
Control of Thyroglobulin Synthesis and Secretion — …
As the nascent chain enters the cytoplasm, it is glycosylated on suitable asparagine residues and forms disulfide bonds. Folding is completed by association with accessory proteins such as BiP, calnexin, peptidyl prolyl cis/trans isomerases, and protein disulfide isomerase (18). Proteins also oligomerize in the lumen of the ER. For example, protocollagens trimerize with an extended coiled-coil configuration, accompanied by cross linking between hydroxyproline and hydroxylysine residues (see Hydroxylation (Lysine, Proline)). If cross linking of collagen protofibrils is inhibited, the improperly folded collagen cannot leave the ER and is quickly degraded. A similar observation is made with the secreted protein al-antitrypsin. In the disease, a-antitrypsin deficiency, a mutation in the protein can prevent its correct folding. These two examples demonstrate that there is a quality-control system that surveys the proteins leaving the ER and prevents the export of improperly folded proteins. Recent data suggest that improperly folded ER proteins are exported from the ER back to the cytoplasm, where they are degraded by proteasomes (19).
Figure 3. Translocation across the ER membrane. (a) Signal recognition particle (SRP) binds to a signal sequence on a nascent protein and halts the protein’s synthesis by the ribosome. (b) SRP brings the protein and ribosome to the translocon on the ER membrane by binding to its receptor. (c) Protein synthesis restarts when the ribosome is correctly positioned and the protein has been threaded into the translocon. (d) Proteins can either be translocated completely into the lumen of the ER to become secretory proteins or translocated partially through the membrane to become integral membrane proteins.
Protein Synthesis and Secretion Flashcards | Quizlet
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.
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).
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A second budding event takes place from the trans region of the Golgi complex. A second coating mechanism causes the formation of a secretory vesicle containing glycosylated proteins for export. The post-Golgi secretory vesicles move to the plasma membrane, either by diffusion or by transport along microtubules. When the secretory vesicles reach the plasma membrane, their membranes and the plasma membrane fuse to form one continuous bilayer, a process known as exocytosis. The fusion step results in the release of the exported protein into the extracellular medium (Fig. 2).
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Figure 5. Sorting of proteins between the ER and GC. (a) Cargo proteins destined for secretion and v-SNAREs needed for carrier vesicle fusion to the GC are sorted away from the ER resident proteins into COPII coated vesicles. After delivery to the GC, the cargo proteins continue through the secretory pathway. (b) However, v-SNARES needed for the ER to GC trafficking step and ER resident proteins that have been mistakenly transported to the GC can be retrieved from the secretory pathway through retrograde transport back to the ER by COPI-coated carrier vesicle. ERGIC-53, a likely carrier protein, cycles between the ER and GC and may assist in sorting the secretory proteins from proteins that are retained in the ER.
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After a carrier vesicle is formed, it must recognize and fuse with its target. Recognition and fusion (see Exocytosis) involve proteins on the vesicle (v-SNARE) and on the target membrane (t-SNARE) (13). When a carrier vesicle leaves the ER, it does not go directly to the plasma membrane but instead fuses with an organelle, the Golgi complex, which is a mandatory way station on the secretory pathway of eukaryotes. The Golgi complex is a stack of membranous cisternae, similar morphologically to a stack of pancakes. The carrier vesicles enter the cis end of a Golgi stack and exit from the trans side. A protein to be exported goes through a series of glycosylation steps in which six- or nine-carbon sugars are added to or removed from oligosaccharide chains attached to serine, threonine (O-glycosylation), or asparagine residues (N-glycosylation). The added sugars can often protect the exported protein from rapid proteolytic degradation after it is secreted into the extracellular world. A unique class of oligosaccharides is added to newly synthesized lysosomal enzymes that allows them to be diverted out of the secretory pathway to primary lysosomes.
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