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| Question | Answer |
| Cytoplasmic pathway; the 'default' pathway for the release of protein from the ribosome. | - Peptides produced by the ribosome fold up and are released into the cytoplasm. - Some (cytoplasmic) proteins remain in the cytoplasm, while others are transported into perioxisomes and/or the nucleus |
| Endoplasmic reticulum pathway: proteins destined for secretion | - proteins produced at the rough ER destined for secreation are transported via a chain of transport vesicles through the stacks of the golgi body before they are released out of the cell or into the cell in vesicles. |
| Protein postcodes: shortsection of DNA at the end of proteins DNA which direct where they end up (can be manipulated by genetic modification). | Reading of the protein postcode: 1. gene is transcribed into mRNA 2. mRNA is released from the nucleus into the cytoplasm, where it is bound to the ribosome. 3. mRNA then starts to transcribe the protein, resulting in it containing a 'hydrophobic stretch'-(sequence of hydrophobic amino acids) . 4. At the cytoplasm, signal recognition protein (SRP), binds to the hydrophobic stretch at the end terminals of the protein 5. This stops synthesis of polypeptide chain at the ribosome, which diffuses and binds to the SRP at the membrane. 6. SRP is released from the ribosome and is replaced by a translocation channel. 7. The polypeptide chain then anchors into the translocation channel; co-translationally (occurs while the protein is being produced). Then there are 2 possibilities for the proteins: - those destined for secretion: entire polypeptide chain goes into the lumen of the ER wher it's folded and transported via vesicles to its destination - those destined to become transmembrane proteins (with hydrophobic stretches in their sequences). Channel opens to allow protein in, making it tran |
| Transport of protein in vesicles | 1. Protein leaves the endoplasmic reticulum in vesicles 2. enters the cis face of the Golgi body 3. enters a chain of vesicles through the golgi apparatus 4. once they reach the trans face of the golgi body the vesicles are released into other vesicles - their fusion with target proteins are mediated by SNARE proteins. - many proteins are kept in an inactive state as they leave the ER- chaperone proteins. Chaperone proteins also aid proper folding of proteins where appropriate. - protein folding results in the formation of disulphide bonds, forming an oxidative enviroment. |
| Elycosidation of proteins at the golgi | - at the golgi glycotransferases (enzymes) add sugars to proteins eg: blood group antigens |
| Destinations for post- golgi proteins: lysosomes | - Proteins end up in lysosomes through a sugar side chain being added. They are acidic, which acts as an additional safety mechanism. When this goes wrong: - Inclusion cell disease- mucoliposis, caused by mutations in gene that codes for the lysosomal postcode;;; proteins fail to make lysosomes so are instead seceated, resulting in developmental defects |
| Destinations for post-golgi proteins; mitocondria | - for lysosomal proteins, they typically have a signal sequence made up of alternating amino acids, known as a amiphatic helix 1. Precursor protein containing signal sequence disassociates from the chaperone protein that is keeping it in a semi-folded state. 2. Signal sequence attached to the protein binds to precursor protein in the surface of the mitochondria. 3. floats around membrane until it finds another contact site in the inner mitochondrial matrix, forming another complex 4. the protein can now be threaded into the mitochondrial matrix - outer membrane complex= tom complex - inner membrane complex= tim complex 5. protein is then reloaded into the mitochondrial matrix, the signal sequence is cleaved and the protein can now execute it's function |
| Nuclear pores | - proteins that need to enter the nucleus enter via nuclear pore - nuclear pores are large multiprotein complexes that let specific large proteins through |
| Active import of macromolecules | - Active import is used for transport between the nucleus and the cytoplasm. - Proteins destined for the nucleus contain the signal sequence NLS (nuclear locaation sequence) - Receptor protein importin in the cytoplasm recognises the NLS, transporting it into the nucleus by binding to the spokes of the nuclear pore complex, so the cargo can be shuttled into the nucleus. - once inside the nucleus importin is replaced by a small GTPase called RanGTP |
| Active export of macromolecules | - exportin shuttles proteins with an NES in the nucleus out of the pore in the cytoplasm. - this too is driven by the GTPase Ran, giving directionality as hydrolysis at GTP is unidirectional |
| Summary; cellular roadmap of protein traffick |
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