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| Question | Answer |
| Overview of endocytosis | - lipid bilayer of the cell membrane is poorly permeable to ions and macromolecules - small and large molecules can enter through protein pumps, carriers or channels - therefore, macromolecules must enter only through membrane bound carriers=endocytosis |
| Types of endocytosis | - phagocytosis - macropinocytosis - clatherin-coated endocytosis - cavaeolae endocytosis - noncoated vesicle endocytosis |
| Phagocytosis | - Function: ingestion/removal of bacteria, single celled organisms use it to feed. EG yeast. - multicellular organisms use it in the immune response, professional phagocytic cells have receptors which can bind to antibodies that tag bacteria. - pathogenic bacteria use phagocytosis as an entry mechanism into non-phagocytic cells |
| Mechanism of phagocytosis | 1. receptors that bind bacteria become activated, activating a cascade, which results in the activation of the actin cytoskeleton. 2. the activated actin cytoskeleton then provides the force to push the membrane forwards, protrudes and 'zippers' around the particle, sealing the phagosome off 3. f-actin then disassembles 4. phagosome is transported into the cell 5. inside the cell the phagosome fuses with the lysosome to produce a phagolysosome 6. contents is degraded. |
| Macropinocytosis | - macropinocytosis is a way of feeding for single-celled eukaryotic organisms - it also removes large numbers of growth factor receptors from the plasma membrane, important in cell division; the prevention of cancer formation. - occurs when growth factors stimulate actin driven protrusion of the plasma membrane called ruffles, so they can be taken up in larger vesicles. - can also be used by pathogenic bacteria |
| Clatherin mediated/non-clatherin mediated endocytosis | - clatherin mediated endocytosis is dynamin dependent. - clatherin mediated endocytosis is receptor mediated, it requires clatherin and adaptor molecules and is done through the caveolae - takes up molecules bound to more than 25 receptors eg: iron uptake on the transferrin receptor and cholesterol uptake on the LDL receptor |
| mechanism of clatherin-mediated endocytosis | - clatherin has 3 light chains and 3 heavy chains, forming a 3-legged structure that self-assembles into a football-like structure. - clatherin then gathers together and pinches off to form a vesicle. 1. ECF binds to receptor, activating it 2. Adaptin is recruited, which then in turn recruits clatherin 3. dynamin is then recruited, which polymerises actin 4. it is then uncoated by auxillin, force produced by the actin cytoskeleton |
| Dynamin Actin | - dynamin is a large spiral protein with a GDPase domain, which is required for making the spirals. - actin has a supporting role in vesicle scission, aiding the transport of the vesicle away from the plasma membrane |
| Calveolin- mediated endocytosis | - caveolae are small 50nm invaginations enriched with cholesterol and caveolin. - calveolin self-associates to form a coat and together with cholesterol in the membrane causes membrane invagination - this internalisation of caveolae requires dynamin and f-actin |
| After the vesicle has been taken up: membrane traffic along the endocytic pathway | 1. vesicle trafficked to early endosome, pH drops to 6.5, mediated by protein pump. 2. drop in pH leads to disassociation of the cargo from some of it's receptors, so the receptor can recycle back via vesicles. 3. vesicles more tightly bound can be trafficked into the late endosome, where the pH drops further - then the receptor is either transported to the trans-golgi membrane to be modified or to the lysosome, where it is degraded. |
| Compartmental specificity: Rab proteins | How Rab proteins regulate compartmental specificity: - Rab-GTP recruits motor proteins that transport carriers on actin filaments or microtubles - Rab GTP can also recruit effectors; including targeting and docking proteins, for example SNARES. - Rabs and effector proteins are primary determinants of compartmental specificity. |
| Example of SNARE proteins as effector proteins | - SNARES are transmembrane proteins, specific for compartments, recruited by Rab. - Conversion of trans-SNARE into cis-SNARE produces ATP to provide energy for the fusion of the vesicle with the target membrane. - SNARES are then recycled through the disassociation of NSF and ATPase |
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