Chapter 8 by Karp (part2)

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ppt done by Allado

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<ul><li>1.Luigi Karlo M. AlladoBio - 4</li></ul> <p>2. The biosynthetic pathway of a eukaryotic cellconsists of a series of distinct membrane-bound organelles Materials are carried between compartmentsby vesicle that bud from donor membranes andfuse with acceptor membranes 3. These electron micrographs show themembranes of these vesicles to be covered ontheir outer (cytosolic) surface by a distinctprotein coat a protein coat formed from soluble proteinsthat assemble on the cytosolic surface of thedonor membrane at sites where budding takesplace Activation of a small G protein recruited to thesite 4. Act as mechanical device that causes membrane tocurve and form budding vesicle Provide a mechanism in selecting the componentsto be carried by vesicle cargo consisting of secretory, lysosomal, andmembrane proteins to be transported The machinery required to target and dock thevesicle to the correct acceptor membrane 5. move materials from the ER forward to theERGIC and Golgi complex. Contains a number of proteins that were firstidentified in mutant yeast cells that were unableto carry out transport from the ER to the Golgicomplex select and concentrate certain components fortransport in vesicles 6. enzymes that act at later stages in thebiosynthetic pathway, such as theglycosyltransferases of the Golgi complex membrane proteins involved in the docking andfusion of the vesicle with the target compartment Membrane proteins that are able to bind solublecargo Mutations in one of these cargo receptors hasbeen linked to an inherited bleeding disorder 7. a small G protein called Sar1, which is recruitedspecifically to the ER membrane Sar1 plays a regulatory role 8. Sar1-GDP molecules have been recruited to the ERmembrane by a protein called a GEF (guanine-exchange factor) that catalyzes the exchange of thebound GDP with a bound GTP each Sar1-GTP molecule has extended a finger-likehelix along the membrane within the cytosolicleaflet. This event induces the curvature of the lipidbilayer at that site a dimer composed of two COPII polypeptides(Sec23and Sec24) has been recruited by the bound Sar1-GTP. 9. Transmembrane cargo accumulates within theforming COPII vesicle as their cytosolic tails bind tothe Sec24 polypeptide of the COPII coat the remaining COPII polypeptides (Sec13 andSec31) have joined the complex to form an outerstructural scaffold of the coat. 10. Two copies of the Sar1-Sec23-Sec24 complex(shown in red, magenta, and blue, respectively)that would form the inner layer of the COPII 11. move materials in a retrograde direction fromthe ERGIC and Golgi stack backward toward the ER and from transGolgi cisternae backward to cis Golgi cisternae first identified in experiments in which cellswere treated with molecules similar instructure to GTP Unlike GTP, cannot be hydrolyzed 12. accumulate in the presence of anonhydrolyzable GTP analogue the coat contains a small GTP binding protein,called ARF1, whose bound GTP must behydrolyzed before the coat can disassemble 13. proteins are maintained in an organelle by a combination of two mechanisms: Retention - may be based primarily on thephysical properties of the protein ex. Solubleproteins that are part of large complexes ormembrane proteins with short transmembranedomains are not likely to enter a transport vesicle. Retrieval of escaped molecules back to thecompartment in which they normally reside 14. trans Golgi network Major sorting stationSorting and transport of lysosomal enzymes Lysosomal proteins are synthesized onmembrane-bound ribosomes of the ER andcarried to the Golgi complex along with othertypes of proteins 15. soluble lysosomal enzymes are specificallyrecognized by enzymes that catalyze the two-stepaddition of a phosphate group to certain mannosesugars of the N-linked carbohydrate chains Lysosomal enzymes are transported from the TGNin clathrin-coated vesicles Lysosomal enzymes are escorted from the TGN bya family of adaptor proteins called GGAs The outer ends of the GGA adaptors bind toclathrin molecules GGA adaptors bind to a sorting signal in thecytosolic tails of the mannose 6-phosphatereceptors 16. MPRs in the TGN membrane and lysosomalenzymes within the TGN lumen becomeconcentrated into clathrin-coated vesicles production of clathrin-coated vesicles beginswith the recruitment to the membrane of asmall GTP-binding protein 17. Lysosomal proteins are not the only materialsthat are exported from the TGN membrane proteins destined for the plasmamembrane and secretory materials destined forexport from the cell are also transported fromthe TGN, but the mechanisms are poorlyunderstood 18. requires specific interactions between differentmembranes Selective fusion is one of the factors thatensures a directed flow through themembranous compartments of the cell 19. 1. Movement of the vesicle toward the specific target compartment2. Tethering vesicles to the target compartment3. Docking vesicles to the target compartment4. Fusion between vesicle and target membranes 20. fusion of a secretory vesicle or secretorygranule with the plasma membrane andsubsequent discharge of its contents continual basis in most cells, as proteins andother materials are delivered to both theplasma membrane and extracellular space membrane fusion produces an openingthrough which the contents of the vesicle orgranule are released into the extracellular space 21. an animal cells digestive organelles contains at least 50 different hydrolytic enzymesproduced in the rough ER and targeted tothese organelles The enzymes of a lysosome share an importantproperty: allhave their optimal activity at anacid pH and thus are acid hydrolases. Optimum pH is 4.6 breakdown of materials brought into the cellfrom the extracellular environment 22. Lysosomes also play a key role in organelleturnover, that is, the regulated destruction ofthe cells own organelles and their replacement Process is called autophagy Once the digestive process in theautophagolysosome has been completed, theorganelle is termed a residual body 23. 90% in a plant cell Solutes and macromolecules are temporarily stored May also store host of toxic compounds tonoplast,contains a number of active transportsystems that pump ions into the vacuolarcompartment to a concentration much higher thanthat in the cytoplasm or the extracellular fluid Osmosis sites of intracellular digestion have some of the same acid hydrolases found inlysosomes 24. Endocytosis - is primarily a process by whichthe cell internalizes cell-surface receptors andbound extracellular ligands bulk-phase endocytosis - is the nonspecificuptake of extracellular fluidsremoves portions of the plasma membrane andmay function primarily in the recycling ofmembrane between the cellsurface and interiorcompartments 25. in contrast, brings about the uptake of specificextracellular macromolecules (ligands)following their binding to receptors on theexternal surface of the plasma membrane provides a means for the selective and efficientuptake of macromolecules that may be presentat relatively low concentrations in theextracellula fluid 26. Cell eating In most animals, phagocytosis is a protectivemechanism rather than a mode of feeding Not all bacteria ingested by phagocytic cells aredestroyed. some species hijack the phagocytic machinery to promote their own survival in the body 27. Have 2 subcompartments in which importedproteins take place: boundary membrane andthe internal matrix Proteins destined for a peroxisome possess aperoxisomal targeting signal, either a PTS for aperoxisomal matrix protein or an mPTS for aperoxisomal membrane protein PTS receptors bind to peroxisome-destinedproteins in the cytosol and shuttle them to thesurface of the peroxisome 28. The PTS receptor apparently accompanies theperoxisomal protein through the boundarymembrane into the matrix and then recyclesback to the cytosol to escort another protein Unlike mitochondria and chloroplasts, whoseimported proteins must assume an unfoldedstate, peroxisomes are somehow able to importperoxisomal matrix proteins in their native,folded conformation, even those that consist ofseveral subunits. The mechanism by whichperoxisomes are able to accomplish thisdaunting assignment remains a matter ofspeculation. 29. have four subcompartments into whichproteins can be delivered an outer mitochondrial membrane (OMM), inner mitochondrial membrane (IMM),intermembrane space, and matrix 30. Chloroplasts have six subcompartments intowhich proteins can be delivered: an inner andouter envelope membrane and intervening intermembrane space, as well asthe stroma, thylakoid membrane, andthylakoid lumen Chloroplast and mitochondrial importmechanisms exhibit many similarities,although their translocation machineries haveevolved independently 31. 1. the vast majority of chloroplast proteins are imported from the cytosol, 2. the outer and inner envelope membranes contain distinct translocation complexes (Toc and Tic complexes, respectively) that work together during import, 3. chaperones aid in the unfolding of the polypeptides in the cytosol and folding of the proteins in the chloroplast, and 4. most proteins destined for the chloroplast are synthesized with a removable N-terminal sequence (termed the transit peptide). </p>