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Lecture 7Golgi to LysosomesWhat is a lysosome What does it look like Why do they not all look the same celullar organelles found in animal cells that contain acid hydrolase enzymes to break up waste materials and cellular debriso contains proteases nucleases glycosidases lipases phospholipases phosphatases sulfatases diverse in size and shape morphologically diversediversity is due to different types of activity that lysosomes may undertake o also reflects formation from fusion of late endosomes with preexisting lysosomes or newly synthesized acid hydrolases different stages of the cycle what we see as the familiar lysosome is one particular stage in cycle of late endosomelysosome fusion How does a lysosome acquire the material it digestsdigestive enzymes meet their substrates in lysosomal compartners in 3 different waysendocytosis endocytic vesicle are initially delivered to small irregularly shaped intracellular organelles known as early endosomes o early endosome also fuses with vesicles from Golgi loaded with lysosomal hydrolaseso late endosomes acidify and mature into lysosomesphagocytosis professional phagocytes macrophages specialized for engulkment of large particles and microorganismso resulting phagosome is coverted changed to lysosome by fusion with existing lysosome What is autophagy How would you recognize an autophagosome substrate delivery to lysosomes for jobs that are bigger where proteosome degradation system that is too small o second method in which lysosome acquire material it digestsdegradation of old wornout mitochondria and large macromolecules structure of unknown origin called isolation membrane surrounds target o forms double membrane structure called autophagosome o fuses with lysosome to form autolysosome o inner membrane of former autophagosome is broken down and contents are digestedWhy and how does the lysosome maintain a reduced pH association between acid hydrolase cargo and M6P receptor is pH sensitive o binding occurs at pH 6567 o dissociation release occurs at 60 approximate pH of late endosome as endosomal compartment matures pH drops acid hydrolases are released and start functioning o acid phosphatase also removes phosphate from mannose6P thus no longer able to bind the M6P receptorHow is a certain type of protein the acid hydrolases targeted to lysosomal compartments lysosomal proteins are selectively taken up by transport vesicles through o addition of mannose6phosphate M6P exclusively to Nlinked oligosaccharides of soluble lysosomal hydrolases as they pass through cisGolgi lumen o signal patch of AAs on surface of lysosomal hydrolase helps with recognition Can you explain why there is a whole class of genetic disorders that are known as lysosomal storage diseases certain acid hydrolases are mutated and are not recognized by Nacetylglucosamine phosphotransferase o result in accumulation of undigested material in lysosomes there is whole class since lysosomes serve breakdown functions for various different molecules Could you trace the steps of an acid hydrolase from the onset of its translation to its activation in the acidic environment of the lysosomelysosomal proteins are cotranslationally translocated into ER lumen transported through Golgi to TGN budding off as transport vesicles to be targeted to early endosomesduring transit through Golgi apparatuso residue of Nacetylglucosamine1phosphate GlcNAc1P is added to carbon 6 of one or more specific mannose residueso second reaction removes GlcNAc leaving mannose residues phosphorylated in the 6 positions Man6Pprotein o specific Man6P receptor is present in membranes of Golgi apparatus binding of Man6P to this receptor targets proteins to lysosomes 2 receptors have been identified both integral membrane proteinso M6P receptors are present in trans Golgi Network TGNlysosomal acid hydrolases are packed into clathrin coated vesicles o M6P receptors bind lysosomal hydrolases on the lumen side but clathrin coat adaptor proteins on the cytosolic sideas endosomal compartment matures pH drops acid hydrolases are released and start functioning o acid phosphatase also removes phosphate from mannose6P thus no longer able to bind the M6P receptoro removal of phosphate resules in lysosomal hydrolase precursor in early endosomeo empty M6P receptors are recycled back to TGN through retromer coated vesiclesWhat is the difference between phagocytosis and pinocytosis phagocytosis makes big ones 250 nm diameter o left to professional phagocytes such as neutrophils and macrophagespinocytosis makes little ones 100 nm diameter o most cells continually take in fluid and soluteso often through clathrin coated pit or caveolae mechanism What are caveolae How do caveolae specify cargo How are caveolae different from clathrin coated pitvesicle system seen present in plasma membrane of most cell types as deeply invaginated flasks o thought to be formed from plasma membrane microdomains lipid rafts involves protein called caveolin which has a hydrophobic loop inserted into membrane from cytosolic side but does not extend across membrane unlike clathrin COPI or COPII coated vesicles caveolae specify cargo by virtue of lipid composition of the invaginating membrane rather than assembly of vesicle coat and its associated adaptor proteinso unlike clathrin cavolin coat is not shed o like clathrin coated pits caveolae pinch off from plasma membrane using dynamincaveolae deliver contents to endosomelike structure called caveosome or pass through cell and fuse on opposite side transcytosis What is receptor mediated endocytosismacromolecules bind to complementary transmembrane receptor proteins which accumulate in coated pits and are taken into the cell through endocytosis of clathrin coated vesicleso receptor functions to concentrate ligand o ligand is just thing that binds to receptor