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bio230 final notes.docx

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University of Toronto St. George
Kenneth Yip

BIO 230 Section 2 Summaries, key points, examples Lecture 1 Membrane Trafficking - Membrane trafficking is important for cells to o Communicate with other cells o Acquire resources - Principles of biosynthetic-secretory and endocytic pathways o Involve polarized trafficking routes vesicles have origins and destinations, go both ways o Presence of sorting stations e.g., TGN to endosome and plasma membrane o Retrieval mechanism and general balance among routes - Secretory pathways o Constitutive secretory pathway Operates in all cells Keeps on secreting: soluble proteins, plasma membrane lipids and proteins o Regulated secretory pathway Operates in specialized cells Requires signal Provides extra plasma membrane when needed Cleavage furrow from endosomes Phagocytosis from endosomes Wound repair from lysosomes Provide extracellular signals Endocytosis of synaptic vesicle components Endocytosis of synaptic vesicle and budding from endosome o Mature secretory vesicle made by retrieving Golgi components and concentrating cargo: e.g., clathrin coat bud from vesicle, brings lipid components back to TGN, does not take contents, membrane of vesicle gets smaller more concentrated - Endocytosis o Endocytosed vesicle joins early endosome, routes to various destinations Recycling moves back to original domain Transcytosis moves to opposite domain (apical or basolateral) Degradation moves to lysosome o Collect resources E.g., LDL uptake LDL receptors endocytosis uncoating fusion with endosome separation of LDL from receptor fuse with lysosome o Down regulate cell surface signalling Unwanted receptors tagged by ubiquitin endocytosis forms multivesicular body degraded by lysosomal protease and lipase o Internalize pathogens by phagocytosis Pseudopods extend out and grab substances - engulf targets - Local membrane changes o Fusion SNAREs (v and t SNAREs, forms helices, excludes water, hemifusion- fusion) o Invagination Clathrin (3 heavy 3 light chains), COPI and COPII, Dynamin drives fussion after invagination bud off) o Budding ESCRT complex (ESCRT-0 binds to PI(3)P and cargo, binds ESCRT-1, transfers cargo, floats away, ESCRT-1 to ESCRT-2 to ESCRT -3, needs PI(3,5)P2, forms huge complex sequestering cargo to vesicle) o Fusion, invagination and budding occur at specific cites to control polarized trafficking and retrieval Specificity regulated by: SNAREs Signaling Lipids Small GTPases Signal sequences Membrane composition Example: inositol sugar head group of inositol phospholipids can be phosphorylated at specific cites Kinases and phosphatases make a variety of PIP species, each PIP binds to specific proteins PIP at distinct sites, specific proteins recruited to these cites Rab Small GTPases localize to distinct sites: o Specific Rab phosphorylates specific PIP o Rab 5: plasma membrane, clathrin-coated vesicles, early endosome o Rab 7A: late endosomes o Rab 11: recycling endosomes Rabs are recruited to specific membranes by RabGEFs Specific Rab works with specific SNARES Lecture 2 Cytoskeletal Networks - Microtubules o Inherently polarized o Each protofilament made of asymmetric heterodimers of alpha and beta tubulin Beta tubulin plus end, can bind GTP or GDP Alpha tubulin minus end, stuck to GTP Tubulin monomers bind and hydrolyze GTP o 13 protofilament 1 microtubule o Gamma tubulin complexes nucleate microtubules at minus ends, plus ends grow away from nucleation sites Gamma tubulin often associates with large microtubule organizing centres like centrosome o Dynamic instability Single microtubules switch between growing and shrinking Growing microtubules have a protective cap of GTP-bound tubulin Once hydrolyzed catastrophe occurs Regaining GTP cap rescues growth o Cargo transported along microtubules by motors Motor activity is polarized: Dynein moves to minus ends, towards centrosome responsible for cargo positioning Kinesin moves to plus ends, away from centrosome E.g., African cichlid fish: o Black: kinesin and dynein compete for colour pigment o White: kinesin inhibited, dynein moves pigments to centre (minus end) - Actin o Inherently polarized o Monomers are asymmetric o Bind and hydrolyze ATP o Monomers assemble and form polarized filaments o After polymerization: ATP ADP ->increases dissociation Net assembly at plus end Net disassembly at minus end Treadmilling ARP complex nucleates actin filament o Polarized treadmilling provide protrusive power, need stationary anchor to move forward Anchored to extracellular matrix by integrins o Chemoattractant receptors orient actin networks Neutrophil chases bacterium Lecture 3 Cell Adhesion Cells are directly connected with epithelial tissues, and disperse within connective tissues - Epithelial o Requires junctional complexes Adherens junction via cadherin clusters Hemophilic interaction between cadherin receptors, calcium needed to keep intrusion regio
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