CBNS 101 lecture10 - V e s ic u la r T ra n s p o rt...

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Vesicular Transport
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TODAY-MORE TRANSPORT!! GATED TRANSPORT VESICULAR TRANSPORT TRANSMEMBRANE TRANSPORT
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Budding Fusion Donor membrane Target membrane How is their cargo specified? How do vesicles pinch off (bud)? How are vesicles targeted to their destination? How do they fuse with their target membrane? Transport vesicles ferry specific cargo to specific compartments
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Different coats are used in different transport pathways Clathrin: Vesicle transport from the Golgi and the plasma membrane. COPI: Vesicle transport within Golgi cisternae. COPII: Vesicle transport between ER and Golgi. Although given coat functions in more than one place, variations in subunit composition make each coated vesicle distinct.
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clatherin coat adaptin cargo receptor vesicle formation cargo molecules uncoating dynamin How does assembly of a coat drive vesicle formation? (clathrin shown). bud formation Transmembrane receptors bind specific soluble cargo molecules. Adpatins bind receptors and bring clathrin molecules to cytosolic surface of vesicle. As coat assembles, membrane is forced to bend into bud. Dynamin proteins assemble around neck of budding vesicles. Once assembled, dynamins hydrolyze bound GTP and pinch off vesicle. After budding, coat proteins removed so vesicle can fuse with target membrane.
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Drosophila shibire mutant: (translation = numbness) Membranes fuse in process of pinching bud off membrane - requires dynamin (binds GTP), energy (hydrolysis of GTP)
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How is vesicle formation initiated at the right time and place? Role of Coat-recruitment GTPases in vesicle budding: Sar1-GTP in membrane recruits COPII coat proteins. •ARF proteins recruit COPI coat proteins or clathrin. Sar1
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So what’s this about a fatty acid tail on a protein? •Sar1 and ARF are myristylated proteins- covalently linked to a fatty acid tail by an amide linkage. •The ability of the fatty acid group to insert into the lipid bilayer is controlled by GTPase activity.
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A large (>30) family of Rab GTPases are incorporated into vesicles and assist in docking. GEF in donor compartment converts Rab to GTP form, fatty acid tail inserts in membrane. Specific Rabs for specific vesicles. Bind to specific Rab effectors on target membrane. How does a vesicle recognize its correct target compartment?
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Vesicles carry specific marker proteins called vesicle SNAREs ( v-SNAREs ) on their surface (>20). The v-SNAREs bind to complementary target SNAREs ( t-SNAREs ) on the target membrane. Complementary pairs of v-SNAREs and t-SNAREs guide docking and play a role in fusion . v-SNAREs t-SNAREs SNARES are also important in vesicle targeting and docking.
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SNARES may play a direct role in mediating bilayer fusion. SNAREs have transmembrane domain and extended alpha helical domain.
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