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Lecture 5: "Vesicular Traffic Of Proteins & The Golgi Apparatus"

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Western University
Biology 2382B
Robert Cumming

Cell Biology Lecture No. 5: Vesicular Traffic Of Proteins & The Golgi Apparatus rd Wednesday January 23 , 2013 The Golgi Complex: -The Golgi complex consists of flattened, disk-like cisternae (stacked tubules) with no ribosomes (somewhat extension of RER, but distinct). There are three types of cisternae in the Golgi complex: Cis- Golgi (CGN), medial-Golgi and trans-Golgi (TGN). This gives us two flanked networks of tubules: the CGN faces the RER and the TGN is opposite the RER (faces the plasma membrane). The importance of the Golgi complex lies in its processing and sorting function of secretory, membrane and lysosomal proteins. The Budding & Fusion Of Transport Vesicles: -Budding is initiated by the recruitment of a small GTP-binding protein to a patch of donor membrane. Complexes of coat proteins in the cytosol then bind to the cytosolic domain of membrane cargo proteins, some of which also act as receptors that bind soluble proteins in the lumen, thereby recruiting luminal cargo proteins into the budding vesicle. After being released and shedding its coat, a vesicle fuses with its target membrane in a process that involves the interaction of cognate SNARE proteins. GTP Binding Proteins & COPII Coat Proteins: -Membrane associated GTP binding proteins (in combination with membrane cargo proteins) promote the association of COPII coat proteins (used for RER to Cis-Golgi transport) on ER membrane. Once the COPII vesicles are released from the donor membrane, the hydrolysis of GTP occurs which triggers the disassembly of COPII coat proteins. The “naked vesicle” now has its v-SNARE (vesicle snare) proteins exposed for the interaction with the t-SNARE (target snare) proteins on the cis-Golgi. RER To Cis-Golgi Transport: -Anterograde (forward) transport is mediated by COPII vesicles, which are formed by the polymerization of soluble COPII coat protein complexes on the ER membrane. V-SNAREs and other cargo proteins in the ER membrane are incorporated into the vesicle by interacting with these coat proteins. Soluble cargo proteins are recruited by binding to appropriate receptors in the membrane of budding vesicles. Dissociation of the coat recycles free coat complexes and exposes v-SNARE proteins on the vesicle surface. After the uncoated vesicle becomes tethered to the cis-Golgi membrane, pairing between the exposed v-SNAREs and cognate t-SNAREs in the Golgi membrane allows vesicle fusion, releasing the contents into the cis-Golgi compartment. -Retrograde (reverse) transport, mediated by vesicles coated with COPI proteins, recycles the membrane bilayer and certain proteins, such as v-SNAREs and wrongly-sorted ER-resident proteins, from the cis- Golgi to the ER. Important to note is that ATP hydrolysis is required for snare proteins to dissociate. Cystic Fibrosis: -Cystic fibrosis is a recessive genetic disease characterized by the abnormal transport of chloride and sodium across epithelium, leading to thick, viscous secretions. CF is caused by a mutation in the gene for the protein cystic fibrosis transmembrane conductance regulator (CFTR), the most common being ΔF508 mutation. Specifically, the disease interferes with the ability of CFTR proteins to interact/bind COPII coat proteins. It is important to understand that this ΔF508 CFTR protein is still functional at the membrane, but sorted incorrectly and is retained in ER and degraded. Cis-Golgi To RER Transport & The KDEL Sorting Signal: - ER luminal proteins (e.g. chaperones like BiP and lectins like calreticulin), can be passively incorporated into COPII vesicles and transported to the Golgi. Many such proteins bear a C-terminal KDEL (Lys-Asp- Glu-Leu) sequence that allows them to be retrieved. The KDEL receptors, locat
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