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Lecture 9

BIOL 2021 Lecture 9: L9 Chapter 13b

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BIOL 2021
Patricia Lakin- Thomas

Lecture 9: Ch 13, Vesicular Traffic Coated vesicles - Regulation of vesicle formation 1) lipid markers -> phosphoinolitides (most common) -> different proteins bind different PIs 2) coat-recruitment GTPases  Fig 13-14a Formation of COPII coated vesicle o COP2 protein shown exiting the ER o Protein binding to PIs o Active when GTP bound and inactive when GDP bound (molecular switches)  Activated via GEF and inactivated via GAP o Examples  Arf -> helps make COP1 coat + clathrin at Golgi  Sarl -> COP 2 at ER exit site  Acivated via Sar1-GEF membrane -> picks up GTP which causes a conformational change -> binds to membrane as a Sar1-GTP complex  Fig 13-14b Formation of COPII coated vesicle  Fig 13-14d Formation of COPII coated vesicle o Sec24 and Sec23 (coat proteins) are induced by Sar1- GTP to bind to Sar1 and cargo receptors bind to adaptor to make an inner coat shown above  Fig 13-14c Formation of COPII coated vesicle (COP2 coat formed) o More coat proteins that form the outer coat (Sec23/24 and Sec13/31) coats around the inner coat and then budding occurs and the vesicle pinches off. o How to remove the coat/uncoating process o Sar1 hydrolyzes GTP -> GDP + Pi o Sar1 pops out of the membrane and the coat falls off (conformational change b/c of the GTP->GTP which causes the tail to pop out of the membrane) o No GAP needed… spontaneous process o REMEMBER -> Triskelions ONLY for Clathrin o Sar1-GTP complex induces all the coat proteins to come to the membrane, without it coats fall off - Vesicle targeting and fusion  Fig 13-16 tethering of vesicle to target membrane  Rab proteins (GTP binding protein)  RAB = ID marker for organelles and is on both donar and target membranes  They bind to RAB effector proteins on the opposite membrane -> TETHERING o Vesicle is tethered and docked o Link vesicle to target and helps ID  Membrane fusion: done using SNARES -> fuse vesicles to membranes  SNARE proteins o Fig 13-2 budding and fusion -> fusion uses SNARES o Fig 13-16 tethering of vesicle  ABOVE  V-snare is on vesicle and T-snare on target  Both specifically interact together -> snap together -> spontaneous process  many diff snares on different organelles and this adds specificity  v-snare must match t-snare o Fig 13-20 Dissociation of SNARES Question 1: RAB proteins and SNARES identify vesicles and ensure the right vesicles fuses with the right target. Is the identity problem solved? ANSWER: NO Question 2: How do the right RABs and SNARES get into the right membrane in the first place ANSWER: we don't know Golgi Apparatus  Fig 13-3A Roadmap ER to Golgi - Golgi function and structure  Fig 13-26 B: EM of Golgi  Fig 13-26A Golgi structure o Cis Golgi network: vesicles moving from ER to Golgi o Cis face is near the ER o Medial cisternae = middle = the middle flattened sacs o Trans face is near PM o Cisternae = flattened sacs o Trans golgi network (TGN): vesicles moving AWAY from golgi o Functions of golgi:  Carbohydrate synthesis site  Sorting and transfer station for products of ER - Transport between ER and Golgi  Fig 13-22 ER exit sites o At the ER exit site, there are COP2 coated vesicles o Membrane proteins have exit signals on the cytosolic surface which bind to COP2 coat proteins (inner coat) o Soluble proteins that you want to get out of EF have exit signals that bind to membrane receptors, these receptors bind to coat proteins o Resident proteins: proteins that are supposed to stay in
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