Unit 4: ER to Golgi Flashcards
ERES
ER exit sites
- distinct subdomain of the ER
- located next to cis face of Golgi complex
ERES is enriched with molecular machinery responsible for ____
- formation of budding of membrane bound vesicles destined for Golgi
- proper packaging of vesicles with the correct lumenal and membrane cargo proteins destined for Golgi
Do resident ER proteins enter the Golgi destined transport vesicles?
- prevented from entering Golgi-destined transport
What are the three major classes of coat proteins?
- COPII (anterograde) ER to Golgi
- COPI (retrograde) Golgi to ER
- Clathrin (from Golgi or PM to endosomes)
Where do COPII-coated vesicle assemble?
ERES
Transport Vesicle Assembly at the ERES:
Step One
- soluble COPII component Sar1-GDP (Sar1 = Gprotein) recruited from cytosolic surface of the ERES
- Sar1-GDP binds a guanine-exchange factor, which generates Sar1-GTP
What is GEF?
Guanine Exchange factor
- ER integral membrane protein that catalyzes the exhange of GDP for GTP on Sar1
Transport Vesicle Assembly at the ERES:
Step Two
- Sar-1-GTP integrates into the ER outer leaflet at ERES
- Results in membrane curvature of the ERES membrane
Transport Vesicle Assembly at the ERES:
Step Three
- Sar1-GTP recruits other soluble COPII components
- from cytosol to surface of ERES membrane
- Sec23 and Sec24
…
Transport Vesicle Assembly at the ERES:
Step Three
What happens to Sec 23 and Sec 24
- form a dimer to promote further bending ERES membrane
Transport Vesicle Assembly at the ERES:
Step Three
What does Sec24 do?
- Sec24 also binds to cytosolic-facing domains of selected integral membrane including
- membrane cargo proteins
- membrane cargo receptors proteins
- membrane receptor proteins required for trafficking and docking
Transport Vesicle Assembly at the ERES:
Step Four
- additional soluble COPII components recruited to cytosol to surface of growing coated vesicle ‘bud’
Transport Vesicle Assembly at the ERES:
Step Five
What happens after COPII coat assembly?
- vesicle bud pinches off from ERES and vesicle begins to traffic to proper recipient membrane
Transport Vesicle Assembly at the ERES:
Step Five
What happen prior to vesicle fusion with the Golgi?
- COPII coat disassemble
- SarI-GTP is converted back to Sar1- GDP
- and released along with all of the other COPII proteins
- into the cytosol for another round of COPII coat assembly at ERES
What is a Rab protein?
- large family of lipid-anchored membrane proteins
- located on all transport vesicles and recipient membranes
- vesicle targeting specificity and unique rabs to different membranes
What does the association of the Rab protein with a membrane require?
GTP
Vesicle targeting and fusion at CGN
Step 1
Recognition of the incoming vesicle and recipient membranes
Vesicle targeting and fusion at CGN
Step 2
Tethering of the incoming vesicle to recipient membrane
What do GTP-Rabs do?
- complementary membrane bound
- recruit various recipient factors
- act like tethering proteins
- from cytosol to surface of the vesicle and recipient membranes
What are tethering proteins?
- highly elongated (fiber-like, rod-shaped)proteins
- or components of large multi-protein complexes
What do tethering proteins do?
- form a molecular bridge
- mediate vesicle membrane-recipient membrane contact by bringing the two membranes close together
What is step 3 of vesicle targetting and fusion at CGN?
docking of the vesicle at recipient membranes
What mediates the 3rd step of vesicle targetting?
- SNARE proteins help docking
What are SNARE proteins?
- large family of integral membrane-bound proteins located on all transport vesicles AND all recipient membranes
- specificity (unique vesicle targeting)
What is a SNARE motif?
- all SNARE proteins have a SNARE motif
- cytosolic facing domain that extends from membrane surface
- SNARE-SNARE protein binding
2 main classes of SNARE proteins
v snares
t snares
Explain v-SNARES
- vesicle membranes
- incorporated into vesicle membrane at site of budding on donor comparment
- i.e. at ERES vSNARE incorporated into golgi-destined VESICLE MEMBRANE
What are t-SNARES
- target acceptor membranes
- i.e. at the cis Golgi
What is the SNARE complex? What mediates it?
v-snare + t=SNARE
- cytosolic facing SNARE motif mediates
What is step four of vesicle targeting fusion at CGN?
fusion of vesicle and acceptor membranes
What does the SNARE complex do? (4th step of vesicle targeting and fusion)
- pulls opposing vesicle and recipient membranes together = fusion
What does the vesicle-recipient membrane fusion result in?
- Lateral movement of vesicle membrane proteins (cargo and receptors) into recipient membrane
- Release of vesicle soluble ‘cargo’ proteins into the interior lumen of the acceptor organelle
What is the fate of vesicle specific proteins or proteins that escape from the ER?
returned from the CGN to the ER by specific ER retrieval signals via retrograde transport
What is the direction of retrograde transport?
cis-Golgi to ER
What sequence do most resident soluble ER proteins possess?
KDEL sequence
What are examples of ER retrieval signals?
KDEL sequence KKxx sequence (cytosolic facing)
What is the role of the KDEL sequence?
recognized in the CGN by KDEL receptor
What does the KDEL sequence bind to?
- lumenal side - binds KDEL sequence of escaped protein in CGN lumen
- cytosolic side- binds COPI components
- releases once reaches ER
What is the role of COPI?
- mediates the formation of transport vesicles at the CGN
- return soluble ER protein KDEL receptor complex back to ER
What is the role of the KKxx sequence?
- cytosolic facing
- sequence on escaped ER membrane proteins at the CGN
- recognized by COP1
- COP1 coated vesicles return resident ER to ER
What else has a KKxx ER retrieval signal?
KDEL receptor
