Lecture 12: Endomembrane System Part I Flashcards
Transport vesicles
Transport vesicles with folded proteins pinch off ER membrane and fuse to target membrane (golgi, lysosome, plasma membrane) to deliver proteins and lipids
Transport vesicles bud off membrane of one organelle (donor) and fuse with membrane of another organelle (target)
Endomembrane System
Consists of nuclear envelope, ER, Golgi apparatus, endosomes, and lysosomes to be collectively called endomembrane system
Allows cells to communicate with outside world and respond to environmental changes
Organelles can also communicate with each other due to similar origin
Why are the organelles in endomembrane system grouped together and able to communicate with one another?
All formed by invagination of plasma membrane
Share ancestry
Why is mitochondria not included in endomembrane system?
Different origin
Did not come from invagination of plasma membrane, but endosymbiosis
Transport vesicles are used to deliver ______ to various organelles and the plasma membrane
specific proteins from ER
Receptors in transport vesicles
Receptors in vesicles selectively bind to specific cargo protein that will be delivered to different organelle
When transport vesicles bud off membrane of donor organelle they are ________
coated vesicles
Coated vesicles
proteins surround transport vesicle on cytosolic surface of membrane
not permanent installment as vesicles shed protein coat before fusion so membranes can directly interact
Protein coats on vesicles 2 functions
- Help package cargo in vesicle to aggregate in small area (do not select cargo)
- Causes membrane to curve and form vesicle
For formation of budding to occur for transport vesicle, what proteins are needed?
- Cargo receptors
- Adaptor proteins
- Coat proteins
Critical steps for Vesicular transport
- cargo receptor binds to cargo in organelle
- adaptor protein connect cargo receptor to coat protein (ex. clathrin)
- protein coat promotes vesicle formation as continue to cover bilayer with coat proteins
- vesicle released from donor organelle; coat and adaptor proteins released from vesicle to be left with naked transport vesicle
Different _____ are used to make transport vesicles go to different target organelles
coat proteins
COPII coated vesicles
move materials from ER to Golgi
involved in forward (anterograde) pathway
COPII
coat protein used to make the transport vesicle at the ER membrane (donor organelle)
Sar1
GTP binding protein that controls formation of COPII coated vesicles on ER membrane
Sar1-GDP: inactive as cytosolic protein
Sar1-GTP: active and will help budding and is on ER membrane
Process of Sar1 GTP binding protein (ER to Golgi)
When Sar1GDP (inactive) binds to Sar1 GEF on membrane
Sar1GEF: promotes exchange of Sar1GDP –> Sar1GTP
Sar1GTP (active) anchored to ER membrane
Transmembrane segment protrudes out and provides anchor to ER membrane
Sar1GP recruit adaptor proteins (Sec23/24) to promote formation of COPII coated vesicle
COPI coated vesicles
move materials from Golgi to ER; trans Golgi to cis Golgi
involved in retrieval (retrograde) pathway
Stimulated by Arf1 (GTP binding protein) on Golgi membrane
Once vesicle sheds COPII proteins and fuses with..
cis face of Golgi to transport selected proteins
Why is retrieval pathway for transport vesicles needed?
Since forward pathway from ER to golgi is imperfect
Proteins from ER can get accidently packaged to golgi and need to be sent back