Lecture 25 Flashcards
Vesicular transport
- the movement of material between organelles of eukaryotic cell via membrane enclosed vesicles
- transport protein and lipids to various parts of the cell including the endomembrane system and the plasma membrane
- proteins from the ER are initially sent to the Golgi and then may be delivered to other compartments of the cell
what are coated vesicles?
- membrane enclosed vesicles that wear a distinctive layer of proteins on it cytosolic surface
- helps shape the membrane into a bud and captures molecules for onward transport
Clathrin-coated vesicles
- found budding from the Golgi to the plasma membrane and lysosomes via endosomes as well as from the plasma membrane to the endosomes
COP-coated vesicles
- found in vesicles between the ER and Golgi, as well as from one part of the Golgi to another
vesicles have a unique combination of which proteins
- Rab GTPases
- v-SNARES
describe the process of tethering
- Rab proteins are recognized and bound by tethering proteins found on the target membrane bringing the two closer in proximity
describe the process of docking
the v-snares on the vesicles interact with complementary t-snares to firmly dock the vesicles in place
describe the process of fusion
- the vesicle fuses with the target and the cargo protein is delivered to the interior of the aircraft organelle (or secreted if at the plasma membrane)
Docking and fusion
- Docking does not always lead to fusion, sometimes it requires a stimulatory signal
- The fusion membrane is energetically unfavourable
- fusion occurs when the v-snares and t-snares wrap tightly around each other, bringing the vesicle closer to the membrane
where are proteins modified
- in the lumen of the ER
- for example: formation of disulphide bonds
Why aren’t disulphide bonds added in the cytosol
- cytosol is a reducing environment which reduces disulfide bonds
Glycosylation
- covalent attachments of short branched oligosaccharides
glycosylation function
- protects against protein degradation
- hold protein in the ER
- recognition of protein for packaging or cell-cell interaction
glycosylation is rare on the ______
cytosolic side
How are oligosaccharides added
- they are not added one at a time they are added “en bloc” (all together)
A _____ sugar oligosaccharide is originally attached to the lipid _________ and is transferred to the ________________________ as the peptidase is translocated
- 14
- dolichol
- an amino group of an asparagine side chain
Exit from the ER
- some proteins remain in the ER an will contain appropriate signal sequence
- if proteins escape the Golgi, they will be recognized by receptors and sent back to the ER
exocytosis
- vesicles from the Golgi fuse with the plasma membrane
what happens when to many proteins in the ER accumulate
- the unfolded protein response is triggered
- more chaperone proteins and quality control related proteins are produced
- may inhibit protein synthesis
- the size of the ER can be expanded to cope with the load, but if limit is exceeded the cell can die
cisternae
- flattened membrane enclosed sacs
The cis face
is adjacent to the ER
the trans face
points towards the plasma membrane
the medial cisterna
is in the middle
vesicles from the ER enter the Golgi at the _________
cis Golgi network
proteins exit from the ________
trans Golgi network
proteins are further modified in the ____
- Golgi
- oligosaccharide chains are added, removed and modified
constitutive exocytosis pathway
- supplies plasma membrane with lipids and proteins. Some proteins are secreted
- entry to this pathway does not require a specific signal sequence other than the one to enter the ER
- operates continually in all eukaryotic cells
regulated exocytosis pathway
- only operates in specialized cells
- proteins are sorted and packed into the trans Golgi network which has conditions that cause proteins to aggregate (low pH, high Ca2+)
- proteins are stored in vesicles where they accumulate near the PM and wait for a signal to stimulate their fusion with the PM
what does aggregation allow
- it allows secretory proteins to be at very high concentration
What is the difference between exocytosis and endocytosis
- exocytosis delivers phospholipids to the PM, but is balanced by removal from endocytosis
Endocytosis
- the uptake of material from the invaginations of the plasma membrane
endocytosis can be broken down into 2 types based on size
- phagocytosis: ingestion of large molecules, performed by specialized phagocytic cell
- pinocytosis: ingestion of fluid and molecules via small vesicles, performed by all cells
Why is phagocytosis important
- important for the uptake of food and defence against infection
after vesicles are engulfed, they are enclosed in vesicles called _______
phagosomes
Example of pinocytosis transport
Receptor mediated endocytosis
- LDL receptors bind to LDL and are internalized as clathrin-coated vesicles
- they fuse with endoscope and are delivered to the lysosomes
- LDL receptor recycled to PM
