Flashcards in Protein Traficking Deck (19)
What are the 3 main types of vesicles
Clathrin, COPI and COPII
What proteins are associated with clathrin coat? How do they function?
AP2 proteins lie on the other side of the membrane. They bind to the cargo and leads to the exvaginaiton of the membrane to form the clathrin coat
How does the endocytotic vesicle form
There are receptors and the AP proteins lie underneath these receptors. When ligand binds to these receptors, clusters of AP proteins form, clathrin binds to these AP proteins forming a vesicle.
It is the specific arrangement of the calthrin binding to the AP that leads to the formation of the vesicle
What happens once the vesicle if formed
AP2 proteins and the clathrin dissociate and suspends in the cytoplasm, waiting for another signal for endocytotic vesicle formaiton
How does the COPII forms a vesicle coat
COPII have 2 major proteins called the SEC12 and SAR1. SEC12 is a GEF, it causes the binding of SAR1 with GTP (initially it is bound to GDP). When SAR1 binds to GTP it is active now and leads to the formation of a vesicle
What is the structure of COPII
Unlike clathrin coat, COPII is a double membrane stuctured
How does the vesicle fuse with a membrane? What proteins are involved?
RAB-GTP attaches itself to the membrane. This interacts with the RAB effector which is like a long strand. The interaction of RAB GTP with the RAB effector causes docking of the vesicle. Then V SNARE which is sticking out of the vesicle fuses with the V SNARE protein leading to fusion of the vesicle.
Remember in tetanus these snare proteins are cleaved so neurotransmitters cannot be secreted into the post synaptic cleft
Where are clathrin, COPI and COPII found in the cell
What is their assciated directionality
1. Clathrin is involved in endocytosis so it is reponsible for taking the stuff back to trans Golgi, and trans Golgi only. It is also found in the early endosome vesicles
2. COPII is found in the ER and is responsible for taking stuff to the Golgi, so the direction is away from the cell
3. COPI is found in the cisternae of the golgi so it takes stuff back to the cell, towards the nucleus, however it is only found in the cisternae of the golgi and its associated vesicles
What is the RAB cycle
RAB GTP is formed by the GEF which then attaches itself to a newly formed vesicle. When the vesicle reaches its desired destination, there is a RAB Effector protein that hydrolyzes the RAB GTP into GDP. The V snare and the T snare interact to cause fusion of the vesicle. RAB GDP is released into the cytoplasm. This can then be reused for vesicle trasnport when a GEF makes it a RAB GTP again and the cycle continues.
What is the significance of Rab proteins
There are different RAB proteins involved in different transport directionality and locations.
How does the vesicles get directed
RAB proteins interact with the motors that are connected to the cell's cytoskeleton. Specific RAB proteins interact with specific motors that determines how the vesicles are transported in different destinations inside the cell.
The specificity of the RAB determines where it will go. This is determined by the associated proteins (perhaps the motor proteins).
What are the 2 ways proteins can be taken to the plasma membrane
1. Direct pathway
2. Indirect pathway: proteins at the wrong location are endocytosed, and then are enveloped in the right vesicle to be moved to the right membrane location of the cell
What are the proteins involved in transport across moitochondria
TIM and TOM, know which one is in the inner and outer mitochondrial membrane.
Also a protein called PAM is involved that provides the driving force for a substance to enter the mitochondrial matrix
How does Nuclear export and import happens
With the help of RAN GTP/GDP and the import and export receptors. In the import receptor binds with RAN GTP, goes out, gets RAN GDP and releases it, binds with cargo and then comes back in.
In export, the receptor binds with cargo and the RAN GTP and follows this process.
Review of transport to lysosome
We then reviewed the transport to lysozome. Lets review this again.
In the golgi, there is addition of glcnac (remember that process). Once the M6P is attached to the desired protein, in the trans golgi a clathrin coat forms the vesicle and takes it to the early endosome which is being prepared by the V type ATPase. The enzyme get active at low pH. The receptors that held the enzyme in the vesicle dissociate and comes back to golgi. The phophate leaves the enzyme as soon as it is introduced in the early endosome.
How does stuff gets into Peroxisome
PTS1 which is conserved, binds to the C terminal and PTS2 which is not conserved, it binds to the N terminal
Directions of Clathrin, COPI and COPII
Looks like COPI is strickly back from golgi to ER, COPII is strickly from ER to Golgi, Clathrin is from endocytosis back only to Golgi
How is cholestrol transported to the cell
LDL is in the blood, there are LDL receptors on the surface of the cell, receptors with the help of AP1 protein forms clathrin coat, fuses or forms an early edosome, hydrolytic enzmes work on LDLs, making cholestrol that can diffuse through the membrane