Week 27 / Protein Sorting and Secretion 2 Flashcards
(51 cards)
Q: What is the unifying principle of both the secretory and endocytic pathways?
A: The use of membrane-bound vesicles to move “cargo” between cell compartments.
Q: How do vesicles function in cellular trafficking?
A: They bud off from one membrane and fuse with another to transport molecules.
Q: What is a key point about protein orientation in vesicular trafficking?
A: Proteins integrated into a vesicle membrane maintain the same orientation relative to the cytosol.
Q: Why do proteins only need to be translocated across a membrane once?
A: Because their orientation remains unchanged during both exocytosis and endocytosis.
Q: What are the three types of vesicles involved in intracellular transport?
A: COPII, COPI, and Clathrin-coated vesicles.
Q: What is the function of COPII vesicles?
A: They transport proteins from the rough ER to the Golgi.
Q: What is the function of COPI vesicles?
A: They transport proteins in a retrograde direction between the Golgi and ER, as well as within different portions of the Golgi.
Q: What is the function of Clathrin-coated vesicles?
A: They transport proteins from the plasma membrane to endosomes.
Q: What initiates vesicle budding?
{where do they bind?
what do they cause?
}
A: Recruitment of small GTP-binding proteins to the cell membrane, causing invagination.
Q: What role do coat proteins play in vesicle formation?
A: They bind to cytosolic membrane cargo receptor proteins.
Q: How are cargo proteins incorporated into a vesicle?
A: They are recruited into the budding vesicle via cargo receptor proteins.
Q: What happens after the vesicle buds off?
A: The membranes fuse, releasing the vesicle into the cytosol.
Q: What happens to coat proteins after vesicle formation?
A: They are lost and recycled for future use.
Q: How does a vesicle fuse with its target membrane?
A: Through the interaction of SNARE proteins.
Q: What does SNARE stand for?
A: Soluble NSF Attachment Receptor protein.
Q: What are the two types of SNARE proteins?
A: v-SNARE (vesicle SNARE) and t-SNARE (target membrane SNARE).
Q: What is NSF, and what is its function?
A: NSF (N-ethylmaleimide-sensitive factor) is an ATPase enzyme involved in vesicle docking and unloading.
Q: How do SNARE proteins ensure vesicle fusion with the correct membrane?
A: They function in specific pairs, ensuring accurate docking.
Q: What are the key steps in COPII vesicle formation from the ER to the Golgi? [6]
A:
- Sec12 → GTP on Sar1
Sec12 activates Sar1 by helping it swap GDP for GTP. - Sar1 inserts tail into ER
Activated Sar1 sticks into the ER membrane using its hydrophobic tail. - Recruits Sec23/24 (coat proteins)
Sar1 calls over Sec23/24 to start forming the vesicle coat. - Budding → Vesicle forms
The coat helps the vesicle bud off from the ER. - GTP → GDP = Coat comes off
Sar1 hydrolyzes GTP to GDP, and the coat disassembles. - Vesicle goes to Golgi
Sar1, a GTP-binding protein, interacts with Sec12 on the ER membrane, triggering GDP-GTP exchange and membrane anchoring via a hydrophobic tail. Sar1 then recruits coat proteins (Sec23/Sec24), driving vesicle budding. GTP hydrolysis provides energy to remove the coat, allowing the vesicle to fuse with the Golgi.
Q: How does vesicle docking and fusion occur at the plasma membrane?
A: A Rab protein on the vesicle binds to an effector protein on the plasma membrane, ensuring correct targeting. The vesicle’s v-SNARE (VAMP) then interacts with the membrane’s t-SNARE complex (syntaxin and SNAP-25), leading to stable docking and membrane fusion. Finally, NSF, an ATPase, hydrolyzes ATP to dissociate the SNARE complex, completing the process.
Q: How does the KDEL receptor facilitate the retrieval of ER resident proteins from the Golgi?
A: ER luminal proteins are transported to the Golgi via COPII vesicles.
ER resident proteins contain a KDEL sequence (Lys-Asp-Glu-Leu), which binds to the KDEL receptor in the Golgi,
where the receptor has a high affinity due to the low pH. COPI vesicles then transport these proteins back to the ER, where the receptor releases them due to the higher pH.
Q: What is the endocytic pathway, and what are some examples of its function?
A: The endocytic pathway is used to bring proteins and other molecules into the cell across the plasma membrane. Examples include the uptake of cholesterol via LDL particles, iron transport via transferrin proteins, and the removal of receptor proteins from the cell surface.
Q: What is the role of vesicles in endocytosis?
A: Vesicles form at the plasma membrane to internalize extracellular or membrane-bound molecules into the cell.
Q: What are the key protein complexes involved in clathrin-coated vesicle formation?
A: Dynamin (a GTPase) and Clathrin (a fibrous, three-legged protein).
