Vesicle Trafficking Flashcards
How are vesicles formed
- Coat proteins assembly at membrane forces bilayer to begin to bend
- As they gather at the membrane, coat proteins may also select cargo to be packaged into the vesicle
- More coat protein binding results in the formation of the sphere of membrane
- Once coated, vesicle pinched off. Coat is then detached and cargo-filled vesicle is transported to destination
This process helps to concentrate proteins into a special patch on the membrane for designated transport
What are the 3 types of archetypal vesicle coat
Clathrin, COPI and COPII drive the formation of transport vesicles by polymerising on cellular membranes
COPII covers vesicles emanating from the ER
COPI surrounds vesicles originating from Golgi
Clathrin surrounds those from the plasma membrane
(Coats around vesicles)
What is a vesicle
Small lipid membrane sack that carried cargo between intracellular compartments
What is the ERGIC
The ERGIC, or endoplasmic reticulum-Golgi intermediate compartment, is a key station in the secretory pathway of eukaryotic cells. It functions as a sorting and processing hub between the endoplasmic reticulum (ER) and the Golgi apparatus
Is between Golgi and ER
What does COPI do
Transport from Golgi to ER
What does COPII do
Transport from the ER to the Golgi
What are the core principles of vesicular trafficking
Cargo selection-> specific molecules inside the cell are identified for transport
Cargo receptor binding
Coat proteins ecruitment
Membrane budding
Unloading
Vesicle transport
Fusion with target compartnment
How is cargo transport from the ER to the Golgi via COPII
Cargo protein binds cargo receptor
GTPase Sar1 is activated by the guanine nucleotide exchange factor (GEF) Sec12, which facilitates the exchange of GDP for GTP on Sar1
Once activated, Sar1-GTP embeds into the ER membrane, initiating the recruitment of COPII coat proteins
activated Sar1-GTP recruits the Sec23/24 complex. Sec24 binds directly to cargo proteins or cargo receptors, while Sec23 interacts with Sar1-GTP
The Sec23/24 complex then recruits the Sec13/31 complex, which polymerizes to form the outer layer of the COPII coat. This assembly promotes membrane curvature and vesicle budding
After budding, the GTP bound to Sar1 is hydrolyzed to GDP, leading to the disassembly of the COPII coat. This uncoating process is necessary for the vesicle to fuse with the Golgi apparatus.
How is cargo transported back to the ER from the M. COPI
-
What is the structure of clathrin
- more complex that COPI and COPII
- Arf GTPase also involved to recruit adaptors
- clathrin layered onto coating to crease clathrin-coated vesicles composed of triskelions
- each triskelion composed of three clathrin heavy chains interacting at their C-termini
- each heavy chain has a 25kDa light chain tightly bound
- heavy chains provide the structural backbone of the clathrin lattice and the light chains thought to regulate formation and disassembly of lattice
What are the steps for the formation of a clathrin coated vesicle
Cargo molecules bind to specific cargo receptors
Adaptor protein complexes (such as AP-2 for endocytosis at the plasma membrane or AP-1 for transport between the trans-Golgi network and endosomes) recognize and bind to both the cargo receptors and the cytoplasmic tails of the cargo molecules.
Clathrin proteins, which consist of three heavy chains and three light chains forming a triskelion structure, are recruited to the membrane by the adaptor proteins.
Lattice Formation: Multiple clathrin triskelions polymerize into a lattice structure, creating a coated pit on the membrane. This lattice structure provides the mechanical force needed to bend the membrane into a vesicle.
As more clathrin and adaptor proteins accumulate, the membrane begins to curve, forming a budding vesicle.
Accessory Proteins: Various accessory proteins (such as epsin, amphiphysin, and dynamin) assist in the bending and stabilization of the curved membrane.
The GTPase dynamin forms a collar around the neck of the budding vesicle. Through GTP hydrolysis, dynamin constricts and pinches off the vesicle from the donor membrane.
Scission Completion: The vesicle is released into the cytoplasm, still coated with clathrin and adaptor proteins.
What is Dynamin and what is its role (how is it involved in clathrin-coated vesicle formation)
-The GTPase dynamin forms a collar around the neck of the budding vesicle. Through GTP hydrolysis, dynamin constricts and pinches off the vesicle from the donor membrane
When is Dynamin not functional
Not functional at high temperatures
Therefore it can’t be used to pinch off vesicles
This could lead to paralysis as there’s no vesicle regeneration at synapses
What is the structure of clathrin
36 triskelions make up a hexagonal clathrin cage
Triskelions are three legged heteropolymers (composed of 3 heavy and 3 light chains)
What pathways can clathrin coated vesicles from the PM take
