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
What would happen if you block vesicle fusion
Paralysis (mentioned in another flash card)
What theories are there for the vesicle fusion process
The SNARE hypothesis
What is the SNARE hypothesis
Series of molecules discovered to play a role in vesicle fusion (NSF, alpha SNAP, and alpha SNAP receptor or SNARES)
SNARE hypothesis= each type of transport vesicle carries a specific v-SNARE that binds to a cognate t-SNARE on the target membrane
Binding between v and t-SNARES makes a stable four helix bundle
1 helix contributed by the v-SNARE the other 3 are contributed by the oligomeric t-SNARE
What do SNAREs do
Promote fusion by overcoming major energy barrier to fusion
Help ensure specificity of membrane fusion
Different/t SNARE complexes form at different steps of intracellular transport
What proof is there of SNARE specificity
In vitro liposomes with SNARE partners
SNAREs alone can fuse with vesicles
Highly specific interaction of v and t SNARES
What is the role of Rab GTPases
Guide vesicle targetting- lots of different forms each specific for different cargos
Active Rab-GTP binds Rab effectors for movement for tethering vesicles to membranes
Includes motor proteins or tethering proteins or SNAREs-coupling tethering to fusion
How can some neurotoxins work
They work by inhibiting SNARE complex formation
E.g tetanus, botulinum toxins hydrolyse peptide bonds with SNARE
What is the extracellular matrix (ECM)
Cells in tissues are
organised into complex structures surrounded by the ECM
Fundamental role in development
is a structural support network made up of many proteins and macromolecules
What is the basal lamina and its role
-The basal lamina is a specialized layer of the extracellular matrix that lies underneath epithelial cells and surrounds some other cell types
It provides structural support
The basal lamina mediates adhesion between cells and the extracellular matrix through integrins and other receptors on the cell surface. This interaction is essential for cell signaling, differentiation, and migration.
It acts as a barrier that separates epithelial cells from underlying connective tissue
Composed of collagen, laminins (Glycoproteins that interact with cell surface receptors, facilitating cell adhesion and communication), perlecan, nidogen
