What is the role of transport vesicles?
They bud off from one cellular compartment (the donor), carry material as cargo, and fuse with another compartment (the target), thereby transporting the cargo.
What is the difference between the secretory and endocytic pathways?
Secretory Pathway: Leads outward from the ER → Golgi → Cell Surface.
Endocytic Pathway: Leads inward from the Plasma Membrane → into the cell.
What are coated vesicles
Most transport vesicles form from specialized, coated regions of membranes.
They bud off as coated vesicles, which have a distinctive cage of proteins. The coating on the vesticule tells it where to go.
4 types of coating
Clathrin-coated
COPI-coated
COPII-coated
Retromer-coated
Each type is used for different transport steps.
Clathrin coating
Makes vesicules from eating.
What is the function of the protein coats on vesicles?
They assemble into a geometrical cage that helps form the vesicle.
They help select specific cargo for packaging.
How are transport vesicles guided to their correct target membrane?
Rab proteins (in their GTP-bound state) on the vesicle surface are recognized by tethering proteins (Rab effectors) on the target membrane. This is the first step in docking. all transport vesicles display surface markers that identify them and target membranes
display complementary receptors .
What proteins mediate the final fusion of the vesicle with the target membrane?
SNARE Proteins. A v-SNARE on the vesicle and a t-SNARE on the target membrane form a tight trans-SNARE complex, which pulls the membranes close together and catalyzes fusion. Water between and needs to be expelled by bring membranes together.
How do proteins leave the ER?
Proteins Leave the ER in COPII-coated Transport Vesicles that form at the ER exit site (ERES).
How is specific cargo loaded into vesicles?
Cargo receptors inside vesicles ensure they are loaded. Need receptors to know where to make the vesicules. Must be transmembrene proteins to signal thing outside the cell.
● Cargo-specific receptors
● Activated receptors recruit adaptor proteins that
recruit the coat proteins.
How can vesicles transport very large proteins?
COPII vesicles transport vesicles can accommodate the large cargoes by
assembling tubes instead of vesicles. They have different pattern and is very important in collagen.
What is the structure of the Golgi apparatus?
It consists of a stack of flattened, membrane-enclosed compartments called cisternae. They are not connected. It has two poles:
cis Face: Receives vesicles from the ER.
trans Face: Dispatches vesicles to other destinations.
What is a key function of the Golgi apparatus?
Glycosylation and phosphorylation of proteins. These sugar coats act as destination tags, directing proteins to their final cellular location. Different modifications happen in different cisternae.
What are the two models for how cargo moves through the Golgi?
Vesicular Transport Model and Cisternal Maturation Model. These two models can happen at the same time.
Vesicular Transport Model:
Stable cisternae; cargo moves between them via transport vesicles. This is hard for large proteins.
Cisternal Maturation Model
Cisternae themselves mature, moving from the cis to the trans position while carrying cargo, and COPI vesicles bring Golgi enzymes backward. The last part of cisterae just becomes many vesicules. The cisternae change as they move.
Evidence of Vesicular Transport Model
● Vesicles bud and fuse.
● COPI vesicles surround the Golgi ( 60–90 nm ).
Evidence for Cisternal Maturation Model
Large cargo like procollagen (300 nm) is found in enlarged Golgi cisternae but is too big to fit inside standard COPI vesicles (60-90 nm).
How has modern technology confirmed Golgi dynamics?
Using fluorescent protein tags (like GFP) fused to Golgi proteins, scientists can watch cisternae mature in real-time in living cells, showing that cis-Golgi markers are replaced by trans-Golgi markers in the same cisterna over time.
● Make fusion proteins , e.g., Golgi protein fused to GFP.
● Track GFP through the Golgi.
● See processes in real time and in different colors.
Is the Golgi always a stacked structure?
No. In some yeasts (e.g., Saccharomyces cerevisiae), the Golgi consists of dispersed, individual cisternae rather than a neat stack.
What is the role of COPI-coated vesicles?
Bring ER proteins back from the Golgi
What is the role of COPI II coated vesicles?
Bring ER proteins back to the Golgi
How are ER resident proteins recognized for retrieval?
They often have a KDEL sequence (Lys-Asp-Glu-Leu) at their C-terminus. The KDEL receptor in the Golgi binds to this sequence and packages them into COPI vesicles for return to the ER.
What is the default pathway for proteins in the Golgi?
A nonselctive Constitutive Secretory Pathway. Proteins without a specific sorting signal are automatically transported directly to the cell surface and secreted.
