HomeWork - Endomembrane System & Vesicle Transport

Question 1

Describe the structure of the membrane vesicle

Answer 1

• The membrane vesicle is a small, spherical structure enclosed by a lipid bilayer, containing the same material as the cell membrane and functions in the transport of materials within a cell.
• A phospholipid bilayer, similar to the plasma membrane, forms the outer boundary of the vesicle.
• The hydrophilic (water-loving) phosphate heads of the phospholipids face outwards and interact with the aqueous environment.
• The hydrophobic (water-hating) fatty acid tails of the phospholipids are oriented towards the interior of the vesicle.
• The bilayer is fluid, allowing movement and flexibility of the membrane.
• Specific proteins can be embedded within the membrane to facilitate transport or other cellular processes depending on the function of the vesicle.
• Vesicles vary in size depending on their function (small transport vesicles versus larger storage vacuoles).
• The fusion capability of the vesicle allows it to fuse with other membranes within the cell like the plasma membrane to release their contents.
• Cells have different types of vesicles such as secretory vesicles, transport vesicles and lysosomes which all have their specific roles.

Question 2

List the three types of vesicles and the transport directions of these vesicles

Answer 2

• The dynamic transition form of cellular materials within eukaryotic cells, carrier of target transition of cellular materials within the cell are described as transport vesicles.
  1. Clathrin-coated vesicle transports molecules from Golgi complex or cell
     membrane to lysosome.
  2. COPII-coated vesicle transports molecules from ER to Golgi complex.
  3. COPI-coated vesicle transports molecules from Golgi complex to ER.

Question 3

What is the mechanism of secreted proteins transport out of the cell

Answer 3

Secreted proteins are transported out of a cell through a process called the secretory pathway.

1. Signal Recognition Particle (SRP)
   A SRP on the newly synthesized protein targets it to the ER membrane to complete translation.
   - the rod-like complex of the SRP binds to the ribosome having the mRNA and growing polypeptide chain with ER signal sequence attached
   - the SRP binds to its receptor in the ER membrane and is then displaced and recycled
2. Translocation into ER lumen
   The protein is translocated across the ER membrane into the lumen where it folds and may undergo initial modifications
   - A translocation channel in the ER membrane facilitates the translocation of the protein after cleavage of the SRP
3. Vesicle Transport to Golgi
   Once the protein is processed in the ER, it is packaged into vesicles that bud off and travel to the Golgi apparatus
4. Golgi Modification
   Proteins undergo more modifications within the Golgi stacks and are sorted for transport to their final destination
   - Two models of transport explain how proteins move through the Golgi complex: the cisternal maturation model which states that Golgi cisterna mature as they migrate outward through the stack and the vesicle transport model
5. Trans-Golgi network sorting
   The vesicle transport model supports the fact that at the trans-Golgi network, proteins are packaged into secretory vesicles based on their target location.
6. Degradation in lysosome
   There are four pathways for degradation in lysosome: endosome maturation, macropinocytosis, phagocytosis and exocytosis
   - In exocytosis, the secretory vesicles travel to the plasma membrane and fuse with it, releasing the protein into the extracellular space.

Question 4

Please list the proteins involved in the vesicle assembly. And describe the functions of these proteins

Answer 4

1. Adaptin
   Binds with clathrin and target molecules respectively. Peripheral membrane protein that helps to form intracellular transport vesicles and mediates the interaction between transmembrane proteins and the clathrin skeletons of coated vesicles.
2. Dynamin
   A small GTPase involved in the scission of newly formed vesicles from the membrane of one cellular compartment. Dynamin is required for vesicle formation during synaptic vesicle endocytosis (SVE) and clathrin-mediated endocytosis.
3. Hsp70
   Heat shock protein 70 acts as a molecular chaperone that regulates protein synthesis and is involved in the process of vesicle assembly. The use of Hsp70 requires calcium ions and ATP.