Endocytosis

Question 1

Talk about clathrin

Answer 1

• clathrin: 3 large & 3 small polypeptide chains - that form triskelion structure
• clathrin triskelions assemble to basket-like framework, to form clathrin-coated pits on regions of plasma membrane on the cytosolic side.
• that is coated in the protein, clathrin, on the cytosolic face.
• curvature of the triskelions induce curvature of the plasma membrane, which helps to form uniformed sized vesicles.
• clathrin bind to adaptins, which are bound membrane bound cargo receptors
• clathrin regions are continually forming & budding off by endocytosis, to form clathrin coated vesicles.
• intracellular clathrin coated vesicles containing extracellular fluid - containing any substances dissolved in the extracellular fluid.
• clathrin coated vesicles shed coat when they fuse with early endosomes.

Question 2

What is pinocytosis?

Answer 2

• ingest bits of their plasma membrane in the form of pinocytoic (endocytoic) vesicles.
• the vesicles are then returned to the cell surface
• a cells surface area remains constant - same amount of membrane being removed by endocytosis, and added by exocytsosis.

Question 3

What is the mechanism of endocytosis

Answer 3

• formation of clathrin coated pit on the intracellular of the plasma membrane
• the curvature of the clathrin triskelions induces curvature of the plasma membrane (leading to uniform sized buds).
• adaptins bind to both clathrin triskelions & membrane bound cargo receptors
• fusion of the plasma membrane is required for the vesicle to pinch off the bud.
• fusion is helped by GTP-binding protein dynamin - which assembled round the neck of the bud
• dynamin recruits other proteins to the neck of the budding vesicle, and together with dynamin, bend the membrane by distorting bilayer structure or by changing lipid composition (by enzymes recruited to the dynamin complex)

Question 4

What is endocytosis

Answer 4

• an energy-using process by which cells absorb molecules by engulfing them
• FORMATION of vesicles (exocytosis is the dismantling of vesicles, releasing molecules).

Question 5

Why is endocytosis important?

Answer 5

• most substances important to the cell are large polar molecules that cannot pass through the cell membrane, which is hydrophobic.

Question 6

What happens to the clathrin coated vesicle after it has budded off the plasma membrane into the cell.

Answer 6

• vesicle quickly loses clathrin-coat
• Hsp70 (chaperone protein) functions as an ATPase, using energy derived from ATP hydrolysis to peel off the coat
• Auxillian activates the ATPase

Question 7

At least three different coats form around transport vesicles. What two principle functions do these difference coats have in common

Answer 7

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Question 8

Clathrin-cated vesicles bud from eukaryotic plasma membrane fragments when adaptor proteins, clathrin, and dynamin-GTP are added. What would you expect to observe if the following modifications were made to the experiment? Explain your answers

a. adaptor proteins omitted
b. clathrin omitted
c. dynamin omitted

Answer 8

a. adaptor proteins mediate the incorporation of specific cargo proteins into clathrin-coated vesicles by linking the clathrin coat to cargo receptors.

Question 9

Specificity of adaptor proteins

Answer 9

• specificity for the transport pathway & the transported cargo come not from the clathrin coat, but from the adaptor proteins .
• several varieties of adaptor proteins allow different cargo receptors, hence difference cargo proteins to be transported along specific transport pathways.