L17 - Endocytosis

Question 1

5 types of endocytosis:

Answer 1

1. Macropinocytosis
2. Phagocytosis
3. Clathrin-coated endocytosis
4. Non Clathrin-coated endocytosis
5. Caveolin mediated endocytosis

Question 2

Phagocytosis:

Answer 2

Ingest/remove bacteria.

Some phagocytic cells have receptors which bind to antibody tagged bacteria.

Question 3

Mechanism of phagocytosis:

Answer 3

Antibody on bacteria activates receptor on phagocytic cell.
Causes signal transaction cascade.
Leads to nucleation (polymerisation) of F-actin cytoskeleton.
Membrane protrudes and forms a “zipper” around bacteria.
Engulfs bacteria and placed in phagosome and F-actin disassembles.
Phagosome binds to lysosome forming a phagolysosome. Content is then degraded.

Question 4

Macropinocytosis:

Answer 4

Remove large amounts of growth factor receptors from plasma membrane.

GF causes actin protrusion of membrane and form ruffles that are taken up as large vesicles.

E.g. Salmonella uses this mechanism to be taken up.

Question 5

Clathrin mediated endocytosis:

Answer 5

Dynamin dependent. Has clathrin and adapter molecules. Use caveolae.

Function: uptake of more than 25 receptors

Constitutive receptor mediated:
- Iron uptake - transferrin receptor
- Cholesterol uptake - LDL receptors
These take up receptor even if there is no ligand.

Ligand induced receptor mediated:
- uptake of EGF-receptor - modulates GF signalling

Question 6

Clathrin mediated endocytosis process:

Answer 6

Adapter molecule binds to clathrin. Adaptin is recruited and forms a spherical structure. Dynamin is recruited and forms a collar around vesicle.
Arp2/3 and N-WASP complex is recruited to cause actin polymerisation. Actin pushes vesicles against membrane and is pinched off.
Dynamin comes off.
HSC70 and Auxillin break down clathrin layer and recycles it.

Question 7

Dynamin:

Answer 7

It is a GTPase and hydrolyses GTP.
Forms a collar around vesicle and is removed when pinched off.
Can oligomerise into spirals and necessary for fission of vesicle.

Question 8

Role of F-actin polymerisation in endocytosis:

Answer 8

Allows vesicle scission.
Causes vesicle to move away from membrane.

Activates N-WASP. This is a nucleation promoting factor and activates Arp2/3 complex. This leads to actin polymerisation.

Question 9

Caveolin mediated endocytosis:

Answer 9

These are small invaginations (50nm).
They self associate to form coat. Cholesterol is used to make invaginations and gives curved structure.
Recruits dynamic and F-actin to cleave vesicle.
Can carry EGF receptor.

Question 10

Membrane traffic along endocytic pathway:

Answer 10

When receptor enter by endocytosis they can go:

• become lysosomes with MVB (late endosome) using Rab7
• become TGN and is exocytosed with Rab9
• can go straight to receptor with Rab4
• can go indirectly to receptor from recycling endosome with Rab11

Question 11

Rab function:

Answer 11

Regulate where vesicles go to.
Can recruit motor proteins which transport carriers.
Can recruit effectors so target and dock components (SNARE’s).

Question 12

SNARE Proteins:

Answer 12

These are transmembrane proteins.
They are recruited by Rab proteins.
V-snare on vesicle and T-snare on compartment fuse and form trans-SNARE complex.
Rab converts trans-SNARE complex to cis-SNARE complex. This provides energy for fusion of vesicle.
NSF then breaks down SNARE complex and recycles SNARE’s.