SFP: endosomes, lysosomes, and endocytes

Question 1

Describe the differences between micropinocytosis and macropinocytosis.

Answer 1

Micro: uses small endocytic vesicles formed by proteins such as flotillins and caveolins in lipid rafts

Macro: uses large endocytic vesicles formed by extensions of the plasma membrane, which depends on actin filaments

Question 2

Does pinocytosis use clathrin?

Answer 2

no

Question 3

Describe the main steps involved in phagocytosis.

Answer 3

-Material binds at the extracellular surface
-Actin filaments form pseudopodia
-Phagosomes form
-Phagosomes fuse with lysosomes to form phagolysosomes

Question 4

Specify the stages of receptor-mediated endocytosis and describe the roles of clathrin, AP2 and dynamin.

Answer 4

-Ligands bind to receptors, which are connected to adaptor proteins in clathrin coated pits
-Pits bud off to form clathrin coated vesicles using dynamin
-Coat dissociates and vesicles fuse with early endosomes
-Receptors are recycled to the plasma membrane and vesicles with ligands transport to late endosomes
-Late endosomes pinch off lysosomes containing ligands and other material

Question 5

Clathrin: solubility and uses

Answer 5

-Insoluble when forming a vesicle, soluble after dissociating
-It distorts the membrane to produce a vesicle and captures membrane receptors and bound molecules

Question 6

Describe the formation of clathrin coated vesicles for endocytosis

Answer 6

-A ligand on the extracellular side is bound to a receptor that extends from the intracellular side to the extracellular side
-An adaptor protein found on the internal surface of the clathrin coated pit binds to the receptor
-The vesicle buds off using dynamin and contains the receptor and ligand, and the vesicle is inside the cell

Question 7

Specify the trafficking steps involved in receptor-mediated endocytosis of a ligand from the extracellular space to the lumen of a lysosome.

Answer 7

-Coat dissociates
-Vesicle goes to and fuses with the early endosome. Ligands dissociate from receptor due to acidic pH
-Transport vesicle pinches off from the early endosome to recycle the receptors and membrane
-Endocytic vesicle with receptor travels to and fuses with the late endosome
-Lysosome pinches off containing ligand and lysosomal enzymes from trans golgi. This processes into the mature lysosome.
-Mannose-6-p receptors get recycled to the golgi

Question 8

Give the pH of the early endosome, late endosome, and mature lysosome

Answer 8

Early endosome: 6.0
Late endosome: 5.5
Mature lysosome: 5.0

Question 9

Describe the structure and function of multivesicular bodies (MVB).

Answer 9

They are late endosomes that contain receptors that have been tagged as opposed to being recycled. They allow for the sorting and degradation of integral membrane proteins and help down-regulate receptors to desensitize ligands

Question 10

Vesicular trafficking is dependent on which cytoskeletal element? Which proteins are involved?

Answer 10

Microtubules for intracellular transport of vesicles using either dynein or kinesins

Question 11

what are caveolae

Answer 11

Small membrane invaginations that are just modified lipid rafts

Question 12

how are caveolae connected to the membrane

Answer 12

palmitoylation (lipid anchored to internal cysteine)

Question 13

what are three components often found in caveolae?

Answer 13

shingomyelin, cholesterol, caveolins

Question 14

describe the possible functions of caveolae

Answer 14

They can prevent mechanical damage to the plasma membrane (most likely), may help with cell signaling, may help with endocytosis, and might help with transcytosis

Question 15

Describe the membrane and lumen of lysosomes

Answer 15

Lumen: acidic with many acid hydrolases

Membrane: contains a proton pump to keep the lumen at Low pH, is glycosylated to prevent the lysosome from digesting itself, and has membrane transporters that release products of digestion such as AAs, sugars, and lipids.

Question 16

Describe the details of lysosome formation including sorting of enzymes in the Golgi
complex, trafficking to late endosomes and maturation into lysosomes

Answer 16

1.Lysosomal enzymes are tagged at their N linked oligosaccharides with mannose-6-P in the cis golgi.

2.These enzymes bind to mannose-6-P receptors in the trans golgi network, which activated the adaptor protein and clathrin. The enzymes then bud from the trans golgi in their clathrin coated vesicles and head to finish becoming lysosomes.

3.The clathrin coat sheds and the vesicle fuses with the late endosome.

4.The mannose-6-P receptors dissociate and are recycled back to the trans golgi

5.The late endosome matures into a lysosome

Question 17

what are 2 methods besides receptor mediated endocytosis for delivering material to lysosomes

Answer 17

-Phagocytosis: phagosomes fuse with lysosomes

-Autophagy: autophagosomes fuse with lysosomes to form autolysosomes for digesting material

Question 18

I cell disease: enzyme involved, reaction impacted, result

Answer 18

I cell: the defective enzyme is N-acetylglucosamine-1-phosphotransferase. The defect in this enzyme causes a lack of processing of mannose-6-P tag lysosomal enzymes. As a result, the lysosomal enzyme accumulates and causes the forming of inclusion bodies. The lysosomes are large and have so much undegraded product.