Endocytosis

Question 1

What are the two types of endocytosis? What are the differences?

Answer 1

Pinocytosis and Phagocytosis. Pinocytosis occurs with structures 150nm>x and may occur nonspecifically or in receptor mediated fashion. Phagocytosis occurs in 250nm

Question 2

What is constitutive pinocytosis? How does it occur?

Answer 2

nonspecific pinocytosis occurring on a continuous basis. It occurs when multiple lipid rafts containing high concentrations of calveoli are located close together- this causes a dent in the membrane. Eventually, the protein dynamin (GTPase) closes the invagination, causing the formation of a vesicle.

Question 3

What is dynamin?

Answer 3

a GTPase protein responsible for cleaving membrane invaginations during pinocytosis

Question 4

What is caveolin?

Answer 4

important in the process of constitutive pinocytosis, protein found in lipid rafts that cause endocytosis to occur.

Question 5

Describe receptor mediated pinocytosis.

Answer 5

Adaptin (AP-2) intracellularly binds receptors that may or may not have cargo bound to them extracellularly. This signal clathrin to bind to adaptin. Clathrin has a skelion shape, and when they form together they form a geodesic dome. The invaginated membrane is cleaved by dynamin into a vesicle headed towards an endosome. The AP-2 and clathrin are recycled en route.

Question 6

What is adaptin?

Answer 6

AP-2; binds transmembrane receptors in receptor mediated pinocytosis

Question 7

What is clathrin?

Answer 7

binds adaptin in RMP. Its skelion shape lends itself to the formation of a geodesic dome around the incoming vesicle. Both clathrin and AP-2 are recycled en route to endosome.

Question 8

What are SNARE proteins and why are they important?

Answer 8

SNARE proteins are found within endocytotic vesicles and are important for the docking of the vesicle to endosomes.

Question 9

What are the Rab proteins? Why are they important? Which one speficially?

Answer 9

Rab proteins label vesicles for transport. In RMP, Rab5 is contained within the vesicle and labels it for transit through the endosomal pathway.

Question 10

List the 4 types of ligands that enter the cell through RMP.

Answer 10

1. Hormones
2. Growth factors
3. Lymphokines
4. Nutrients

Question 11

Give some examples of lymphokines.

Answer 11

tumor necrosis factor, interferon, colon stimulating factor, interleukins

Question 12

Give some examples of nutrients that enter the cell through RMP.

Answer 12

Cholesterol (LDL) and iron (transferrin)

Question 13

Why does transferrin make a good transport protein?

Answer 13

It only binds to its receptor at neutral pH when bound to iron. Once it enters the cell, it disassociates from iron at acidic pHs, but does not leave the receptor until neutral pHs. This allows transferrin to be recycled.

Question 14

Describe the endosomal pathway.

Answer 14

Vesicles are guided by Rab and SNARE proteins to dock with early endosomes, which are slightly acidic (pH 6), but do not have acidic hydrolyses. The vesicles are induced to drop their cargo by the acidity of the early endosome. The contents is then moved via multi-vesicular bodies to late endosomes, which are more acidic (5.5) and do contain acidic hydrolyses for degrading macromolecules. Late endosomes fuse with lysosomes (even more acidic

Question 15

What do Rab 4 and 7 do?

Answer 15

Rab 4 signals for recycling back to the membrane. 7 is for early endosomes to become late endosomes.

Question 16

What is the significance of mannose-6-phosphate?

Answer 16

M6P is a tag placed on membranes in the golgi that signals them to go to early and late endosomes.

Question 17

What is the cause for familial hypercholesterolima?

Answer 17

LDL receptor cannot bind AP-2, so it cannot be taken into the cell.

Question 18

Describe phagocytosis.

Answer 18

targets are opsonized (covered with antibodies or complement). The cell moves towards these targets via terminal web growth. The target becomes engulfed into a phagosome, which then fuses with a lysosome.

Question 19

Describe autophagy

Answer 19

One the cell seeks to destroy one of its own organelles, the ER engulfs it in membrane and it is then fused with a lysosome. Autophagy vesicles are notable for their double membranes.

Question 20

What is lipofuscin?

Answer 20

Lipofuscin is the remnants that lysosomes could not destroy. It can accumulate in cells and is often brown on slides.

Question 21

What is endocytosis?

Answer 21

the process of moving larger structures into the cell