Endocytosis

Question 1

types of endocytosis

Answer 1

pinocytosis: constitutive cell drinking
phagocytosis: entire cells or very large particles are endocytosed
receptor mediated endocytosis: involves coat proteins

Question 2

what cells can perform phagocytosis?

Answer 2

macrophages, neutrophils, dendritic cells. all are white blood cells. These cells have specialized intracellular machinery

Question 3

describe the process of phagocytosis

Answer 3

the cell is extended (forming pseudopods) due to actin filament rearrangements triggered by phosphoinositides and PI-3 kinase

Question 4

what is pinocytosis? how does it work?

Answer 4

pinocytosis is constitutive cell drinking where small endocytic vesicles form that result in extensive internalization of PM.
macropinocytosis uses plasma membrane ruffles (formed by actin filament rearrangements) to engulf large volumes of extracellular fluid

Question 5

what is receptor mediated endocytosis and how does it work?

Answer 5

it involves coat proteins such as clathrin to help with budding of vesicles

Question 6

what is raft mediated endocytosis?

Answer 6

a type of receptor mediated endocytosis that occurs in lipid rafts (caveolin is present I lipid rafts) and is thought to mediate transcytosis.
Caveolin coat protein in association with Cavins mediate budding of Caveolae

Question 7

what is transcytosis?

Answer 7

endocytosed material is delivered to the plasma membrane on the opposite side of a polarized cell

Question 8

how does cholera toxin invade cells?

Answer 8

cholera toxin binds to ganglioside found in lipid rafts of epithelial cells and is then taken in (by raft mediated endocytosis) to the trans golgi network and to the ER. it triggers the response for disposing of terminally misfiled proteins in the ER and exits ER but avoids degradation. This released protein modifies a G protein and leads to rapid production of cAMP and chloride secretion in intestinal cells. increased chloride means massive movement of water, causing dehydration/diarrhea

Question 9

endocytosed material is sorted where? and what are its possible fates?

Answer 9

sorting occurs in the early endosome
fates:
1. recycled to the PM
2. Degraded within lysosome
3. moved across the cell (transcytosis)

Question 10

why is the structure of the early Endosomes important?

Answer 10

they have long tubules which are enriched in membrane proteins and bilayer due to their large surface are to volume ratio. these tubules are easy to pinch off quickly and are important for recycling

Question 11

what is a recycling Endosome?

Answer 11

recycling endosomes serve as intracellular reservoir of transmembrane proteins. The proteins will relocate to the PM when the correct signal is received

Question 12

describe the process of endocytosed material being digested

Answer 12

endocytosed membrane proteins destined for degradation are carried in multi vesicular bodies (formed from early endosomes) to the Late Endosomes where they fuse with lysosomes to become endolysosomes

Question 13

how is a transmembrane protein tagged for degradation?

Answer 13

a ubiquitin is added to mark the protein for sequestration (formation of an intralumenal vesicle where cytosolic domains can be digested)

Question 14

what is ESCRT?

Answer 14

Endosomal Sorting Complexes Required for Transport are responsible for the sequestration event that forms intralumenal vesicles. They recognize both the ubiquitin and the phosphoinositide on a cargo receptor

Question 15

what is receptor down-regulation?

Answer 15

a widespread, general phenomenon that renders cells temporarily unresponsive to certain extracellular signals. The unresponsive (down-regulated) receptors are sorted to lysosomes to be destroyed. This can reduce effectiveness of drugs are receptors are destroyed

Question 16

How does transcytosis work? mother’s milk antibodies example

Answer 16

antibodies bind to cell surface receptors within acidic gut environment. The receptor antibody complex is endocytosed by clathrin-coated pits. It is transported through early endosomes, recycling endosomes, and finally to the PM on the other side of the polarized cell. antibody dissociates from receptors in the neutral pH of extracellular fluid.