Endocytosis III

Question 1

What is phagocytosis

Answer 1

Specialized endocytosis

Question 2

Who can do phagocytosis

Answer 2

Specialized cells=macrophages, neutrophils, dendritic cells - immune cells

Question 3

What is cargo for phagocytosis

Answer 3

> 500mnm diameter several microns in size

Question 4

What does phagocytosis depend on

Answer 4

Actin dependent (rho gtpases) and receptor dependent pathway

Question 5

Describe initial steps in phagocytosis

Answer 5

Ab (constant domain) on cargo binds fc receptor on phagocyte —> reorganization of actin cytoskeleton —> pseudopod formation —> pseudopods surround cargo in zipper like fashion = phagosome

Question 6

Describe image phagocytosis

Answer 6

If No abs = no phagocytosis = bc partial wrapping
Neutrophils do phagocytosis toounder oxidative stress
Phagosome exchanges material freely with endosome
Phagosome —> fuses with lysosome = phagolysosme = degraded bacteria, sometimes bacteria can evade this

Question 7

Describe phagosomal maturation

Answer 7

Phagosomes undergo fusion and protein exchanges first with early endosomes then will late endosomes =
Gradual acquisition of first early endosomal proteins then late endosomal proteins

Question 8

When do phagosomes fuse with lysosomes

Answer 8

Once phagosomes have acquired sufficent lamps = begins to fuse with lysosomes = becomes phagolysosome (degradative structure)

Question 9

Describe autophaggy

Answer 9

Cargo acquired from within cell
Catabolic process for reusing components rather than de novo
Takes something in cell and surrounds with membrane and delivers it to endocytic pathway for degradation

Question 10

Describe cargo for autophagy

Answer 10

Non specific vs specific cargo: long lived proteins,protein aggregates and defective organells

Question 11

When autophagy

Answer 11

Get rig of aggregated = ub misfiled proteins —> ub = signal autophagy
Mitochondria autophagy= quality control mechanism = destroys defective mito
If cell stresssed = starvation conditions = autophagy of non essential parts - lipid droplets, er, organells

Question 12

Name all purposes of autophagy

Answer 12

Obtain nutrients under starvation conditions
Turnover of defective organelles
Removal of protein aggregates from cytoplasm
Removal of bacteria/infectious agents
Specialized purposes (e.g. liberation of cholesterol from lipid droplets)
(Also some viruses = can get into cytoplasm and replicate so autophagy fights this)

Question 13

Name the 3 kinds of autophagy

Answer 13

Chaperone mediated autophagy
Micro autophagy
Macro autophagy

Question 14

Describe chaperone mediated autophagy

Answer 14

KFERQ - tagged proteins bind to hsc-70 and are translocated to lysosomes via lamp2a

Single cytoplasmic proteins containing by KFERQ pentapeptide sequence are recognized by and bind to hsc70 (heat shock cognate-70) —> translocated to lysosome via lamp2a

Question 15

Describe micro autophagy

Answer 15

Proteins/hsc-70 adhering to phosphatidylserine on surface of late endosomes are budded into internal vesicles
Internalization of small regions of cytoplasm

Question 16

Describe macro autophagy

Answer 16

Large structures including protein aggregates and organelles are enclosed by a double unit membrane structure to form an auto phagosome

Question 17

Describe macro autophagy - turnover

Answer 17

Turnover of bulk cytoplasm, damaged or unwanted organelles (mitochondriaàmitophagy,
peroxisomesàpexophagy
Formation and growth of double membrane structure called phagophore/isolation
membrane requires ATG proteins

Question 18

Describe macro autophagy - proteins

Answer 18

Atg8/lc3 = phagophore expansion and closure
Tagged with ub - lc3ii crosses bilayers to get to cargo
P62 = cargo autophagy receptor = binds ub at one end and lc3 at other = attaches

Question 19

Describe lc3 congugation = what is it

Answer 19

Lc3= mt associated protein light chain 3

Question 20

Describe lc3 congugation = describe lc3i

Answer 20

Terminal tail of lc3 cleaved and binds to atg3 - pe clipped off
Soluble
Found in cytoplasm - floating around
Binds isolation membrane if autophagy

Question 21

Describe lc3 congugation = describe lc3ii

Answer 21

Lc3 ii = conjugated to pe (phosphatidylethanolamine) by atg12, 5, 16L complex
Membrane bound - atttached to membrane, recogznises autophagy receptors

Question 22

Name and describe all steps of macro autophagy

Answer 22

Formation of isolation membrane - recognizes defective mito —> elongation, atg4 recruited —> sealing (then becomes early endosome like then late endosome like) —> fusion with lysosomes then target degraded

Question 23

Describe formation - macro autophagy

Answer 23

Formation of phagophore membrane - isolation membrane
= mito? Er?

Question 24

Describe cleavage - macro autophagy

Answer 24

Cleavage of lc3
Conjugation of cleaved lc3 to pe and insertion into phagophore membrane to from lc3ii (have proteins that transfer lipids between them - er/membranes)

Question 25

Describe extension - macro autophagy

Answer 25

Phagophore membrane extends and surrounds an organelle or cytoplasm to produce an early autophagosome Target often ub P62 proteins bind ub and lc3

Question 26

Describe fusions - macro autophagy

Answer 26

Early autophagosome fuses/exchanges membranes with endosomes, lc3ii will remain - not degraded Autophagosome then fuses with lysosomes and contents are degraded

Question 27

Describe maturation of autophagosomes

Answer 27

Autophagosomes merge with endocytic pathway in manner similar to phagosomes - evidence they become first early endosome like then late endosome like Then fuse with lysosomes and contents digested - recognition and fusion dependent on rab7

Question 28

Compare endosomes/phagosomes/autophagosomes

Answer 28

Late autophagosome in many ways acts like reg phagosome Fuses with lysosome

Question 29

Where do isolation membranes originate from

Answer 29

Er