Endocytosis

Question 1

“Pinocytosis”

Answer 1

Ingestion of small particles (<150nm)

Question 2

Constitutive pinocytosis

Answer 2

Caveolin on lipid raft forms caveoli (conformational)
Dynamin (GTPase) separates from membrane
Depends on caveolin concentrations/cell specialization

Question 3

Receptor mediated pinocytosis/endocytosis

Answer 3

Membrane receptor
Recruits adaptin (AP2), recruits clathrin -> conformational sphere
Dynamin (GTPase) separates
AP2 and clathrin recycled to cytosol
Rab5 targets and SNARE fuses vesicle to endosome

Question 4

Clathrin

Answer 4

Triskelion molecule, forms spherical pit when associated

Question 5

Pinocytosis vesicle specificity

Answer 5

Rab5 and SNARE present in raft with receptor
Rab5 - specificity of destination
SNARE - bilayer fusion
AP2 and clathrin non-specific, recycled after separation

Question 6

Examples of receptor-mediated endocytosis

Answer 6

Protein hormones (insuliln, LH, FSH, GH, prolactin, glucagon)
Growth factors
Lymphokines
Nutrients (LDL, transferrin)

• Basically any protein or larger molecule that needs to enter cell!

Question 7

Endosomal system

Answer 7

Tethering proteins/receptors specific to Rab
SNAREs (t) mate with vesicle SNARE (v)
Slight acidity prompts release of cargo from receptor
Receptors recycled to membrane in vesicles

Question 8

Endosome vs lysosome

Answer 8

Slightly acidic
No hydrolase activity in early endosome
Endosome does not become lysosome

Question 9

Endosome/lysosome pH

Answer 9

Early endosome - 6, no hydrolases
Late endosome - 5.5-6, hydrolases, H+ ATPase
Lysosome - <5, hydrolases, H+ ATPase

Question 10

Endocytic pathway

Answer 10

Contents transported in multivesicular bodies (MVB)
Guided by ESCRT proteins
Late endosomes can fuse with lysosomes

Question 11

Acid hydrolases

Answer 11

Only function at low pH (protective if lysosomal membrane failure)
Labelled with mannose-6-phos to target to late endosome or lysosome
Wide range for catabolism of various molecules

Question 12

Appearance of lysosomes

Answer 12

Depends on cell activity
Can be large, usually homogenous vs MVB
Ex granules in WBC

Question 13

Familial hypercholesterolemia

Answer 13

Dysfunction of LDL receptor - binds LDL but can’t bind AP2 (not endocytosed)
Grossly elevated serum, early CAD, stenosis, xanthomas

Question 14

Phagocytosis

Answer 14

Opsonization of bacteria by antibodies, complement
Receptors trigger actin->pseudopodia
Lysosomes fuse with phagosome to expose hydrolytic enzymes
Neutrophils and macrophages are “professionals”

Question 15

Autophagy

Answer 15

Old/damaged organelles
ER encircles organelle, fuses with lysosome
Triple membrane vs phagosome (double)

Question 16

Lipofuscin

Answer 16

Vesicles of lysosomal products that can’t be broken down
Often visible in older cells
“residual bodies”