1. Lecture 11 Vesicular Trafficking: Lysosomes;
endocytosis and exocytosis
From cell exterior to lysosome
What’s taken in? Macromolecules, particulate substances, other cells!
Process: a small portion of the plasma membrane, invaginates,pinch off and form “endocytic vescile
Tow typpes based on size: phagocytosis (phagosome)--cellular eating! >250 nmAnd pinocytosis (pinocytic vescicles)--cellular drinking! 100 nm

3. macromolecules are degraded in lysosomes
pH is one of the mechanisms
for protecting contents in the cytosol
All major classes of
macromolecules are degraded
in lysosomes
40 hydrolytic enzymes
Lysosomal glycoproteins
are unusually highly glycosylated
Vacuolar H+ ATPase:
Lysosomes
Endosomes
Selected compartments of
Golgi Apparatus
Transport vesicles
Secreted vesicles

4. Lysosomes are highly heterogeneous
Shape and size
But all have acid hydrolases
The “stomach” of a cell
Stained for acid phosphatase:
Phosphatase substrate

5. Huge vacuoles of plant cells
Related to lysosomes
Storage organelle
Degradative compartments
Turgor pressure
Homeostatic device
Rubber
Opium
Garlic
Proteins
Pigments
Noxious molecules

6. Three pathways to degradation in lysosomes
pH 6.0
pH 5.0

7. The structure of mannose 6-phosphate on a lysosomal enzyme
A unique marker for lysosomal proteins

8. The transport of newly synthesized lysosomal hydrolases
to lysosomes
M6P receptor
pH6.0 pH
Recycling of M6P

9. A signal patch in the hydrolase polypeptide chain
provides the cue for M6P addition
Defects in the GlcNac phosphotransferase cause a
lysosomal storage disease in humans, inclusion-cell
Disease (I-cell disease)
N-acetylglucosamine
Lysosomal storage diseases
Some lysosomes may undergo exocytosis

11. Phagocytosis by a macrophate Phagocytosis by a neutrophil pseudopods
5 um cell!
Professional phagocytes: macrophages, neutrophils, dendritic cells
Phagocytosis is a triggered process, need receptor recognition and signaling; antibodies are good triggers, Fc region, neutriphil and bacteria
Pinocytosis is constitutive
phosphatidylserine is a trigger for phagocytosis of the dead cell
Phagocytosis of inanimated particles: glass, latex beads, asbestos fibers,
Living cells:don’t-eat-me signal, inhibitory receptors recrcuit tyrosine phosphatases that antagonize the intracellular signaling events required to initiate phagocytosis
Pinocytosis: cell drinking; Phagocytosis: cell eating

12. clathrin-coated vesicles from the plasma membrane
The formation of
clathrin-coated vesicles
from the plasma membrane
100% plasma membrane turn over in half an hour
Endoytic-exocytic cycle
Clathrin-coated pits
Shed coat and fuse with early endosomes

13. Caveolae in the plasma membrane of fibroblast
Lipid rafts
caveolin
Non-calthrin coated: caveolae(little cavities)
Form from lipid rafts, rich in cholesterol, glycosphingolipids, GPI-anchored membrane proteins
Caveolin
Caveolae pinch off by virtue of the lipid composition of the membrane
Endosome-like compartments or transcytosis
Virus entry

14. A low-density lipoprotein (LDL) particle
Receptor-mediated
Endocytosis
Example: cholesterol
uptake
3,000 Kd
1500 cholesterol
Molecules
800 phospholipid
500 unesterified
cholesterol
Atherosclerotic plaques;
When uptake is blocked
Receptor-mediated endocytosis
Cholesterol take up by LDL
LDL particle
LDL receptor gene mutation--high cholesterol in blood, artherosclerosis
Atherosclerosis
Cholesterol esters
Are hydrolyzed to
Free choleterol in lysosomes
500 Kd

15. Normal and mutant LDL receptor
Common endocytic
signal: YXXY
(binding to adaptin)
But LDL receptor:
Asn-Pro-Val-Tyr
Coronary artery disease

16. Possible fates for transmembrane receptor proteins
Different Rabs
associate with
early and late
endosomes
Recycling:
LDL receptor

17. Receptor-mediated endocytosis of LDL

18. Sorting of transferrin (red) and opioid receptors (green)
in the recycling endosomes
Transferrin receptor recycles with
its ligand
Different
Rabs
Opioid receptors and EGF receptors
Are not recycled but degraded in
Lysosomes along with ligand: receptor
Down-regulations

19. Some early endosomes
Migrate slowly along MT
Toward the cell interior
And pinch off Vesicles to
form MVBs
MVBs may fuse with a late
endosomal compartment or
they fuse with each other
to become late endosome

20. Lysosomes form when vesicles
from trans Golgi network fuse
with late endosomes

21. Receptors and their ligands
are fully accessible to
digestive enzymes in MVBs
Ubiquitin tagging facilitate
the uptake of receptors
into endocytic vesicles and
and sorting into the
internal membrane vesicles
of MVBs

22. Transcytosis: transferring macromolecules across
ephithelial cell sheets
A newborn rat obtains antibodies from its mother’s milk
Remember transcelluar
transport of glucose?
Early endosome to
recycling endosome
Receptors have
sorting signals

23. Exit of membrane proteins from the recycling endosomes can be regulated

24. Two early endosomal compartments and one common late endosomal
compartment in epethilial cells

25. Exocytosis: fusion of vesicles
with the plasma membrane

26. Two mechanisms of secretion

27. Three pathways of protein sorting in the trans Golgi network

28. Secretory proteins form
selective aggregates
Secretory proteins become
highly concentrated:
aggregation and membrane
retrieval

29. Proteins are often proteolytically processed during
the formation of secretory vesicles
A prohormone
In different cells
Size of the final products
Activity

30. Regulated exocytosis can be a localized response of
the plamsa membrane and its underlying cytoplasm
Mast cell
secreting histamine
all over the
cell surface when
on a solution of
soluble stimulant
Localized exocytosis when
stimulant is presented from a
solid bead

31. Two types of polarized cells
Polarized cells direct proteins
from the trans Golgi
network to the appropriate
domain of the plasma membrane

32. Two ways of sorting plasma membrane proteins
Gut epithelial cells
Live cells

33. Lipid rafts in the trans Golgi network
Glycosphingolipids and cholesterol form rafts in the lipid bilayer
May mediate sorting of glycosphingolipids and GPI-anchored proteins
To the apical plasma membrane

34. The formation of synaptic vesicles
50 nm

35. Summary
Lysosomes are where most of the intracellular degradation
occurs. Formation of lysosomes, sorting into lysosomes;
Phagocytosis, pinocytosis, receptor-mediated endocytosis,
early endosomes, late endosomes, recycling endosomes;
Two types of exocytosis, sorting during exocytosis, synaptic
vesicles.