1. The ER and Golgi Modify & Sort Proteins1

2. Glycosylation Affects Protein & Lipid Function
Serine (S) / Threonine (T)
Asparagine (N)
Folding
Stability
Recognition
Virtually all cells in body carry glycosylated proteins or lipids.
Virtually all secreted or membrane proteins contain sugar modifications.
Functions:
Protein folding & stability
Blood type determinants – A, B, O blood groups – glycosylation of glycophorin
Microbes & other pathogens attach and invade
Cell-cell adhesion – white blood cells adhere to endothelial cells
Bind growth factors – signaling involved in development & disease
Regulates protein sorting to different compartments 2

3. Glycosylation Begins in the ER
OST 3

4. Glycosylation Continues in the Golgi
Unique enzymes carry out each step of glycosylation! 4

5. Two Models for Progression Through Golgi
(ERGIC)
(ERGIC)
Cisternal maturation: COPI vesicles would contain glycosyltransferases that carry out 5

6. Model 24: COPII Coat Protein Assembly in Vesicular Transport
COPII Coat Formation
GEF
GTP
Sec13/31 coat proteins
GEF
GTP
Sar1 GTPase
GTP
Sec23/24 coat proteins
GEF
GDP
Sar1
GEF
GTP
GTP
GEF
Transmembrane & soluble cargo recruitment
Cytoplasm
GEF
ER lumen
Define site of vesicle budding
Recruit coat proteins
Recruit specific vesicle cargo to the budding site
Deform the membrane
Pinch off to form a vesicle 6

7. Model 23: Vesicular Transport Through the Secretory Pathway
Rough ER
COPI Coat
(Anterograde)
COPII Coat
(Anterograde)
Golgi Complex
cis medial trans
COPI Coat
(Retrograde)
KDEL/KKXX proteins
ERGIC
Endocytosis
Dynein
MTOC
Kinesin
Kinesin
Unknown Coat?
Exocytosis
Dynein 7

8. Retrieval of ER-Resident Proteins
[H+]
ER Retention/Retrieval Signals
KDEL: Lys-Asp-Glu-Leu
Soluble ER proteins
KKXX: Lys-Lys-aa-aa
Transmembrane ER proteins
KDEL
ER-resident protein K X
KDEL Receptor
Golgi
Cytoplasm
ER retrieval signal is KDEL – constitutive retrieval
KDEL binds KDEL Receptor – conditional retrieval
KDELR has KKXX at C-terminus that can interact with COPI coat proteins. This signal is hidden until the receptor interacts with a KDEL protein, which causes a conformational change that exposes the KKXX motif.
KDELR interacts with KDEL at lower pH in the Golgi. When transported back to the ER, the pH increases and the ligand and receptor dissociate.
COPI coat proteins 8

9. Clathrin Coats Help Vesicles Bud Off from Golgi9

10. Intracellular Transport
Yildiz, Tomishige, Vale & Selvn. Science (2004) 10

11. Kinesin, Dynein & Myosin are Molecular Motors
Often (not always) (+) end directed
Moves along MTs
Requires ATP hydrolysis
>45 different varieties
Dynein
(-) end directed
Only 1 cytoplasmic dynein heavy chain
Myosin (not shown)
Moves along actin
17 classes, lots of members 11

12. Secretory Vesicles Fuse with the Plasma Membrane During Exocytosis12

13. Clathrin-Mediated Endocytosis
Clathrin (triskelion) – the coat
Adapter proteins – recruit cargo & clathrin coat
Dynamin (a GTPase) – pinches vesicle off membrane
Adapter protein 13