Transport of Proteins - ER --> Lysosome

Question 1

Where do Proteins go After ER?

Answer 1

• Golgi Apparatus
• Outside the Cell
• Lysosome or Endosome

Question 2

Golgi Complex Structure

Answer 2

Cisterna = cis, medial, trans

Tubules

• cis = faces ER
• trans = opposite ER – secretory vesicles pinch off

Question 3

Transport Vesicles

Answer 3

Coat proteins = budding of vesicles

• membrane cargo proteins (integrated into ER)
• soluble cargo protein (free floating)

GTP-binding protein
- promote coat protein binding

SNARE proteins
- fusion of vesicles with target membranes

Question 4

Types of Vesicles

Answer 4

COPII vesicles – RER to Golgi = anterograde transport
COPI vesicles – Golgi to ER = retrograde transport

Clathrin vesicles – Golgi to PM

Question 5

GTP-Binding Proteins

Answer 5

• promote COPII coat proteins on ER

- hydrolysis of GTP = disassembly of COPII coat proteins

Question 6

Rough ER to Golgi

Answer 6

• COPII vesicles + GTP-binding proteins
• cargo membrane proteins use DXE sorting signal (recognized by COPII proteins)
• ATP hydrolysis = dissociation of SNARE complexes

Question 7

Cystic Fibrosis

Answer 7

• abnormal transport of chloride and sodium
• mutation can’t bind COPII coat proteins
• no transport via COPII vesicles to Golgi

Question 8

Golgi to Rough ER

Answer 8

• mis-sorted ER resident protein (KDEL sequence)
• KDEL receptor - acidic in Golgi = bind KDEL
• KDEL receptor - basic in RER = release KDEL
• KDEL receptor (KKXX sequence) bind COPI coat proteins

Question 9

Glycosylation in Golgi

Answer 9

• Glycosidases removes sugars

- Glycosyltransferases adds sugars

Question 10

Pathways from trans-Golgi (4)

Answer 10

1. COPI vesicles – return to ER
2. Adapter Protein coated vesicles – fuse to lysosome
3. Clathrin coated vesicles (triskelion structure) – fuse to endosome
4. Secretory vesicles – move to PM

Question 11

trans-Golgi to Lysosomes

- Sorting Signal

Answer 11

GlcNAc Phosphotransferase
- adds phosphorylated GlcNAc to mannose sugar of lysosomal enzyme = M6P

Mannose-6-Phosphate

• targets soluble proteins to lysosome
• requires M6P receptor

Question 12

trans-Golgi to Lysosomes

- Vesicles

Answer 12

Lysosomal enzyme bind to M6P receptor

• AP complex + clathrin triskelion added
• go to late endosome
• remove coat proteins + fuse with SNARE proteins

M6P detataches

• lysosomal enzyme delivered to lysosome
• M6P receptor returned to Golgi

Question 13

Inclusion Cell Disease

Answer 13

No GlcNAc Phosphotransferase = no M6P signal

• lysosomal enzymes secreted NOT sorted to lysosome
• accumulated undigested glycolipids

Question 14

Internalize Extracellular Materials

Answer 14

• Phagocytosis = phagocytes engulf large things
• Pinocytosis = buds sip fluids
• Receptor-mediated Endocytosis — selective internalization of ligands

Question 15

Low-Density Lipoprotein

Answer 15

• transport lipids – mediate cholesterol transport

LDL Receptors

• in clathrin-coated pits
• clathrin helps internalize LDL

Question 16

Low-Density Lipoprotein Structure

Answer 16

Amphipathic shell – phospholipid monolayer + apolipoprotein
Apolar core – hydrophobic

Apoliprotein B – guide particles to certain cell types
- mediates binding to LDL receptor

Question 17

Clathrin/AP coated Vesicles

Answer 17

Clathrin – pinch off using dynamin + GTP hydrolysis

AP Complex – recognizes sorting signals of cargo proteins

Question 18

Low-Density Lipoprotein Receptors

Answer 18

• short cytosolic with sorting signal (NPXY)
• long exoplasmic with ligand binding domain
• β-propeller domain

Question 19

Low-Density Lipoprotein Receptors and pH

Answer 19

Normal pH – ligand binding arm bound to ApoB

Acidic pH – β-propeller domain gets (+) charge and bind to ligand bind arm (-)

Question 20

Acidification of Lysosomes

Answer 20

V-class Proton Pumps = transport H+ across membranes (ATP)
Cl- Channels = acidification of lumen

Question 21

Internalizing Low-Density Lipoprotein (5)

Answer 21

1) LDL receptors (in clathrin-coated pits) bind to ApoB
2) Dynamin + GTP hydrolysis pinch off vesicle
3) Shed vesicle coat and fuse to late endosome (pH = release LDL)
4) Late endosome fuse with lysosome (LDL broken down)
5) LDL receptor recycled to PM + ready bind new LDL

Question 22

Familial Hypercholesterolemia

- high cholesterol levels

Answer 22

LDL receptor mutations

• no LDL receptor
• can’t bind and internalize LDL

Question 23

Transferrin Cycle

Answer 23

Transferrin = transport iron

1) Ferrotransferrin binds receptor and internalized
2) Dynamin + GTP hydrolysis pinch off vesicle
3) Shed vesicle coat and fuse to late endosome
4) Fe3+ ions released from apoprotein
5) Apoptransferrin still bound to receptor + recycled to PM

Question 24

Autophagy

Answer 24

Lysosomal degradative pathway

• clear out misfolded proteins
• clear out damaged organelles

Question 25

Autophagic Pathway

Answer 25

Autophagosome = enclosed in double membrane vesicle
Atg8 – bring lipid to problematic material
- forms autolysosome
- degrades into constituents to be reused

Question 26

Loss of Atg8

Answer 26

• neurodegeneration + early death

- protein aggregates

Question 27

Over-expression of Atg8

Answer 27

• promotes lifespan extension

- suppresses protein aggregate accumulation

Question 28

Loss of Atg5

Answer 28

• neurodegeneration + early death

- protein aggregates

Question 29

Over-expression of Atg5

Answer 29

• activates autophagy and extends lifespan