Transport of Proteins - ER --> Lysosome Flashcards
Where do Proteins go After ER?
- Golgi Apparatus
- Outside the Cell
- Lysosome or Endosome
Golgi Complex Structure
Cisterna = cis, medial, trans
Tubules
- cis = faces ER
- trans = opposite ER – secretory vesicles pinch off
Transport Vesicles
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
Types of Vesicles
COPII vesicles – RER to Golgi = anterograde transport
COPI vesicles – Golgi to ER = retrograde transport
Clathrin vesicles – Golgi to PM
GTP-Binding Proteins
- promote COPII coat proteins on ER
- hydrolysis of GTP = disassembly of COPII coat proteins
Rough ER to Golgi
- COPII vesicles + GTP-binding proteins
- cargo membrane proteins use DXE sorting signal (recognized by COPII proteins)
- ATP hydrolysis = dissociation of SNARE complexes
Cystic Fibrosis
- abnormal transport of chloride and sodium
- mutation can’t bind COPII coat proteins
- no transport via COPII vesicles to Golgi
Golgi to Rough ER
- 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
Glycosylation in Golgi
- Glycosidases removes sugars
- Glycosyltransferases adds sugars
Pathways from trans-Golgi (4)
- COPI vesicles – return to ER
- Adapter Protein coated vesicles – fuse to lysosome
- Clathrin coated vesicles (triskelion structure) – fuse to endosome
- Secretory vesicles – move to PM
trans-Golgi to Lysosomes
- Sorting Signal
GlcNAc Phosphotransferase
- adds phosphorylated GlcNAc to mannose sugar of lysosomal enzyme = M6P
Mannose-6-Phosphate
- targets soluble proteins to lysosome
- requires M6P receptor
trans-Golgi to Lysosomes
- Vesicles
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
Inclusion Cell Disease
No GlcNAc Phosphotransferase = no M6P signal
- lysosomal enzymes secreted NOT sorted to lysosome
- accumulated undigested glycolipids
Internalize Extracellular Materials
- Phagocytosis = phagocytes engulf large things
- Pinocytosis = buds sip fluids
- Receptor-mediated Endocytosis — selective internalization of ligands
Low-Density Lipoprotein
- transport lipids – mediate cholesterol transport
LDL Receptors
- in clathrin-coated pits
- clathrin helps internalize LDL
Low-Density Lipoprotein Structure
Amphipathic shell – phospholipid monolayer + apolipoprotein
Apolar core – hydrophobic
Apoliprotein B – guide particles to certain cell types
- mediates binding to LDL receptor
Clathrin/AP coated Vesicles
Clathrin – pinch off using dynamin + GTP hydrolysis
AP Complex – recognizes sorting signals of cargo proteins
Low-Density Lipoprotein Receptors
- short cytosolic with sorting signal (NPXY)
- long exoplasmic with ligand binding domain
- β-propeller domain
Low-Density Lipoprotein Receptors and pH
Normal pH – ligand binding arm bound to ApoB
Acidic pH – β-propeller domain gets (+) charge and bind to ligand bind arm (-)
Acidification of Lysosomes
V-class Proton Pumps = transport H+ across membranes (ATP) Cl- Channels = acidification of lumen
Internalizing Low-Density Lipoprotein (5)
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
Familial Hypercholesterolemia
- high cholesterol levels
LDL receptor mutations
- no LDL receptor
- can’t bind and internalize LDL
Transferrin Cycle
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
Autophagy
Lysosomal degradative pathway
- clear out misfolded proteins
- clear out damaged organelles
Autophagic Pathway
Autophagosome = enclosed in double membrane vesicle
Atg8 – bring lipid to problematic material
- forms autolysosome
- degrades into constituents to be reused
Loss of Atg8
- neurodegeneration + early death
- protein aggregates
Over-expression of Atg8
- promotes lifespan extension
- suppresses protein aggregate accumulation
Loss of Atg5
- neurodegeneration + early death
- protein aggregates
Over-expression of Atg5
- activates autophagy and extends lifespan
