Endocytosis & Protein Degredation Flashcards
What are the 2 major routes of small volume endocytosis? How do they work?
Phagocytosis
- Multicellular organisms
- Usually by specialized cell (i.e. macrophage/ neutrophil)
- Cells recognize foreign organisms -> engulf-> deliver to lysosomes for degradation
- Recognize apoptotic cells
Pinocytosis
- Associated with vesicle uptake of (by) specific ligands and receptors
What are the types of vesicles used in pinocytosis?
Clathrin coated &
Caveolae
How are clathrin coated vesicles formed?
Cargo molecules (LDL) bind transmembrane receptor (LDLR) -> transmembrane receptor recognizes and binds adaptor protein (w/ motif in cytoplasm) -> Adaptor complex of proteins forms -> Enables clathrin coat to assemble on vesicle budding from plasma membrane (or golgi to secretory pathway)
Vesicle is pinched off from membrane by dynamin
- adaptor complex and clathrin rapidly dissociate
What is the function of dynamin
Pinch clathrin coated vesicles off from golgi or plasma membrane
What is the progression of phagocytosis of LDL?
LDL-receptor binding -> coated vesicle (clathrin and adaptor complex dissociation) -> early endosome -> late endosome (LDLR returns to membrane) -> lysosome
What types of molecules enter through caveolae?
cholera, tetnus, folic acid,
How many caveolins are in each vessicle?
144
Where is caveolin 3 expressed? and what diseases can its mutation cause?
in skeletal and cardiac muscle
- limb girdle disese
- Rippling Muscle disease
What can macrophages degrade?
EVERYTHING (except cholesterol)
mis-folded protiens
damaged organelles
good and bad endocytosed materials
Why must improperly folded proteins be degraded?
They may aggregate and wreak HAVOC!
What are the 3 major pathways of protein degradation?
Ubiquitin-proteasome system (UPS)
lysosome
autophagy
What is autophagy used for?
- degradation of long lived proteins & entire organelles
- important in development
- Req for adaptation to environmental stresses (i.e. starvation)
Describe the ubiquitin-proteasom system (UPS)
- Proteins deglycosylated after retrotranslocation out of ER
- Ligase enzymes E1, E2, and E3 bind & activate ubiquitin
- Polyubiquinated=> targeted to proteasome for degradation
Describe the action of a proteasome
huge complex of proteis -> unwinds misfolded proteins, feed into compartment -> where cut into 7–9 amino acid lengths
What % of cellular protein is proteasome?
~1%
How many E1, E2, & E3 genes do humans have? What is the function of each?
E1: 1 - activate ubiquitin
E2: ~50 - Receive U from E1, transfer to E3
E3: ~500 - attaches U to lysine in protein - substrate specificity
How many ubiquitins are required for degradation?
at least 4 in a chain (polyubiquitination)
What are the other functions of ubiquitin?
multiubiquitination: mutiple single ubiquitins at several locations on a protein
aka: mono/multi ubiquitins- regulatory signals (u of histones, TFs to reg transcription
What is the function of chaperone proteins?
Help with proper folding of newly synthesized proteins
What are 2 examples of chaperone proteins?
hsp60
hsp70
(heat shock protein)
How does hsp70 function?
prevents aggregation by binding to exposed hydrophobic areas in incompletely folded amino acids
How does hsp60 function?
acts as an isolation chamber (prevent aggregation and refold) in barrel-shaped structure
What is the generic name for proteins that bind sugas? Give 2 examples
lectins: calnexin & calreticulum
What percentage of proteins are folded in the rER? Where are these proteins destined?
~30%
transmembrane proteins, vesicular transport and secretion
Describe the protein folding pathway involving calnexin, calreticulum and a glucosyltransferase.
calnexin and calreticulum bind oligosaccharide chain if a glucose is present-> When glucose reomved by glucosidase (GT) C & C release protein -> if correctly folded allowed to exit ER
- > if incorrectly folded-> glucose added back on by glucosyltransferase and cycle repeats
- at a certian point (unclear when/why) protein is identified as unsaveable, “times out” and is translocated from ER, destined for degredation
Describe a proteasome
~2million dalton protein
- central cylinder with cap a each end (recognize polyubiqutiinaiton) -> use ATP to unfold protein- feed into cylinder
Degrades only proteins!
How many amino acids does ubiquitin have?
76 - HIGHLY CONSERVED!!
~1% of cellular protein
Describe the function of a lysosome
- degrade extracellular materials& some intracellular components
What ca lysosomes degrade
everything: proteins, lipids and sugars
What is the path to a lysosome?
monoubiquinated plasma proteinstansferec by late endosome/ multivesicular body to lysosome
- multovesicular body (prelysosomal compartment with molecules slated for destruction in lumen)
lysosomal membrane proteins and transporters protected from degradation (error here = lysosomal storage disorder)
What is autophagy?
when components of cell are broken down and reused
- normal turnover, damaged organelles, starvation
- uses double membrane around organelle- fuses with lysosome-> contents degraded
What is ERp57
protein folding enzyme
thiol oxoreductase - allows formation of disulfide bonds
What is UDP?
Folding sensor involved in quality control
glucose:glycoprotein glucosyltransferase
