AC Lecture 32: Protein quality control Flashcards
What kinds of damage leads to misfolding and aggregation of proteins?
physical forces (e.g. heat) chemical forces (e.g. oxidation
What is proteostasis?
cellular quality control processes that continually monitor the proteome for misfolded or aggregated proteins and target them for degradation and disposal
What are the stages for polypeptide folding?
burst phase (0-5ms) intermediate phase (5-100ms) rate-limiting
Why did molecular chaperone proteins evolve?
to promote folding and prevent protein aggregation in physiological conditions (which are make it difficult to refold denatured proteins due to molecular crowding and relatively high temperatures)
What happens in the burst phase of polypeptide chain folding?
formation of secondary structure and collapse of the hydrophobic core
What happens in the intermediate phase of polypeptide chain folding?
formation of molten globule intermediate which has characteristics of both folded and unfolded proteins
What happens in the rate-limiting phase of polypeptide chain folding?
attainment of native structure
marked by conversion of molten globule via global repacking of hydrophobic side chains and the association of domains that were folded independently in the intermediate stages
List some families of molecular chaperones.
Heat Shock Proteins:
Hsp70, Hsp60 (chaperonin), Hsp90 families
Describe the Hsp chaperones.
chaperone proteins that bind and release polypeptides in a manner that is dependent on ATP binding, hydrolysis, and nucleotide exchange
Describe the process of protein folding in a chaperonin family molecular chaperone.
- unfolded protein binds the rim of a chaperonin (which looks like a barrel with 2 stacks of rings, each 7 subunits)
- Lid structure displaces protein into the cavity of the barrel (which is lined with hydrophilic amino acids to drive folding)
- polypeptide can then fold in a sequestered and protected environment
- lid dissociates due to changes in conformation of the large subunit as ATP is hydrolyzed
How is gene transcription for molecular chaperones controled?
Heat Shock Transcription factor (Hsf)- which responds to the presence of unfolded proteins or heat shock or other types of proteotoxic stress
Describe the structure of the proteasome (26S).
central catalytic core (20S) regulatory cap (19S)- recognizes ubiquitinylated substrates and deubiquitinylated substrates as well as prepares them for proteolysis (via protein unfolding if necessary)
How are proteins degraded in cells?
in both the cytosol and nucleus it is largely done by proteasomes (large gated proteases)
target substrates via covalent linkages to multiple copies of ubiquitin
List the activities of the eukaryotic proteasome.
chymotrypsin-like (cleaves after hydrophobic amino acids)
trypsin-like (cleaves after basic amino acids)
peptidyl-glutamyl peptide hydrolyzing activity (cleaves after acidic amino acids)
List the additional activities of mammalian proteasomes.
cleavage after branched chain amino acids
cleavage between small, neutral amino acids
What enzymes are necessary for ubiquitinylation?
E1 (ubiquitin-activating enzyme; small amount)
E2 (ubiquitin-conjugating enzyme)
E3 (ubiquitin ligase; specifies substrate selection)
Describe the process of ubiquitinlyation.
- ubiquitin-adenylate is formed
- transfer of activated ubiquitin to thiol site in E1
- E2 accepts ubiquitin from E1
- E2 transfers ubiquitin to the protein substrate with the help of E3
How is ubiquitination linked to molecular chaperones?
CHIP (C-terminal Hsp interacting protein)
binds to Hsp70 and catalyzes ubiquitinylation of misfolded proteins
What causes protein aggregation?
when misfolded proteins overwhelm the ubiquitin/proteasome pathway
What are the types of aggregates?
amorphous assemblies of misfolded proteins (bound together by hydrophobic interactions)
ordered assemblies of amyloid fibers (formed by stacked beta-sheet structures)
How are aggreagates cleared?
largely inaccessible to proteosome
cleared by the autophagic system
Which human disorders are characterized by the presence of organized protein aggregates?
amyloid diseases (e.g. Alzheimer's disease) Lewy Bodies (e.g. Parkinson disease) polyglutamine repeat diseases (e.g. Huntington, SMA, DRPLA, SCA) prion diseases (e.g. Creutzfeld-Jakob, fatal familial insomnia)
What are the characteristics of Alzheimer’s disease?
extracellular amyloids of APP intracellular deposits (or neurofibrillary tangles) of phosphorylated Tau (microtubule binding protein)
What causes PD?
loss of dopaminergic neurons in the substantia nigra
aggregates of Leqy Bodies enriched in alpha-synuclein
