Chaperones Flashcards
(29 cards)
Chaperone
A protein that helps other proteins fold up correctly
Highly adundant in cell
What are the proteins that chaperones help called?
Clients
Percentage of proteins that need help folding up
50%
or more in stressed cells
What do all chaperones use to power activity?
ATP hydrolysis
Hspxx
Chaperones often produced in response to heat shock
Names determined using this
Hsp = heat shock protein
xx = protein size on SDS-page gel
How do chaperones find proteins that need help folding?
Chaperones recognise exposed hydrophobic regions of unfolded proteins
Anfinsen hypothesis
native structure of a protein is determined only by its sequence
TRUE but cells are crowded so things may bind to proteins - preventing them from folding
Why are unfolded proteins bad for cell?
Loss of function
- unfolded protein cannot carry out function
Gain of function
- aggregation causes toxicity or blockage
Waste of energy for cell
How do unfolded proteins waste energy in cell?
Made a protein that doesn’t do its intended job
- need to make more
ATP used by chaperone to fix it
Takes 4-6 ATP/GTP to add each aa in translation
Hsp70/40 (DnaK/J in bacteria)
ATP bound Hsp70 binds weakly to exposed hydrophobic regions of unfolded protein
Hsp40 hydrolyses ATP –> ADP causing the ‘lid’ to fold down - binding client tightly
Nucleotide exchange factor (Hsp110) catalyses exchange of ADP –> ATP
Hsp70 re-formed, protein released
What is the role of Hsp70/40?
Binds the protein and releases
Just gives the protein a chance to fold up correctly
If it doesn’t work the process is repeated
- only uses 1ATP so can go again
Mitochondrial Hsp70
Used to drive protein import into mitochondrial matrix
- acts as ratchet to prevent protein from moving back out of channel
- allows it to fold up once in the mitochondrion
Mitochondrial protein control by chaperones
When mt-proteins are expressed from ribosome they get coated in ctHsp70
Protein carried to mitochondria and fed through channels
Inside the N-terminus is cleaved off and mtHsp70 binds to exposed region
How are mt-proteins recognised?
Have very +ve N-terminus
Pulls them into mitochondria due to strong membrane potential
Hsp90
Dimer
Does not require any additional proteins
ADP-bound form is fairly open
ADP-free form is very open
- both bind weakly to unfolded proteins
What happens when ATP binds to Hsp90?
The monomers come together and close up over the hydrophobic regions of client
ATP hydrolysis then reopens the dimer
- client leaves
Role of Hsp90
bind and release
give protein another chance to fold correctly
- repeats process in protein still doesn’t fold properly
What does Hsp70 bind best to?
completely unfolded proteins with exposed unfolded sequences
What does Hsp90 bind best to?
hydrophobic surfaces
such as on proteins that started folding but didn’t finish
e.g. molten globules
Hsp70 and Hsp90 cooperation
Hsp70 binds first to start client folding and if necessary Hsp90 joins to help in later stages
Due to their preferred binding
Holdase Activity
Hsp90 binds weakly to hydrophobic surfaces of proteins that have them intentionally - for function
Holdase activity sheilds the hydrophobic residues to stop them getting ‘clogged up’
What kind of proteins have hydrophobic residues for function?
Kinases, TFs and steroid hormone receptors
Steroid hormone receptors
Hydrophobic ligands pass through cell membrane to carry out their function so do not need cell-surface receptor
SHR bind these hydrophobic ligands
Hsp90 protects it until ligand arrives
Once ligand is bound, Hsp90 comes off and the SHR moves to nucleus to act as TF
Hsp60/Hsp10 (GroEL/GroES)
Chaperonins
Isolate unfolded proteins to allow them to fold up
