Transcript From gene to protein 2

PROTEIN FOLDING AND
DEGRADATION
Kanokporn Boonsirichai
Proteins must fold up into their
unique 3-D conformation
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To be able to perform their specific
function
To assemble correctly with other
proteins
To bind with small-molecule cofactors
that are required for their activity
To be appropriately modified by
protein kinases or other proteinmodifying enzymes
When a protein folds:
Most of its hydrophobic residues
are buried into an interior core.
 A large number of noncovalent
interactions are formed.
 The final conformation is usually
of the lowest free energy.
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Proteins begin to
fold as they exit
from the ribosome
Secondary structures
are formed and
aligned roughly within
a few seconds
  “Molten globule”
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Cytochrome b562
Side-chain adjustment
(slow) to form the
appropriate tertiary
structure
By the time it is released from the ribosome,
much of the folding has already been completed.
Molecular Chaperones
A class of proteins
which mediate
protein folding
Heat-Shock Proteins (Hsp)
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Firstly identified in E. coli
Increased synthesis at elevated temperature
(42oC)
What happens at elevated temperature? Why
does the cell need more chaperones?
Heat-Shock Proteins (Hsp)in Eucaryotes
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Two major families: Hsp60 and Hsp70
Members are organelle-specific: cytosolic, ERassociated, mitochondrial
Work with their own set of assiciated proteins
Show an affinity for exposed hybrophobic
patches
Hydrolyze ATPs
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Hsp70
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Works with Hsp40
Performs its function as the target protein is being
translated
Binds to a string of seven hydrophobic amino acids
Hydrolyzes an ATP as it binds; releases ADP and
rebinds ATP as it dissociates
Hsp60 (Chaperonin)
•Acts after its target protein has been fully synthesized.
•Provides a favorable environment for the target protein to refold.
•Binding of ATP and GroES may transiently stretch the misfolded
protein.
•ATP hydrolysis triggers the release of the target protein.
Choices of Protein Quality Control
Proteasome
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A machinery for protein destruction
An abundant ATP-dependent protease (= 1% of cellular proteins)
Found in the cytosol and the nucleus
Only marked proteins are targeted to the proteasome
Ubiquitin
• A 76-amino-acid tag
• Activated through a high-energy
thioester linkage to a cysteine
residue
• Is transferred to the lysine residue
of the target protein
~ 30 distinct
kinds
Targets
•Denatured/
misfolded
proteins
•Proteins with
oxidized/
abnormal
amino acids
biology.caltech.edu/Members/Deshaies
•E3 recognizes specific degradation signals in
the target proteins.
•Multiubiquitin chain is recognized by the
proteasome.
Regulated Destruction of Proteins
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Some proteins turn over rapidly at all time.
Some proteins are stable most of the time but
become unstable under a certain condition.
Mechanisms
•Activation of specific E3
•Activation/ exposure of
the degradation signal in
within the target protein
Activation of Ubiquitin Ligase
Activation of Degradation Signal
Abnormally folded proteins may form
protease-resistant aggregates
•Some protein
aggregates form a
fibril structure
•Cross-beta
filaments: layered
of polypeptide
chains with
continuous stacks of
beta sheets
Prion disease
Protein-aggregates-induced Diseases
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Huntinton’s disease
Alzheimer’s disease
Creutzfeldt-Jacob disease (CJD) in
humans
Bovine spongiform encephalopathy
(BSE) in cattle (i.e. mad cow disease)
Prion
diseases
Regulation of
cellular protein levels
can occur at many
different points
•Transcription
•RNA processing
•RNA transport
•Translation
•Protein degradation