Protein Quality Control Flashcards
There are ___ quality controls systems in the cell
2
one for ER proteins
one for cytoplasmic proteins
Transmembrane proteins are subject to ____ quality control
ER and cytoplasmic
ER proteins are degraded by. . .
Pumping out to the cytoplasm! There is no machinery for degradation within the ER itself.
Translocation to the cytoplasm is managed by the ER-associated degradation (ERAD) pathway.
Anfinson experiment
Anfinson showed that if you take a purified protein existing in its mature, folded state, then unfold it by adding a reagent that disrupts hydrogen bonding (urea), the protein will quickly recover its correct function when the urea is removed.
The conclusion: the three-dimensional structure of a protein is dictated by one thing, its amino acid sequence. No other information is necessary.
But. . . it was done on a simple protein that did not require chaperones.
efficiency of the folding reaction is determined by competition between . . .
intramolecular interactions and intermolecular interactions
Amyloid
Amyloid refers to the structure of a particular form of aggregate. Whereas aggregates are in general highly disordered, amyloid has a fibrillar morphology, which results from a partial β-sheet structure. The protein within an amyloid is partly ordered and partly disordered, but where ordered, the structure may not resemble the native state.
Key chaperone systems of the cytosol
Hsp70, Hsp90, and chaperonins
(Where HSP = heat shock protein)
About ~___% of proteins require the chaperone system
About ~5-10% of proteins require the chaperone system
Cytoplasmic Chaperone Pathways
Hsp70 Mechanism
Hsp40 delivers an unfolded protein.
Hsp70 clamps around it by hydrolyzing its ATP, allowing Hsp40 to diffuse.
The protein may fold a bit.
ATP binding causes diffusion.
If the protein fails to fold, it will be once again sequestered by a chaperone until it does eventually fold.
Kinetic Partitioning of Chaperones
When Kon is greater than Kfold, chaperone systems function to sequester a bit of unfolded protein to prevent aggregation which stabilizes the unfolded state.
If folding is inefficient, the protein is targeted for proteasomal degradation.
Decision to Degrade or to Fold
It is not currently known what the trigger is, but Hip and Hop binding represents a decision to fold while BAG-1 and CHIP (CHIP being an E3) represents a decision to degrade via the proteasome.
Hsp90 Mechanism
Glucocorticoid Receptor Signaling
The glucocorticoid receptor, the protein remains inactive while bound to Hsp90, and only becomes folded and active when the steroid hormone cortisol binds to the receptor. It is then released from Hsp90, where it goes to the nucleus to activate transcription.
Takes advantage of the fact that Hsp90 has certain preferred substrates.
Chaperonin Mechanism
Effectively acts as a small chamber that allows small proteins to fold outside of influences from other cellular proteins.
ER chaperones
Bip (Binding Protein) is similar to Hsp70 and associates with proteins as they are translated into the ER, allowing them to fold via the same kinetic partitioning model.
ER “Dislocons”
Transporters that transit unfolded proteins out of the ER for proteasomal degradation in the cytoplasm.
ER Quality Control
Calnexin System
As long as the carbohydrate chain contains at least one glucose, calnexin associates with the protein. Calnexin, a resident ER protein, anchors the protein to the ER while it is bound.
The mannosidases act as a sort of timer, cutting off one sugar at a time. When the sugars run out, Calnexin can no longer bind, and so the unfolded protein is no longer anchored to the ER and will be exported for proteasomal degradation by a dislocon.
The calnexin system, thus, effectively operates as a timer for ER processing on the protein, and when it runs out, the protein is degraded.
Summary of plasma membrane permeability
Types of transporter mechanism
Protein Families of Transporters
Background on Cystic Fibrosis
Current median survival ~37 years, obstructive lung disease responsible for ~80% of mortality.
CFTR and CFTR expression regulators
Cystic Fibrosis Transmembrane conductance Regulator
Note that the role of the transporter in the sweat gland is to absorb chloride, hence the high chloride content in CF patient’s sweat.
CFTR membrane assembly
R must be phosphorylated by a cAMP-dependent kinase in order for the channel to be active. May also serve a scaffolding role for other membrane channels.
CFTR Gating
One ATP molecule (green) is thought to be bound to the channel all the time. A second ATP molecule can bind the channel, to form the complex on the upper right. This can then lead to the opening of the channel (O1). ATP hydrolysis leads to the formation of O2, which is then converted to the closed state.
Detailed Regulation of CFTR
See CBB7 final reading slide for full explanation.
ERAD Pathway
Translocation Mechanism
