RER/glycosylation Flashcards
Which end is the signal sequence located on the newly made peptide?
N-terminal end
It targets the peptide to the RER
In the signal hypothesis…
The role of the SRP
Binds to the signal sequence of the peptide and stalls translation
Also…binds to the SRP receptor in the RER membrane
The SRP receptor
Associated with the closed translocon channel in the RER membrane
Binding of the SRP to its receptor —>
- Mediates the binding of GTP to both the SRP and its receptor
GTP hydrolysis in the signal hypothesis
GTP is bound to SRP and its receptor
Promotes the opening of the translocon…and the dissociation of the ribosome to the SRP
The SRP also dissociates from its receptor
After the SRP is dissociated from its receptor and the ribosome…
SRP is recycled
Ribosome becomes associated with the open translocon
Translation continues again…and the peptide is translated through the open channel
Co-translation/translocation
Refers to the continuation of translation of the peptide through the open translocon and into the RER lumen
(Signal hypothesis)
Signal peptidases in the signal hypothesis
As signal peptide is translocated into the RER lumen
It is cleaved by signal peptidases
As translation finishes….
Ribosome falls off the mRNA and RER surface
Ribosome dissociates into its respective 40S and 60S
Translocon channel closes and peptide folds into its native structure
Soluble matrix proteins vs. integral proteins
Soluble = translocated through ER membrane and into lumen or matrix
Integral = inserted directly into ER membrane (have hydrophobic nature of amino acids that form the transmembrane spanning
Glycosylation (general)
Enzymatic addition of carbohydrates (glycans) to proteins
Glycation (general)
NON-enzymatic addition of carbs (glycans) to proteins
Ex: glycation of Hgb (Hemo A1c) —> relevant in diabetes
Glycosylation of proteins aids in…
Primarily protein folding and targeting
Also…protein stability (if go to E.C leaflet of plasma membrane)
Also…cell-to-cell contact
Lectins
Carbohydrate-binding proteins
Can interact with other glycosylated proteins
ABO blood typing
Results of EC glycosylation (antigenic)
2 types of glycosylation
O-linked
N-linked
O-linked glycosylation
Carbs added individually
Sequentially to -OH groups (O-linked) of serine and threonine
Enzyme: gylcosyl transferase
Example: adding carbs to proteins to generate ABO blood groups
N-linked glycosylation
Carbs added as a performed oligosaccharide complex to Asparagine (Asn) residues
…in an Asn-X-(Ser/Thr) consensus sequence
Occurs in ER as the peptide is translocated through the ER membrane
Complex is further processed in ER and Golgi….and can serve as a mechanism for targeting of the protein to its final cellular destination
Where does synthesis of oligosaccharide complex start
Cytosol
Then further in ER lumen/Golgi after added to peptide in N-linked gly.
Dolichol phosphate
Membrane bound molecule that the oligosaccharide precursor is added upon
Activated nucleotide-sugar donors
UDP-sugar or GDP-sugar
Add sugars to the dolichol phosphate sequentially until the oligosacc. Precursor is complete
Tunicamycin
Analog for the first nucleotide sugar substrate (UDP-GluNAc)
Inhibits first reaction of oligosacc precursor formation
Prevents formation of complex
Dramatic effects in protein folding, stability, targeting, and secretion
***used to study eukaryotic protein folding…NOT a clinical drug
Oligosaccharyl transferase
Enzyme that transfers the complex from the dolichol phosphate to the Asn residue of the consensus sequence
Once attached the complex is further processed by glycosidases
Disulfide bonds primary role
Primarily found in secreted proteins or in proteins found in the EC leaflet of the plasma membrane
Also aid in proper folding and provide a strong covalent structure and stability to the protein
Where are disulfide bonds formed?
ONLY in the ER lumen
Only under oxidative conditions as the protein is translocated through the ER membrane
Protein disulfide isomerase (PDI)
Transfers oxidized disulfide bonds, found in the enzyme structure to the reduced protein substrate
In doing so…
Cysteine residues in PDI = reduced
Cys in substrate = oxidized
PDI can also rearrange disulfide bonds in the substrate protein to allow the correct conformation for foldiing
Alpha-1-antitrypsin deficiency (A-1AT)
General info
Made and secreted from the liver
Genetically inherited
Most common genetic cause of emphysema in adults (familial emphysema) and liver disease in children (jaundice, cirrhosis)
A-1AT deficiency mechanism and role of neutrophils
Neutrophils secrete elastase in order to move through the lung tissue matrix…
A-1AT is needed to inhibit typsin and unregulated elastase —> otherwise tissue damage in lungs
Mechanism = point mutation in protein that prevents proper folding….crystalline aggregates in ER lumen
—> improper secretion of A-1AT and other proteins from lumen —> damages liver tissue
A-1AT deficiency treatment
Goal to minimize symptoms
Immunizations for flu and pneumonia to prevent symptoms
No smoking/pollution
A-1AT replacement therapy = no persuasive data
Broncodilators and steriods to open up airways
O2 if lungs really fucked
Liver transplants (most common cause of this in peds)
