ER Flashcards
Start of Endomembrane system
RER
Polypeptides are synthesized at two location
-1/3 at RER
-2/3 on free cytosolic ribosomes
Protein synthesized at RER
- secreted proteins
- transmembrane proteins
- soluble proteins reside in Endomembrane system
Protein synthesized on free cytosolic ribosomes
-proteins destined to remain in ribosomes
-peripheral proteins of cytosolic surfaces
-proteins transported to nucleus
-proteins for chloroplast/chloroplast/peroxisome
All proteins begin synthesis in
Ribosomes in cytosol
How proteins know where to go
ER signal sequence OR sorting signal
TIGHTLY TIE PROTEIN SEQUENCE,SYNTHESIS,TARGETING
Translation start at
Cytosol
Most polypeptide synthesis take place in
Cytosol
After translation two pathways separated
Free ribosome vs ER-docked ribosome
Protein sorting pathways
1- Co-translational import
2- post-translational import
Co and post translational import difference
Co:
Proteins carry ER signal sequence direct to RER
——ER signal sequence
Post:
Lack of ER signal sequence, complete synthesis on free ribosome
——sorting signal
The proteins are released in cytoplasm from post-translational import and ———
Those who have organelle-specific sorting signal are imported into organelle
**cytoplasmic proteins do not have sorting signal
Multiple ribosome synthesizing same mRNA
Polysome
Co translational translocation of SOLUBLE PROTEIN
Deposit polypeptides in lumen of ER
Signal Recognition Particle (SRP)
Binds ribosome-mRNA-polypeptide complex to ER membrane
SRP bind ER signal sequence to new forming polypeptide
SRP contain
Protein and RNA
Polypeptide synthesis proceed until
ER signal sequence has been formed
Srp bind to signal sequence and ———
Block further translations
ER signal sequence inserted into translocation once ———
SRP has been released
How the channel to ER lumen opens?
ER signal sequence contact interior of translocon
When polypeptide synthesis is complete
- polypeptide released into lumen
- ribosome detaches from ER membrane
-translocon pore closes
G protein (GTPases)
Molecular switch between GTP- and GDP- bound state
Different conformations= different affinities
G-proteins play roles in
- cell signaling
- cell division
- proteins synthesis
- vesicle fusion
SRP and SRP are
Both G protein
One mahor group of polypeptide synthesized on RER is molecules destined to be
Integral Membrane Proteins
Transmembrane proteins can be
Single or multi pass
Oligosaccharide chains are always ———
On non-cytosolic side
Translocon and membrane have
Charge asymmetry
—— more negative on cytosolic side
Hydrophobic transmembrane domain can ——— in lipid bilayer
Dissolve
Orientation of multi-pass protein integral membrane protein is determined by ———
Charge/orientation of first transmembrane domain
—— each subsequent transmembrane domain must have opposite charge
RER is site of
-Protein modification
-recognition and removal of misfolded proteins
-lipid synthesis
Glycosylation in (ER and Golgi) =
Glycoproteins
Most proteins synthesized in RER are glycosylated on ———
amide nitrogen of asparagine
Glycosylation is then modified by
Golgi
Carbohydrate groups in glycoproteins function as :
-macromolecule binding site
-aid protein folding
-increase stability
Co-translational glycosylation
Oligosaccharides are added to protein during synthesis
Processing of the N-linked oligosaccharide in ER lumen
Initial process when 3 glucose residue and 1 mannose are removed
WHY?
Possible sort of signal indicating
As polypeptides enter ER lumen
Fold into final shape
Chaperones of N-linked oligosacvharides
Calnexin and calreticulin
Binding of calnexin and calreticulin ———
Prevent aggregation and drives disulfide bond formation
Bip
Binding proteins chaperone
Binds to hydrophobic regions and prevent interactions
Bip release the polypeptide chain then:
If fold correctly, hydrophobic region is buried in the interior
If fold incorrectly, interacts with BiP again
Disulfide bond formation enzyme
Protein disulfide isomerase PDI
In eukaryotic cells, disulfide bonds are only formed in
ER lumen
Disulfide bond formation occurs between
Adjacent cysteines
Proteins can be —— faster than they can be ——
Made faster than fold
Higher made than fold activate
Unfolded Protein Response (UPR)
UPR will
Sensor molecules in ER detect misfolded proteins
When UPR is activated
1- phosphorylate translation factor
(Inhibiting protein synthesis)
2- upregulate the expression:
— er based chaperones
— transport proteins out of ER
— protein degradation machinery
Component of UPR that recognize misfolded or unassembled proteins
ER-associated degradation (ERAD)
What happen to misfolded proteins
Export to cytosol and degraded by proteasomes
Proteasomes
Bind ubiquitin-labeled proteins
Remove ubiquitin
Fed the protein in central channel
Ubiquitin is joined to target protein by
3enzymes
E1—— ubiquitin activating
E2—— ubiquitin conjugating
E3—— ubiquitin ligase
Primary source of membrane lipid
ER
Why ER is primary
Because other doesn’t have the required enzymes
Except ER who can produce lipid
Mitochondria, chloroplasts, peroxisomes
Fatty acids required for lipid synthesis
Are synthesized in cytoplasm then go to ER membrane on cytosolic side
Then, transfer to lumenal side by flippases
Most organelles have enzymes modifying lipids
Converting one type to another
Exchange lipids between compartment
Lipid Transfer Protein
