Term
| What organelles make up the endomembrane system? |
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Definition
endoplasmic reticulum
golgi
endosomes
lysosomes/vacuoles
secretory granules
plasma membrane |
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Term
True or False
The endomembrane system organelles are highly conserved in eukaryotes |
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Definition
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Term
| What are the general steps of vesicle transport? |
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Definition
1. Vesicle which contains cargo buds off from donor membrane compartment
2. Vesicle is transported to recipient membrane compartment
3. Vesicle fuses with recipient membrane
4. Budding & fusion repeats in either direction |
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Term
True or False
Vesicles are somewhat selective in terms of which membrane & lumenal proteins can enter |
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Definition
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Term
| How are nascent vesicles transported through the cytosol? |
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Definition
| via molecular motors and cytoskeletal elements |
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Term
| What are some examples of trafficking pathways that exist within the endomembrane system? |
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Definition
Biosynthetic pathway
Secretory pathway
Endocytotic pathway |
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Term
| What is the biosynthetic pathway? |
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Definition
| ER to Golgi to endosomes then o lyososomes OR plasma membrane |
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Term
| What are the two types of secretary pathways? |
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Definition
| constitutive & regulated secretion |
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Term
| What is constitutive secretion? |
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Definition
| materials from the ER are continuously being transported to the Golgi and then the PM, and/or released outside of the cell |
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Term
| What is regulated secretion? |
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Definition
| materials from ER go to Golgi then are stored in secretory granules, which will fuse with the PM in response to specific signals and then release their cargo into the extracellular space |
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Term
| Is the release of neurotransmitters into the synaptic cleft an example of constitutive or regulated secretion? |
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Definition
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Term
| What is the endocytic pathway? |
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Definition
| materials from the PM or extracellular space are incorporated into the cell then transported to endosomes or lysosomes |
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Term
| What is the starting point for the biosynthetic & secretory pathways? What happens here? |
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Definition
endoplasmic reticulum
site of lipid & protein synthesis, protein folding, processing |
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Term
| Describe the structure of the endoplasmic reticulum |
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Definition
| Network of rod-like tubules and sheet-like cisternae with multiple subdomains |
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Term
| What mediates the shape of tubules & cisternae? |
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Definition
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Term
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Definition
| integral proteins of the ER membrane which have a V-shaped secondary structure, and regulate the curvature of the membrane |
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Term
True or False
The ER is a very stable, constant network |
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Definition
False
It is dynamic: cisternae & tubules are constantly undergoing bending, fission, fusion etc. |
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Term
| Describe the structure & function of the RER |
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Definition
| mainly cisternae, has ribosomes, involved in protein & phospholipid synthesis |
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Term
| Describe the structure & function of the SER |
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Definition
| mainly tubules, no ribosomes, involved in Ca2+ sequestration and hormone synthesis |
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Term
| Describe the relationship between the outer nuclear membrane & RER |
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Definition
| -outer nuclear mmb is continuous with RER, contains NUPs, is attached to ribosomes |
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Term
| What is the term for the regions where transport vesicles bud off and head to the Golgi? |
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Definition
| endoplasmic reticulum exit sites (ERES) |
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Term
True or False
There are a large number or ribosomes localized around ERESs |
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Definition
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Term
| What is the function of MAM & PAM? |
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Definition
| Allow easy transfer of phospholipids from the ER to mitochondria or the plasma membrane, without a need for vesicles |
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Term
| What are the two main sites of translation in the cell? |
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Definition
| free ribosomes in cytosol & ER membrane-bound ribosomes |
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Term
| Where do proteins synthesized by free ribosomes go? |
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Definition
-remain in the cytosol e.g. glytolitic enzymes
or
-targets to an intracellular destination e.g. to chloroplasts |
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Term
| Where do proteins synthesized by membrane-bound ribosomes go? |
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Definition
-remain in RER or go to another ER subdomain
or
-targets to another compartment in the endomembrane system e.g. Golgi |
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Term
| Describe co-translational translocation of soluble proteins into the RER lumen (brief summary) |
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Definition
1. N terminus of a growing polypeptide emerges from ribosome, signal sequence is recognized by an SRP, SRP binds to ribosome & stops translation
2. SRP targets complex to ER surface
3. SRP is released, ribosome binds to cytosolic side of translocon, translation continues
4. Growing polypeptide moves through translocon, signal sequence enters lumen is cleaved, translation stops, ribosome is released |
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Term
| What is a signal sequence? |
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Definition
| A sequence of hydrophobic amino acids in the N-terminus of a nascent polypeptide, which acts as an ER targeting signal to indicate where the protein needs to go |
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Term
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Definition
Signal Recognition Particle
6 proteins + 1 small RNA |
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Term
|
Definition
| integral membrane protein that forms the pore through which translation takes place |
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Term
| What happens immediately after an SRP binds to a free, active ribosome? |
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Definition
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Term
| After an SRP has stopped translation, what happens? |
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Definition
| complex moves to surface of the ER, SRP binds to an SRP receptor |
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Term
|
Definition
| an integral membrane protein complex with cytosolic domains which act as a docking site for an SRP |
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Term
|
Definition
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Term
| What happens to the SRP after it is released? What happens as the SRP is released? |
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Definition
It returns to cytosol for another round of protein import
Simultaneous to the release, the ribosome binds to Sec61 |
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Term
| What does the 'release' step of co-translational translocation depend on? |
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Definition
| GTP hydrolysis which causes a conformational change in the SRP and the SRP receptor (G proteins!) |
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Term
| Describe the structure of a translocon |
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Definition
| multi-protein complex with several subunis, forming an hour-glass shaped pore with a central ring which acts as a gate |
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Term
| Describe the closed state of a translocon |
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Definition
-opening of the pore ring is too narrow for translocation to occur
-alpha helix plug blocks opening |
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Term
| What is the function of the alpha helix plug in the closed state of a translocon? |
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Definition
| prevents ions & small molecules from moving from the ER lumen to the cytosol or visa-versa |
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Term
| What happens when the ribosome binds to Sec61? |
|
Definition
-translation resumes
-signal sequence interacts with interior of translocon, displacing the plug and widening the pore ring |
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Term
| Describe the events of translocation once the translocon has been opened |
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Definition
-signal sequence enters lumen & is cleaved by signal peptidase
-cleaved protein enters lumen, is glycosylated, reticuloplasmins begin folding it |
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Term
| What is glycosylation and why is it important? |
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Definition
| The addition of sugars to a polypeptide, it is essential or proper folding of proteins |
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Term
| What is signal peptidase? |
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Definition
| a protease found next to the translocon, it will cleave the signal sequence of a polypeptide as it enters the lumen of the ER |
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Term
| What are reticuloplasmins? Give an example of a reticuloplasmin |
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Definition
| chaperone proteins which mediate folding and prevent protein aggregation e.g. Binding immunoglobulin protein (BIP) |
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Term
| What happens to the ribosome after it is released from the translocon? What happens to the translocon? |
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Definition
It returns to cytosol for another round of import
Translocon pore ring closes, alpha subunit plug returns |
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|
Term
True or False
All membranes form from pre-existing membranes |
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Definition
|
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Term
| Which membrane proteins & lipids are not synthesized in the ER? |
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Definition
glycolipids (synth. in Golgi)
unique chloroplast & mitochondrial proteins & lipids |
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Term
| Where is asymmetry of membrane proteins & lipids established? |
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Definition
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Term
| Where will the lumenal domain of an integral membrane protein be found if it is incorporated into the plasma membrane? |
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Definition
| on the extracellular face of the plasma membrane |
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Term
| Summarize the steps of co-translational insertion of an integral membrane protein into the RER |
|
Definition
1. N terminus enters translocon
2. TMD interacts with hydrophobic pore ring, stops translocation, signals pore ring to open laterally (TMD is flipped if needed for proper orientation)
3. Synthesis of the cytosolic-facing C terminus resumes
4. Signal sequence is cleaved at the N-terminus in the lumen
5. Glycosylation
6. Protein folding & assembly
7. Quality control to handle misfolded proteins |
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Term
|
Definition
transmembrane domain
alpha-helical, hydrophobic stretch of amino acids whih interacts with the hydrophobic pore ring of a translocon |
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|
Term
| What determines the topology of all membrane proteins? |
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Definition
| positively-charged AA's found upstream of the TMD, which interact with the negatively charged translocon |
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Term
| When is the signal sequence cleaved from the N-terminus of a nascent protein? |
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Definition
| As soon as it enters the ER lumen |
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Term
| What are oligosaccharides? What is their function? |
|
Definition
short chains of sugar monomers linked together
-assist in binding with macromolecules
-assist in protein folding
-important in intracellular trafficking |
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Term
| When an oligosaccharide is added to a protein it becomes a.... |
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Definition
|
|
Term
| What is the most common type of glycosylation? |
|
Definition
N-linked glycosylation
-addition of short chains of monomers to the N-terminus of an Asparagine |
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|
Term
| What are the 2 stages of N-linked glycosylation? |
|
Definition
1. Core glycosylation
2. Core modification |
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|
Term
| What is a core oligosaccharide? |
|
Definition
| highly branched oligosaccharide chain of 14 sugar residues, including 3 terminal glucose units |
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Term
| What is core glycosylation? |
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Definition
| -synthesize of a core oligosaccharide which culminates with the linking of this core to a specific Asn (N) amino acid on the lumenal-facing terminus of a protein |
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Term
| What enzyme links the core oligosaccharide to a specific Asn residue in core glycosylation? |
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Definition
| oligosaccharyl transferase |
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|
Term
| BiP, calrexin & calreticulin are examples of.... |
|
Definition
| reticuloplasmins (molecular chaperones) |
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Term
| What is core modification? What steps are involved? |
|
Definition
Trimming & modification of core oligosaccharide
-first 2 glucose units are trimmed
-glycoprotein is folded
-glycoprotein undergoes quality control
-glycoprotein binds to calnexin
-glucosidase II trims last glucose unit
-protein is released from calnexin |
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Term
| What enzymes trim the two terminal glucose units in core modification? |
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Definition
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Term
| What are the possible ways that a misfolded protein can be handled? |
|
Definition
-correction of the improper folding
-retrotranslocation out of the ER
-ERAD pathway
-UPR pathways |
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|
Term
| Describe how an improperly folded protein might be corrected |
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Definition
-glucosyltransferase recognizes sugars that are normally masked if the protein were folded properly
-GT adds a glucose monomer to the protein
-protein re-binds to calnexin
-protein is refolded |
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Term
| What happens if GT & calnexin cannot fix a single misfolded protein? |
|
Definition
-destruction via ERAD pathway
or
-translocated back out of the ER via he cytosol by retrotranslocation |
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Term
| What is the ERAD pathway? Describe what happens |
|
Definition
ER-Associated Degradation pathway
-misfolded protein is poly-ubiquitinated
-Ub-protein binds to lid of proteasome
-Ub chain is removed & recycled
-protein is threaded into proteasome
-protein is degraded via proteolysis
-AA residues are reused to make new proteins |
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Term
| What is the function of mono-Ub? |
|
Definition
| signals membrane protein import into endosomal vesicles |
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Term
| What is the function of poly-Ub |
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Definition
| signals proteosome degradation of proteins in the ER |
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|
Term
| what happens if misfolded proteins accumulate faster than the ERAD pathway can handle them? |
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Definition
| Unfolded Protein Response Pathways are activated (UPR pathways) |
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Term
| What kind of quality control pathway handles the mutant transporter proteins produced by CF patients? |
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Definition
| ERAD, UPR once accumulation is significant |
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Term
| What kind of quality control pathway handles the misfolded proteins which are produced in patients with Alzheimers? |
|
Definition
|
|
Term
| What activates the UPR pathways? |
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Definition
| ER 'stress' causes PERK and ATF6 sensor proteins to release their bond with BiP |
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Term
|
Definition
Protein RNA-like ER kinase
Activating Transcription Factor 6 |
|
|
Term
True or False
PERK & ATF6 are activated when BiP chaperone proteins bind to them |
|
Definition
False
BiP is bound when these proteins are inactive (ER is not in 'stress' mode) |
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|
Term
| Describe the action of the PERK-mediated UPR pathway |
|
Definition
-BiP is released from PERK
-PERK sensors dimerize, become active
-cytosolic domains of PERK phosphorylate & inhibit the TF eIFa
-protein synthesis in cell is downregulated
-available chaperones can focus on the existing misfolded proteins in the ER
-ER stress is either alleviated, or continues to be dysfunctional and dies |
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Term
|
Definition
| a cytosolic transcription factor which is essential for initiating protein synthesis |
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Term
| Describe the action of the ATF6-mediated UPR pathway |
|
Definition
-BiP is released from ATF6
-ATF6 is activated, moves from ER to Golgi, cytosolic transcription factor is cleaved off
-ATF6 TF is targeted to nucleus
-ATF6 TF up-regulates transcription of various genes
-ER stress is alleviated or cell continues to dysfunction and dies |
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Term
| ATF6 up-regulates the transcription of gene which encode for which proteins? |
|
Definition
-ER molecular chaperones (assist in protein folding)
-ER export components (move properly folded proteins out of ER)
-ERAD components (degrade misfolded proteins) |
|
|
Term
| What enzyme adds a glucose monomer back on to a misfolded protein in ER 'quality control'? |
|
Definition
|
|
Term
|
Definition
binding immunoglobulin protein
(reticuloplasmin) |
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