Protein Targeting: Endoplasmic Reticulum Flashcards
Where is the protein ER signal sequence located?
N-terminus of proteins destined for the ER
When is the ER signal sequence synthesised?
During protein translation
How many amino acids long is the ER signal sequence?
16 - 20
What types of amino acids are ER signal sequences comprised of?
1 or more charged residues and 6-12 hydrophobic residues
What does the Signal Recognition Particle (SRP) do in the targeting of secretory proteins to the ER?
- Binds to the signal sequence of proteins
- Slows elongation of the polypeptide
- Targets polypeptide + ribosome to ER
What is the structure of the Signal Recognition Particle (SRP)?
- Cytosolic ribonucleoprotein (RNA:Protein complex)
- 300 nucleotide RNA + 6 protein subunits
Which protein subunit of the Signal Recognition Particle (SRP) binds to the signal sequence and also GTP?
P54
Which protein subunits of the Signal Recognition Particle (SRP) interacts with the ribosome?
P9 / P14
Where is the SRP Receptor located?
ER membrane
What is the structure of the SRP receptor?
2 subunits (α and β)
Which subunit of the SRP receptor binds GTP?
α subunit
What promotes the interaction between the SRP and the SRP receptor?
Binding of GTP
What is the Translocon in the ER membrane composed of multiple copies of
Sec 61 complex
TRUE OR FALSE:
Translocation of proteins across the ER membrane is CO-TRANSLATIONAL in Eukaryotes?
True.
Occurs as peptide is being made by ribosome
Which enzyme cleaves off the signal sequence as the protein passes through the ER membrane?
Signal peptidase
TRUE OR FALSE:
Translocation of proteins across the ER membrane is CO-TRANSLATIONAL in Yeast?
False.
Post-translational translocation occurs
TRUE OR FALSE:
SRP / SRP receptor is involved during translocation of proteins across the ER membrane in both Eukaryotes AND Yeast?
False.
Not involved in Yeast
Which TWO proteins are involved in bringing the polypeptide chain through the translocon into the ER lumen in Yeast cells?
- Sec 63 complex
- BiP chaperone protein
What determines the orientation of proteins inserted into the ER membrane?
Topogenic sequences
How many Topological classes of integral membrane proteins are there?
4
How do Type I and III of the topological classes of integral membrane proteins differ?
A signal sequence is cleaved during insertion into membrane in TYPE I
What does the translocon do during translocation when an internal sequence of 20-25 hydrophobic amino acids is encountered for Type I integral membrane proteins?
- Stops further transfer of polypeptide through membrane
- Moves polypeptide laterally to anchor protein in the membrane
- Referred to as the Stop-Transfer Anchor (STA) sequence
TRUE OR FALSE:
Type II and Type III integral membrane proteins lack a cleavable N-teminal signal sequence
True
Where is the positive charge located of the signal anchor sequence in Type II integral membrane proteins?
N-teminal side
Where is the positive charge located of the signal anchor sequence in Type III integral membrane proteins?
C-teminal side
TRUE OR FALSE:
Type IV integral membrane proteins have only one transmembrane helices?
False.
They have multipe TM helices
What is the location of the Type IV-A N-terminus?
Cytosol
Signal anchor sequence of Type II (SA-II)
What is the location of the Type IV-B N-terminus?
Exoplasmic space
Signal anchor sequence of Type III (SA-III)
What is used to predict the topology of a transmembrane protein?
Hydropathy plot
Which type of integral transmembrane protein are GPI-anchored proteins initially synthesised and anchored as?
Type I
What is the function of GPI Transamidase?
Cleaves off the stop-transfer anchor sequence and attaches the rest of the protein to GPI anchor
What are TWO reasons to switch from a protein anchor to a lipid anchor?
- Greater mobility in the membrane
- Targeting to particular membrane domains
What THREE proteins assist the folding of proteins in the ER?
- Chaperone proteins
- Calnexin
- Calreticulin
What TWO ways do cells deal with unfolded proteins?
- Unfolded Protein Response
- Endoplasmic Reticulum Associated Protein Degradation (ERAD)
What happens during Endoplasmic Reticulum Associated Protein Degradation (ERAD)?
- Misfolded proteins are translocated backwards into the cytosol
- Degraded by the proteasome
Which post-translational modification is performed on most proteins in the ER?
Glycosylation
