MCB 2050 PT2 EXAM QUESTIONS WITH CORRECT ANSWERS LATEST UPDATE 2025/2026
Describe the steps involved in the trafficking of a protein from the cytoplasm into the nucleus.
Include in your answer how the receptors involved in this process are
'recycled' back to the cytoplasm for additional rounds of nuclear protein import. - Answers Step
1: importin binds to nascent NLS-containing 'cargo' protein; formation of importin /-cargo
complex
Step 2: importin-cargo complex moves to NPC (via cytoskeleton) and importin binds to
cytoplasmic filaments
Step 3: importin-cargo complex moves through FG Nups/'mesh' within aqueous central channel
Step 4: importin-cargo complex binds to nuclear basket and then (via importin ) to Ran-GTP -
causes release & disassembly of cargo protein and importin and into nucleoplasm
NLS on cargo protein not cleaved (allows nuclear re-import)
Step 5: Ran-GTP-importin b moves back to cytoplasm due to [Ran-GTP] gradient
Ran-GTP converted to Ran-GDP (GTP hydrolysis) by GAP
importin b released and Ran-GDP diffuses back into nucleus due to [Ran-GDP] gradient and
converted to Ran-GTP (GDP to GTP exchange) by GEF
Step 5+: exportin binds to exposed NES in importin
* exportin - receptor binds to NES-containing 'cargo' proteins (importin , cyclin, etc) and
mediates nucleus-to-cytoplasm transport
Step 5+: importin -exportin complex binds to Ran-GTP; stabilized complex moves to cytoplasm
due to [Ran-GTP] gradient
Step 5+: in cytoplasm, Ran-GTP converted to Ran-GDP by GAP
Importin a released' from exportin
exportin (w/ exposed NLS) returns to nucleus (via importin)
Ran-GDP diffuses back into nucleus (due to Ran-GDP gradient) and converted to Ran-GTP by
GEF
[Ran-GTP] nucleus > [Ran-GTP] cytoplasm
Describe the steps involved in co-translational insertion of a single-spanning, endoplasmic
, reticulum integral membrane protein that possesses a Type II (Ncytoplasm - Clumen ) topology.
- Answers Co-translational translocation of a soluble protein into RER lumen
(Step 1 & 2)
recognition of exposed signal sequence by SRP; SRP binds to ribosome and translation stopped
(Step 3)
SRP-ribosome complex targets to RER surface and SRP binds SRP receptor
(Step 4)
release of SRP and SRP receptor and transfer of polypeptide and ribosome to Sec61 translocon
('opens' - pore ring & -helix plug); translation resumes; growing polypeptide translocated
(Step 5 & 6)
signal sequence cleaved by signal peptidase; protein translocation continues
(Step 7 & 8)
translation completed; ribosome released/disassembled; translocon closes; nascent protein
processed/folded in ER lumen
Type II membrane protein -
no signal sequence; TMD = SA; translocon recognizes +ve residues N-terminal of SA and 'flips'
SA - N-terminus of protein reorientated towards cytosol; SA moves laterally out of translocon;
remaining polypeptide (C-terminus) translocated into ER lumen
Describe the steps involved in the quality control of a newly-synthesized protein in the
endoplasmic reticulum lumen and, for a protein that is not properly processed, the ERAD
pathway - Answers reticuloplasmins/PDI bind to glycoprotein during synthesis
- ER lumen glucosidase removes last glucose unit in step 3
- Protein released from reticuloplasmins
If protein is proper:
- 1 mannose trimmed by ER lumen mannosidase (4), Functions in ER/transported to Golgi via
vesicles
Misfolded:
- UGGT (monitoring) glucosyltransferase recognizes
Describe the steps involved in the trafficking of a protein from the cytoplasm into the nucleus.
Include in your answer how the receptors involved in this process are
'recycled' back to the cytoplasm for additional rounds of nuclear protein import. - Answers Step
1: importin binds to nascent NLS-containing 'cargo' protein; formation of importin /-cargo
complex
Step 2: importin-cargo complex moves to NPC (via cytoskeleton) and importin binds to
cytoplasmic filaments
Step 3: importin-cargo complex moves through FG Nups/'mesh' within aqueous central channel
Step 4: importin-cargo complex binds to nuclear basket and then (via importin ) to Ran-GTP -
causes release & disassembly of cargo protein and importin and into nucleoplasm
NLS on cargo protein not cleaved (allows nuclear re-import)
Step 5: Ran-GTP-importin b moves back to cytoplasm due to [Ran-GTP] gradient
Ran-GTP converted to Ran-GDP (GTP hydrolysis) by GAP
importin b released and Ran-GDP diffuses back into nucleus due to [Ran-GDP] gradient and
converted to Ran-GTP (GDP to GTP exchange) by GEF
Step 5+: exportin binds to exposed NES in importin
* exportin - receptor binds to NES-containing 'cargo' proteins (importin , cyclin, etc) and
mediates nucleus-to-cytoplasm transport
Step 5+: importin -exportin complex binds to Ran-GTP; stabilized complex moves to cytoplasm
due to [Ran-GTP] gradient
Step 5+: in cytoplasm, Ran-GTP converted to Ran-GDP by GAP
Importin a released' from exportin
exportin (w/ exposed NLS) returns to nucleus (via importin)
Ran-GDP diffuses back into nucleus (due to Ran-GDP gradient) and converted to Ran-GTP by
GEF
[Ran-GTP] nucleus > [Ran-GTP] cytoplasm
Describe the steps involved in co-translational insertion of a single-spanning, endoplasmic
, reticulum integral membrane protein that possesses a Type II (Ncytoplasm - Clumen ) topology.
- Answers Co-translational translocation of a soluble protein into RER lumen
(Step 1 & 2)
recognition of exposed signal sequence by SRP; SRP binds to ribosome and translation stopped
(Step 3)
SRP-ribosome complex targets to RER surface and SRP binds SRP receptor
(Step 4)
release of SRP and SRP receptor and transfer of polypeptide and ribosome to Sec61 translocon
('opens' - pore ring & -helix plug); translation resumes; growing polypeptide translocated
(Step 5 & 6)
signal sequence cleaved by signal peptidase; protein translocation continues
(Step 7 & 8)
translation completed; ribosome released/disassembled; translocon closes; nascent protein
processed/folded in ER lumen
Type II membrane protein -
no signal sequence; TMD = SA; translocon recognizes +ve residues N-terminal of SA and 'flips'
SA - N-terminus of protein reorientated towards cytosol; SA moves laterally out of translocon;
remaining polypeptide (C-terminus) translocated into ER lumen
Describe the steps involved in the quality control of a newly-synthesized protein in the
endoplasmic reticulum lumen and, for a protein that is not properly processed, the ERAD
pathway - Answers reticuloplasmins/PDI bind to glycoprotein during synthesis
- ER lumen glucosidase removes last glucose unit in step 3
- Protein released from reticuloplasmins
If protein is proper:
- 1 mannose trimmed by ER lumen mannosidase (4), Functions in ER/transported to Golgi via
vesicles
Misfolded:
- UGGT (monitoring) glucosyltransferase recognizes
