Protein Trafficking Flashcards
(62 cards)
Which of the following pathways does not utilize vesicular transport
a) Golgi –> ER
b) secretory vesicles –> cell exterior
c) lysosome to late endosome
d) nucleus to cytosol
d) gated transport
What are the two ways that a signal can be built into a protein?
1) signal sequence (continuous linear sequence)
2) signal patch (3D conformation of signal sequence)
What three types of nucleoporins are found in NPC?
TM ring proteins (anchor NPC to envelope) scaffold nucleoporins (form layered ring structures) channel nucleoporins (line central pore)
How do the receptor-like fibrils that protrude from NPC differ?
On nuclear side, converge to form basket like structure
What size of molecules diffuse into nucleus freely via aqueous pores?
5kDa or less
What size proteins cannot enter nucleus via passive diffuse
50kDa and above
Describe the role of Ran in nuclear import
In nucleus, Ran-GTP binds to import receptor and cargo is released.
Once import receptor back in nucleus, Ran-GAP dephosphorylates Ran-GTP and it dissociates.
Import receptor can pick up cargo again.
Describe the role of Ran in nuclear export
Ran-GTP promotes cargo binding to export receptor
What is NFAT?
nuclear factor of activated T-cells
Describe the cycle of NFAT during T cell activation
- T cell activation causes high calcium in active T cell.
- Calcineurin binds and dephosphorylates NFAT, exposing its nuclear import signal.
- NFAT imported, activates IL-2 gene transcription.
- Ca++ eventually decreases
- Calcineurin drops off and NFAT is phosphorylated (using ATP) exposing nuclear export signal
What is the role of IL-2?
Stimulates clonal expansion of memory T cells
How does cyclosporine work to prevent organ rejection?
Inhibits calcineurin, prevents phosphorylation of NFAT and therefore NFAT translocation into T cells
What occurs to collagen polypeptide after translation? (up until ER)
Signal sequence on polypeptide is targeted to ER by signal recognition particle
Describe co-translational translocation
- Signal recognition particle binds to signal sequence, and translation is paused until attaches to SRP receptor in ER membrane
- Translation continues and translocation begins
What does the N-terminal signal sequence also act as?
START-TRANSFER sequence
The start-transfer sequence binds to ___________ and then is discharged into the bilayer once translocation is complete
active translocator
Describe the mechanism by which LDL receptor is placed in plasma membrane (4 steps)
- Start transfer sequence opens translocator
- Stop transfer sequence enters translocator
- Translocator opens and discharges start transfer sequence laterally
(one pass transmembrane protein)
What three changes occur in ER lumen that lead to formation of the triple helix in collagen?
1) signal peptide of N terminus dissolves
2) hydroxylation of lysines and prolines
3) glycosylation (addn of glucose/galactose monomers onto hydroxyl groups) on lysines
Why does lack of Vitamin C cause scurvy?
Because Vitamin C is a cofactor in the hydroxylations of lysines and prolines during the formation of the triple helix of collagen therefore collagen triple helices are loose
Does stop transfer sequence stop translation?
NO
When a single pass transmembrane protein is inserted into the ER membrane, what determine that the N terminus is on the cytosolic side?
If more positively charged AA immediately proceed hydrophobic core of start transfer sequence
How does a multipass transmembrane protein get inserted
same process as any transmembrane protein but translocator does not discharge start transfer sequence laterally until FINAL stop transfer sequence has entered
What is the significance of coated vesicles? Name three types
Different protein coats select different cargo. COPI, COPII, clathrin
After being processed in the golgi apparatus, what happens to procollagen?
It is packaged by COP1 proteins into a secretory vesicle destined for extracellular space
