Nuclear Import & Export Flashcards
What is the name of the macromolecular pore that the nuclear membrane contains? How many?
~500-1000 Nuclear Pore complexes (NPC)
What does the Nuclear Pore complex contain?
- ~ 30 distinct nucleoporins (Nups)
- Membrane ring: (scaffold)
- Core scaffold
- cytoplasmic filaments
- Nuclear basket
3 distinct functions of NPC
Allow movement of moecules through nuclear membrane:
- diffusion through size filtering
- Spontaneous migration of amphiphilic molecules
- Facilitated transport
- cargo associates with an amphiphilic molecule to get it in ;)
What are Karyopherins known as?
Amphiphilic Cargo transporters aka Cargo receptors aka Importins/Exportins
What repeats are found in Nups?
FG repeats
-serve as hydrophobic binding sites (filaments) for NPC
Basic principles of facilitated transport across pore complex (4)
- E dependent
- T dependent
- Signal dependant
- Saturable
What are the two families of karyopherins?
- Receptor family
2. Adapters
Overview of Receptor family Karyopherins
- Contain Ran-GTP binding domain.
- Binds with FG domains of nucleoporins (Nups)
- Another domain binding specific cargo/multiple cargos
Overview of Adapter Karypherins
- Capable of selecting different cargos
2. Increases efficiency of transport
NLS typically consists of which aa?
Basic motif: arginine + lysine rich
NES typically consists of which aa?
hydrophobic motif: Leucine rich
Why is Importin is typically considered as a heterodimer?
Importin usually an alpha beta pair.
Consists of an alpha (adaptor) and beta (transport receptor)
tRNA and miRNA use what type of export mechanism?
Ran dependent transport
tRNA:Exp-t
List different RNA that get exported and their respective export factors
tRNA:Exp-t
miRNA: Exp-5
snRNA: CRM1
mRNA: NXF1 + NXT1, Aly
rRNA: CRM1, NXF1+ NXT1
Describe mRNA export
Note: not part of Karyopherin family
- initiated by Aly linking premRNA to nuclear export
- allows recruitment of transport factors NXF1 + NXT1
- Remodeling by DB helicase removes ALY and forms export competent complex
- Kapα adaptor binds mRNA
- Kapβ transport receptor binds Kapα and mRNA
- Protein complex and nuclear export protein and nuclear basket interact and export occurs
- Once on the other side, a bunch of proteins associate with it and cause the complex to dissociate
mechanisms on how cargo nucleocytoplasmic transport is regulated in:
cargo
- Post translational modification
- Post transcriptional modification
- intermolecular/intramolecular masking
- affinity enhancement
- sequestration
rRNA is ran dependent/independent transport
both
uses CRM1, NXF1+ NXT1
similar to snRNA and mRNA
mechanisms on how cargo nucleocytoplasmic transport is regulated thru:
cargo
- Post translational modification
- Post transcriptional modification
- intermolecular/intramolecular interaction
mechanisms on how cargo nucleocytoplasmic transport is regulated thru:
transport receptors
- expression
- sequestration
- via entropy barriers created by Nups
- Ran GTP/GDP can help overcome barrier
mechanisms on how cargo nucleocytoplasmic transport is regulated thru:
NPC
- pore permeability
2. protein expression and stability
Describe when it is necessary for BRCA2 to get imported into the nucleus and how it is able to stay localized in nucleus
- BRCA-RAD51 complex is transported into nucleus for DNA repair
- Complex is retained in nucleus bc NES on RAD51 is occluded by binding to BRCA2.
- BRCA2 NES is occluded by BSS1 binding during stressful conditions
Give an example of how error in nuclear transport can contribute to breast cancer
A mutation from Asp to His in BRCA2 prevents binding with BSS1
- which means BRCA2's NES is exposed, and the mutant is rapidly exported - RAD51 is also exported
Since BRCA2 is unable to stay in nucleus and function in DNA repair —> genomic instability –> breast cancer
Give an example of how error in nuclear transport can prevent TSGs from doing its job?
- Normally TSG are localized in cytoplasm
- Under stressful conditions, TSG can go into nucleus and do its job
- Many tumors have upregulation of nuclear export proteins
ie: CRM1 or MDM2 (exporting p53) - Tumor suppressors are still functional, so selective inhibitors that can prevent export can restore tumor suppressor function
