Nuclear transport Flashcards
(29 cards)
What is the function of the nuclear pore complex (NPC)?
Bidirectional selective transportation of small polar molecules and large macromolecules.
What is transported across the nuclear envelope by NCP?
- Proteins: histones, DNA polymerases, RNA polymerases, transcription factors, RNA-processing proteins, ribosomal proteins, lamins…
- RNAs: mRNAs, rRNAs, tRNAs…
What is the nuclear pore complex (NPC) comprised of?
A set of 30 different nucleoporins, each nucleoporin occur multiple times, resulting in 500-1000 proteins in fully assembled nuclear pore complex.
Can small water soluble molecules passively through nuclear pore complex?
Yes. Nuclear pore complex contain aqueous passage.
How does the nuclear pore complex stop large macromolecules from freely diffusing in?
The channel nucleoporins with extensive unstructured regions forms a disordered tangle (FG meshwork core), it restricts the diffusion of big macromolecules while allowing small molecules to pass.
What are FG repeats?
phenylalanine-glycine repeats in the unstructured domains of the channel nucleoporins that line the central pore.
What are the functions of FG repeats
- Interact weakly, form protein tangle gel-like properties that impose permeability barrier to macromolecules
- Act as docking sites for nuclear import receptors
How does the properties of phenylalanine and glycine contribute to the function of FG repeats
glycine have no side chain, so confers flexibility to the FG meshwork. NCP needs flexibility to accomodate different cargos
Why is the structure of NPC flexible and dynamic?
To accommodate its diverse cargo.
Describe the structure of the NPC
What is the function of the nuclear localisation signal (NLS)
It is a sorting signal that directs nuclear proteins through the NPC and into the nucleus.
What are the characteristics of the NLS
- Present on nuclear proteins and proteins that transiently enter the nucleus - highly conserved amino acid sequence - precise sequence varying for different proteins - Most contain lots of basic/positively charged amino acids lysine and arginine - Forms patches or loops on protein surface
Where is the NLS located?
Can be located almost anywhere in amino acid sequence. Precise location mostly does not affect function. If one subunit of a multicomponent complex contains NLS then the whole complex gets imported into the nucleus
What is karyopherins?
Karyopherins are nuclear transport receptors (importins + exportins).
What are nuclear import receptors/importins?
Proteins that bind to both NLS and NPC proteins. Sometimes they bind to NLS through adaptor proteins.
What is the function of nuclear import receptors?
Bind to NLS and guide cargo through the NCP
Describe the model of how nuclear import receptors guide cargo through NPC.
- Receptor-cargo complex repeatedly bind, dissociate, and then re-bind to adjacent FG-repeat sequences
- When import receptors bind to FG repeats they dissolve the gel phase protein tangle and thus allowng cargo to pass
- Once inside the nucleus, import receptors dissociate with cargo and go back to cytosol.
How is the directionality of the import process ensured?
Import receptors only dissociate with cargo in the nucleus. So cargo cannot go back out.
How do macromolecules gets exported out of the nucleus?
- cargo have nuclear export signal
- Nuclear export signal binds to nuclear export receptors/exportins
- nuclear export receptors guide cargo through NPC to cytosol
How do nuclear export proteins guide cargo through NPC?
The same way as nuclear import receptors do but in opposite direction.
What is Ran?
- GTPase
- two states: GTP bound or GDP bound
What is RanGAP
- Ran GTPase activating protein (GAP) –> triggers GTP hydrolysis –> turns RanGTP to RanGDP
- cytosolic
What is RanGEF?
- Guanine exchange factor –> turn RanGDP to RanGTP
- nuclear
- anchored to chromatin
What are helper proteins?
Proteins the help facilitate nuclear transport, they include e.g. CAS, NUP50, NTF2
