Nucleus and nuclear import/export Flashcards
Nuclear pore complex major structural features
- 500-1000 total protein subunits
- made from 30 distinct proteins known as nucleoporins (Nups)
- form stacked ring structure with a 40nm central channel
- core scaffold provides stability and set the size of NPC
- sub complexes show 8 fold symmetry
- Nups in core scaffold are very long lived and diminish in aging cells
What anchors the nuclear pore complex (NPC) in the nuclear envelope?
Luminal ring
- Nups in luminal ring are transmembrane proteins (not well conserved across species)
What provides a selective permeability barrier in the nuclear pore complex?
the central pore
- Nups in the central pore and highly dynamic and contain FG repeats ( small, hydrophobic segments)
What are the principles for fast selective transport for nuclear pore complexes?
- diffusion-mediated transport of small molecules
- selective transport of larger cargo
- weak and flexible barrier for efficiency and to prevent clogging
What are FG repeats?
hydrophobic phenylalanine-glycine repeats
How do the centra pore Nups form a selective permeability barrier?
- FG repeats that are intrinsically disordered and highly dynamic
- FG repeats are hydrophobic but connected by flexible hydrophilic linkers
- creates a flexible, hydrophobic environment inside the pore
How does size-filtering diffusion work for nuclear pore complexes?
- driven by diffusion of hydrophilic molecules between the FG repeats inside the pore
- size less than or equal to 40 kDa (water, ions, smaller molecules)
How does spontaneous migration work for nuclear pore complexes?
- amphiphilic proteins (both hydrophobic and hydrophilic) that adjust to show more hydrophobic surfaces, allowing them to interact with FG Nups and pass through
- largest example is Dystrophin (427 kDa)
How does facilitated transport work for nuclear pore complexes?
- large, hydrophilic cargoes selectively bind to hydrophobic transport receptors, which facilitate interactions with FGs
- 230 kDa or more (most RNAs and proteins)
Principles of facilitated transport in nuclear pore complexes
- selectivity - cargoes selectively bind to amphiphilic transport receptors
- cargoes do not unfold - they remain hydrophilic but transport receptors unfold to facilitate migration
- directionality - cargoes are efficiently targeted for nuclear import or export
- transport does not require energy (energy is stabilizing or destabilizing the cargo and transport receptor)
What are nuclear pore complexes?
- transverse the double membrane of the nuclear envelope creating aqueous channels that are freely permeable to small molecules and ions but restrict passage of larger macromolecules
- stabilize sites of membrane fusion and promote fast and selective transport
Describe how transport receptors work with nuclear pore complexes
- cargo proteins contain specific amino acid motifs that are recognized by transport receptors
- transport receptors are necessary for transport and sufficient to cause transport
- Nuclear Localization Sequences (NLS) - bind to receptors that facilitate transport into the nucleus
- Nuclear Export Sequences (NES) - bind to receptors that facilitate transport out of the nucleus
- signal motifs can be re-used
What are karyopherins?
- cargo receptors for nuclear import/export
- amphiphilic proteins that bind to NLS or NES
- binding causes conformational change that exposes hydrophobic surfaces
- Importins - transport into the nucleus
- Exportins - transport out of the nucleus
What is the structure of the nuclear envelope?
- double bilayer, 30-50 nm between inner and outer membranes
- Outer Nuclear Membrane (ONM) is continuous with rough ER
- inner and outer membranes fuse at circular pores
What protein coordinates between the inner and outer nuclear membrane?
KASH (on outer nuclear membrane) connected to SUN (on inner nuclear membrane)
- stabilizes the nuclease
