Exam 3- Protein Transport across the Nuclear Pore Complex Flashcards
What’s the size limit for free diffusion across the NPC?
Less than 40kD
Why is it that the NPC restricts the passage of proteins across its lumen?
it is made with NUPs (nucleoporin proteins) that are rich in FG repeats that are selective in the middle of the channel to nucleic acids and proteins
What’s the amino acid repeat present in the Nucleoporins located in the central region of the NPC?
What environment is produced?
FG repeat (Phenylalanine – Glycine repeat).
Hydrophobic environment giving them a spaghetti like shape, so hydrophilic proteins cannot enter the cell
What are Nucleoporins?
Form the NPC, about 30 of them, have F-G repeats in their structure.
How is the RanGTP gradient across the Nuclear Envelope generated?
this is done through RanGEF which is in the nucleus, bound to chromatin. RanGEF exchanges the GDP for GTP so that it can once again disassociate importin/cargo complexes and carry the importin back out of the nucleus.
Where is the Ran GTPase Activating Protein (RanGAP) localized?
RanGAP- located in the cytoplasm of the nucleus, interacts with Ran/GTP to hydrolyze to RanGDP. when this occurs the complex falls apart making an importin and a RanGDP
Where is the Ran GTP Exchange Factor localized?
RanGEF- found in the nucleus where it is associated to DNA/ chromatin. Converts Ran GDP → RanGTP
Why is the RanGTP gradient important for nuclear traffic?
It favors/prevents the formation of import/export complexes or the disruption of the already existing complexes (for instance, RanGTP is REQUIRED for the formation of Export complexes, whereas RanGTP triggers the DISRUPTION of Import complexes).
What are importins?
assemble in the cytoplasm, in the presence of RanGDP, and fall apart in the nucleus, in the presence of RanGTP.
What are exportins?
assemble in the nucleus, in the presence of RanGTP, and fall apart in the cytoplasm, in the presence of RanGDP.
What’s a Nuclear Localization Signal (NLS)?
a signal that allows proteins larger than 40 kD to go into the nucleus. Consist of 4 or more basic (K / R) amino acids, can be adjacent or spaced.
recognized by Nuclear Transport Receptors
Sequence: KKKK | KRKxKR | KKKxxRR
What’s a Nuclear Export Signal (NES)?
a signal that allows proteins to go out of the nucleus. Consist of a hydrophobic Leucine Residues with space
recognized by exportins
Sequence: LxxxLxxLxL | LxxLxxLxL
What are karyopherins?
importins and exportins
allow proteins larger than the limit to go in and out of the nucleus. Carrier proteins
How is nuclear import/export regulated?
Some, like transcription factor- NF-kB, is complexed with IkB in its inactive form which hides the NLS, when it needs to be activated the IkB phosphorylated an targeted for degradation by poly-Ubiquitin.
Other molecules are regulated typically through phosphorylation at a Ser, Thr, or Tyr residue
