Lecture 9: Protein Sorting Flashcards
(29 cards)
What organelles contain unfolded proteins?
mitochondria, plastids
What organelles contain folded proteins?
nucleus, peroxisomes
What are two types of sorting signals and what kind of transport are they used for
- linear signal sequences for protein translocation into organelles, found at N-terminus
- 3D arrangement of AAs forms the signal patch, used for nuclear and vesicular transport
What identifies an ER resident
a specific signal sequence called an ER retrieval signal of 4AAs (KDEL) at their C-terminus
What proteins is the NPC made up of
nucleoporins
Describe the binding action of nuclear import receptors
bind to NLS
bind to nucleoporins in NPC for transport into nucleus
What is an NLS
Nuclear Localization Signal, a short sequence rich in Lysine and Arginine that is attached to a cargo protein and allows for recognition by nuclear transport receptors
Describe the binding action of Nuclear export receptors
binds to NES
binds to nucleoporins in NPC for transport into cytosol
what is an NES
Nuclear export signal, specific sequences rich in Leucine found on a cargo protein tagged for transport into the cytosol. Recognized by nuclear export receptors
Describe nuclear import of cargo proteins stepwise
1) nuclear import receptor binds only cargo in CYTOSOL
2) receptor recognises NLS and both cargo and receptor move TO NUCLEUS
3) complex reaches nuclear side where RanGTPase concentration is high, causing Ran-GTPase to bind
4) Ran-GTPase binding to receptor causes cargo release
4) Empty import receptor and Ran-GTPase move back to cytosol via facilitated diffusion
5) Ran-GAP promotes cutting of GTP to GDP (GTP hydrolysis)
6) Inactive Ran-GDP releases nuclear import receptor
7) Nuclear import receptor remains in cytosol for furthur transport, Ran-GDP is transported to nucleus via NTF2
8) Ran-GEF converts Ran-GDP to Ran-GTP to facilitate future cargo release
Describe nuclear export of cargo proteins stepwise
1) nuclear export receptor binds both Ran-GTPase and cargo in NUCLEUS
2) Receptor, cargo, and Ran-GTPase move to cytosol
3) Ran-GAP promotes GTP hydrolysis
4) cargo and export receptor are released
5) Empty export receptor returns to nucleus
6) Ran-GDP travels back to nucleus via NTF2
7) Ran-GEF converts Ran-GDP back to Ran-GTP to facilitate future transport
Describe the two possible states of Ran GTP-ase
GDP-bound (off) and GTP-bound (on)
What two proteins regulate RanGTPase and where are they found
Ran-GAP, found in the cytosol, and Ran-GEF, found in the nucleus
RanGAP
Cytosolic GTPase-activating protein. Triggers GTP hydrolysis and converts Ran-GTP to Ran-GDP (turns off)
RanGEF
Nuclear Guanine Nucleotide Exchange factor, promotes exchange of GDP for GTP, converting RanGDP to RanGTP (turns on)
How does the location of RanGAP and RanGEF affect the local GTP concentration
High RanGTP in the nucleus, low RanGTP in the cytosol.
Due to high RanGAP in cytosol, there is higher P concentration in cytosol (profuct of hydrolysis)
Due to high RanGEF in nucleus, there is higher GDP concentration in nucleus (result of exchange)
Describe the movement of RanGTP and RanGDP
RanGTP: moves to cytosol with the help of nuclear import and export receptors
RanGDP moves to nucleus, transported by NTF2
What is NF-AT
Nuclear factor of activated T-cells is an activator of transcription
Describe NF-AT action in an activated T-cell
With high Ca2+ concentration in the activated T-cell, NF-AT is dephosphorylated by calcineurin, a protein phosphatase which then binds to the nuclear export signal to block it, making the nuclear import signal available
Nuclear import signal allows NF-AT to enter nucleus and activate gene transcription
Describe NF-AT action in a resting T-cell
With low Ca2+ concentration, calcineurin detaches from the nuclear export signal site
ATP and active protein kinase phosphorylate the import signal site, blocking it
Exposed export signal site results in export
Gene transcription stops
What is NTF2
Nuclear Transport Factor 2; transports Ran-GDP to the nucleus (after it is formed through GTP hydrolysis from GAP
Must be in the nucleus to permit the action of GEF
in NF-AT example, what affect does calcium concentration have on nuclear import and export
high intracellular Ca2+= nuclear import
low intracellular Ca2+ = nuclear export
Where does transmembrane transport occur
ER, mitochondria, plastids, peroxisomes (proteins usually unfolded)
How do proteins remain unfolded in the cytosol
By associating with hsp70 chaperones
