(L8&9) Protein targeting

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What is the signal sequence for protein targeting to the nucleus?
Nuclear localisation signal
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Describe the nuclear localisation signal
One or two short sequences with basic amino acids, precise location in protein is not critical
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When are proteins folded?
Before import to nucleus
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What do the proteins pass through to get into the nucleus
Nuclear pore complex
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What does the nuclear pore form?
A diffusion barrier, requires GTP hydrolysis
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Describe the process by which proteins are transported into the nucleus through the nuclear pore
Proteins pass through pore, bound to nuclear import receptor, Ran GTPase with GTP binds to receptor, displacing cargo, Ran-GTP plus receptor passes out of nucleus, Ran dissociates, GTP hydrolysed, Ran-GDP transported back to nucleus via own receptor
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What is the opposite of this process?
Export of cargo from nucleus to cytoplasm (e.g. mRNA, Ribosomes), processes correspond to each other
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What does a mitochondria and plastids contain?
~1000 proteins (plastids contrain ~1300), DNA, mRNA, Ribesomes and tRNAs
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Where do they get their proteins from?
They synthesis a minority of them, they tend to get them from import
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What do mitochondria and plastids give evidence for?
Endosymbiosis
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What do most precursors of mitochondrial proteins have?
Amino-terminal signal sequence or transit peptide (some have internal signal sequences)
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What can the signal sequences form?
Amphipathic helix with charged amino acids on one side and hydrophobic amino acids on the other, the hydrophobic face binds to a groove in the import receptor
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When does import occur?
After translation is complete but precursor proteins do not fold
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What do the protein precursors bind to?
Chaperone Hsp70
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What must matrix proteins of the mitochondria pass?
Both the inner and outer membrane
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Where does this probably happen?
Contact sites where membranes are close together
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What does this require?
TOM complex in the outer membrane, TIM complex (TIM23) in the inner membrane, TIM23 spans both membranes
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Describe the process of the peptides moving through the mitochondria membrane
Transit peptide binds to receptor protein in TOM complex, polypeptide passes through TOM complex and TIM23 complex, protein translocated into matrix, polypeptide folds and signal peptide is removed, peptidase is soluble
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When are the chaperons removed?
As the polypeptide enters TOM, this requires ATP hydrolysis
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What does folding inside the mitochondria involve?
Mitochondrial Hsp70 and Hsp60
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What does mitochondrial Hsp70 do?
Hsp70 binding and associated ATP hydrolysis pulls the protein across the membrane, membrane potential also contributes
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Do all proteins pass through TOM and TIM23?
No, there are several routes to the membranes and the intermembrane space, some bypass TOM and TIM23
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Give an example of an alternate pathway
Some inner membrane proteins w/ multiple membrane-spanning domains are inserted into membrane from intermembrane space, includes adenine nucleotide translocator and other metabolite translocators, requires TIM22 complex, proteins-> internal signals
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How id the protein import into plastids similar to mitochondria?
Post-translational, transit peptides (signal sequences) have amphiphatic helices, import requires protein translocators in inner and outer membranes
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Which protein translocators are used in chloroplasts?
TOC (outer membrane) and TIC (inner membrane)
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How is a protein folded inside a chloroplast?
Transit peptide is removed by peptidase, protein folds with assistance from Hsp70 and Hsp60 chaperones, energy source: ATP or GTP hydrolysis
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What is thought to be the result of secondary endosymbiosis?
Three or four membranes
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What organisms have three membranes around their plastids?
Dinoflagellates
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What are the two signal sequences that chloroplast proteins are synthesised with?
Signal peptide (Into EER) and transit peptide (into plastids and mitochondria)
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What is the mechanism that chloroplast proteins are synthesised?
Signal peptide directs protein to the ER, protein moves by vesicular transport to Golgi then to chloroplast, transit peptide directs protein through the inner two membranes
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Other cards in this set

Card 2

Front

Describe the nuclear localisation signal

Back

One or two short sequences with basic amino acids, precise location in protein is not critical

Card 3

Front

When are proteins folded?

Back

Preview of the front of card 3

Card 4

Front

What do the proteins pass through to get into the nucleus

Back

Preview of the front of card 4

Card 5

Front

What does the nuclear pore form?

Back

Preview of the front of card 5
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